BIOLOGICAL OVERVIEW

The Drosophila ovarian tumor gene (otu) encodes a novel cytoplasmic protein crucial to a variety of processes and cells active during oogenesis. Otu ensures the survival of female germ cells in pupae, cyst formation in germ-line cells, the attainment of mature chromosome structure in nurse cells, and egg maturation. Otu function is thought to involve cytoskeletal function, but it is unknown whether Otu is part of the function of the tubulin based microtubule cytoskeleton or the actin based microfilament cytoskeleton. This essay will outline the various functions of Otu and consider evidence of Otu's involvement in either microtubular or microfilament function.

During oogenesis, germ-line cells divide four times to give rise to germ-line cysts, each containing 16 interconnected cells known as cystocytes. 15 of the cells differentiate into nurse cells, which synthesize and transport products required for the development of the remaining cell, the oocyte. The otu gene has long been thought to play a key role in cyst development and growth based on the analysis of three classes of mutant alleles (Storto, 1988). In the presence of quiescent (QUI) alleles, germ-line cells are absent; this has a negative impact on germ cell survival. In contrast, germ cells overproliferate to form benign tumors in the presence of oncogenic (ONC) alleles. When differentiating (DIF) alleles are present, nurse cells display abnormal chromosome condensation, fail to grow normally and do not fully transfer their contents to the oocyte.

The earliest developmental defect associated with otu-null mutants is manifest during the pupal stage in lower survival rates for female germ cells. This otu function is shared with another gene, the transcripition factor ovo, which, when missing, produces ovarian tumors. Mutant phenotypes for females lacking ovo function show up even earlier in development, at the end of the first larval instar. At this stage, a minority of mutant gonads have lost their germ cells. This is in contrast to the phenotype for females lacking otu. Such females have a wild type number of germ cells through all larval stages. In these animals most of the germ cells die during pupariation, although some do survive and divide, but do not differentiate within the adult ovary. Since ovo and otu are required for the survival of XX germ cells, they must control a vital, sex-specific process in these cells. Do the two genes control the same process? Maternal ovo transcripts are present in pole cells (which are prospective germ cells) until embryonic stage 9. In the case of otu, maternal transcripts are also found in 0-4 hour embryos (Geyer, 1993). If required this early in development, maternal ovo and otu products might be thought to assure the survival of XX germ cells before these genes are transcribed zygotically. In this scenario, a vital process served by maternal ovo has already taken place by the end of the first larval instar (Staab, 1996).

With respect to the ovarian tumor phenotype, several genes have been characterized in Drosophila that carry a common defect in oogenesis. In addition to ovarian tumor (otu), these include ovo, benign gonial cell neoplasm, sans fille, Sex lethal, fused and, bag of marbles. Mutations at these loci result in the absence of mature germ cells, and in the overproliferation of small cells with the morphological characteristics of undifferentiated germ cells. Keeping this in mind, the possibility that Otu affects the actin based cytoskeleton will be considered.

A prominent feature of germ cell differentiation is the formation of the cytoplasmic bridges connecting the cystocytes. In oogenesis, germ line stem cells contain a spectrin-rich body known as the spectrosome. As the cystoblast divides, the spectrosome gives rise to an elongated structure known as the fusome, which extends through the ring canals that connect cystocytes to form branching connections between daughter cystocytes. (Cystocytes normally do not undergo complete fission but remain connected via ring canals). Some components of the fusome have been identified, including adducin (the product of the hu-li tai shao gene, a protein that promotes actin assembly), alpha-spectrin and Bag-of-marbles (see alpha Spectrin for a discussion of fusome function).

A comparison was made between the structure of wild-type germaria and the germaria of flies mutant for either strong or weak alleles of otu. The otuPdelta1 allele constitutes a deletion of the otu coding region. Mutant flies are female-sterile and produce either agametic ovarioles or tumorous egg chambers. The otuPdelta1 mutation causes enlargement of germarial region I, resulting from an increased number of germ cells arrested at stages prior to when they would normally interact with follicle cells. Severe hypomorphic alleles of otu form primarily tumorous egg chambers that superficailly resemble the tumors produced by the otuPdelta1 null mutations. However, both hypomorphic alleles otuPdelta3 and otu13 mutant germ cells can undergo further differentiation than do otuPdelta1 mutants; the former two appear to reach stages of development associated with region II of the ovarium, a stage when germ cells normally interact with the migrating follicle cell layer. In normal ovaries, no actin, as revealed by phalloidin staining is apparent in the fusome. The presence of detectable actin filaments in spectrosomes but not fusomes suggests that changes in actin filament localization or accessability normally occur during the spectrosome-to-fusome transition. As with wild type, otuPdelta3 mutants contain spectrin; the cystocytes still have high levels of cortical actin filaments. However, in contrast to wild type, substantial levels of actin filaments were detected in all mutant fusomes examined. These data indicate that otu is required for normal fusome structure, in particular actin filament organization or levels during the conversion of a spectrosome to a fusome (Rodesch, 1997).

Differentiate (DIF) otu alleles disrupt actin organization in stage 10 egg chambers. The cytoplasmic dumping that occurs during stage 10 is believed to require subcortical actin for nurse cell contraction and a complex array of cytoplasmic actin filaments, which anchor the nurse cell nucleus to the plasma membrane. Nurse cells mutant for the DIF allele otu7 fail to form this cytoplasmic actin array. Even in hypomorphic alleles there is a major disruption in the cytoplasmic actin cytoskeleton, while cortical actin appears unaffected. However, in addition to the loss of actin fibers connecting the nucleus to the plasma membrane, actin filaments are observed around the nucleus and partially extending to the plasma membrane. It therefore appears as if otu mutations cause actin polymerization to initiate from the nucleus (Rodesch, 1997).

A different picture of otu function is apparent when considering the gene cup and its effects on meiotic chromosome segregation. In a screen for female sterile P element insertions that effect nurse cell chromosomes, one mutation in the collection (fs(2)04506) proved to be allelic with a previously described female sterile gene, fs(2)cup. In most cup alleles nurse cell chromosomes fail to decondense completely; therefore, individual chromosomal masses remain distinct. In many respects cup phenotypes appear to be remarkably similar to otu phenotypes. As with otu, cup alleles can be grouped into three general classes based on the stage at which their oogenesis is arrested. Class I, the largest set of alleles, causes egg chambers to arrest prior to vitellogenesis. The strongest class I alleles produce enlarged and misshapen germaria where the egg chambers sometimes bud abnormally. Egg chambers from class II females grow larger than class I egg chambers, taking up yolk and sometimes supporting follicle cell migration. However, nurse cell nuclei display abnormal chromatin configurations. Females from class III, the weakest group of cup alleles, produce defective mature eggs, characteristically shaped like cups. Egg chambers that will develop into cup-shaped eggs have normal proportions during early stages, but during stages 9 and 10, their oocytes reach only a quarter to half the size of corresponding wild-type oocytes. The decreased size of the cup oocyte relative to its nurse cell suggests that the transport of materials from the nurse cells into the oocyte is reduced during stages 9 and 10. cup codes for a novel cytoplasmic protein, which accumulates early in the future oocyte within 16-cell cysts of the germarium but which later, during oogensis, is found in large aggregates in nurse cells, around the periphery of the nuclei. Still later these aggregates are found to disperse throughout the nurse cell cytoplasm, still in large aggregates, which eventually move toward the cellular periphery to form particulate structures along the subcortical surface of the nurse cells (Keyes, 1997).

Several observations suggest that cup and otu might interact. Late arresting alleles of otu affect condensation of the nurse cell chromosomes and partially prevent nurse cells from transfering their contents into the oocyte. The distribution of Otu protein also parallels Cup expression: Otu p104 becomes enriched in early oocytes and both p104 and Otu p98 move toward the periphery of the nurse cells during oocytic stages 9-10. There is a strong genetic interaction between cup and otu. In otu;cup double homozygous combinations, neither gene is epistatic to the other. Instead, the double mutant combinations produce defects that are much more severe than either alone. One finds further evidence for an interaction between cup and otu in females homozygous for cup but heterozygous for otu. A single copy of otu11 had a strong dominant effect on three of the four cup alleles tested. In the presence of one copy of otu11, the fertility of females homozygous for cup15 is restored. The ability of a reduced dose of otu to rescue an intermediate cup allele suggests that the balance of these gene products is important (Keyes, 1997).

Despite the diversity of the effects resulting from perturbations in cup function, many of them are known to involve microtubules. The distribution and relocalization of Cup protein during egg chamber maturation suggests that it is directly or indirectly associated with microtubules, at least during the previtellogenic stage. The ability of cup to disrupt chromosome segregation suggests that cup may also affect the ability of chromsomes to associate with the meiotic spindle (Keyes, 1997).

Neither of the above studies provides definitive proof of an interaction between Otu and either the actin based or the tubulin based cytoskeleton, but they do suggest an association. Because of the intimate associations of the two cytoskeletal systems during oogeneis (see cytoskeleton), a complete understanding of Otu's roles in oogenesis awaits a more thorough understanding of Otu's protein interactions. a name="Mercer">

Bourbon and Mycbp function with Otu to promote Sxl protein expression in the Drosophila female germline

In Drosophila ovaries, germ cells differentiate through several stages of cyst development before entering meiosis. This early differentiation program depends on both the stepwise deployment of specific regulatory mechanisms and on maintenance of germline sexual identity. The study of female sterile mutations that result in formation of germ cell tumors has been invaluable in identifying the mechanisms that control these developmental events. This study characterized the germ cell-enriched gene bourbon (bbn), null mutants of which cause the formation of a mixture of agametic ovarioles and cystic germ cell tumors. Proteomic analysis found Bbn forms a complex with Ovarian tumor (Otu), a protein previously linked with regulation of the sex determination factor Sex lethal (Sxl), and the Drosophila ortholog of c-Myc binding protein (Mycbp). Loss of Mycbp also results in the formation of cystic germ cell tumors. Bbn promotes the stability of Otu and fosters interactions between Otu and Mycbp. Germ cells from bbn and Mycbp mutants display a loss of Sxl expression specifically in the germline. Transgenic rescue experiments show the bbn sterile phenotype is independent from Sxl splicing defects. Further evidence suggests Otu physically interacts with and promotes Sxl protein stability. This function does not depend on Otu's deubiquitinase activity. Last, this study found the human orthologs of Otu and Mycbp, OTUD4, and MYCBP, also physically interact, suggesting conservation of function. Together these data provide insights into how a conserved complex promotes the germline expression of Sxl protein and the differentiation of Drosophila germ cells (Mercer, 2025).

The Drosophila ovary has long served as a useful system for studying adult stem cells and the regulation of germline differentiation. Female Drosophila have two ovaries composed of tube-like structures called ovarioles. Germ cell differentiation starts at the tip of each ovariole in a structure called the germarium. Within germaria, germline stem cells (GSCs) reside in a niche formed by cap cells that produce BMP ligands. These ligands act upon receptors on the surface of GSCs and initiate a signal transduction cascade that results in the transcriptional repression of bag-of-marbles (bam), a gene both necessary and sufficient for germ cell differentiation. Germ cells initiate bam expression once they move away from the cap cell niche. Bam protein physically interacts with Benign gonial cell neoplasm (Bgcn), Meiotic P26 (Mei-P26), and Sex lethal (Sxl) and together repress nanos mRNA translation once germ cells have exited the cap cell niche This repressive activity depends on the ability of Sxl to bind to specific elements within the 3'UTR of nanos mRNA. This complex also likely regulates other mRNAs (Mercer, 2025).

Once differentiation has been initiated, germ cell development proceeds through several discrete steps. Characterizing the mechanisms that regulate the stage-specific expression of different factors, including Sxl, will help inform how germ cells prepare to enter meiosis. Sxl and the Nanos interacting protein Pumilio continue to be expressed until the two-cell cyst stage at which time their expression decreases as the expression of Rbfox1 increases. Rbfox1 is a member of a family of RNA-binding proteins that play roles in the regulation of both alterative splicing and mRNA translation. Loss of Rbfox1 prevents female germ cells from entering meiosis and is accompanied by expanded expression of both Sxl and Pum. Further experiments show that Rbfox1 directly binds to pum mRNA and represses its translation. Sxl mRNA has several potential Rbfox1 binding sites within its 3' UTR, but direct binding has not been demonstrated (Mercer, 2025).

In the context of somatic cells, Sxl acts as a master regulator of sex determination. A widespread model has been that Sxl transcription is turned on from an early promotor in response to the ratio of the X chromosome to autosomes. However, more recent results indicate that the maternally provided protein Groucho (Gro) functions to repress Sxl transcription. Only embryos with two X chromosomes accumulate enough activator to overcome Gro-mediated repression and allow for transcription from Sxl PE. Sxl regulates both alternative splicing and mRNA translation of several downstream targets and promotes its own expression through a feed-forward loop. Later in somatic development, when Sxl is transcribed from its maintenance promotor, cells in females expressing Sxl protein will continue to make functional splice variants of Sxl. By contrast, cells in males produce Sxl mRNA that contains its third exon, which carries a premature stop codon, preventing the production of functional Sxl protein (Mercer, 2025).

What promotes Sxl expression in the female germline is less well understood. Factors including ovo, ovarian tumor (otu), sans fille (snf), and fused (fu) have been shown to function upstream of Sxl. For example, snf encodes a splicing factor and specific mutations in this gene result in loss of female Sxl isoforms specifically in the germline. Ovo promotes the transcription of otu), the founding member of a family of deubiquitinases. In addition to Ovo transcriptional regulation of otu, unknown signals downstream of the female isoform of double sex (dsx) in the soma also promote the expression of otu in the germline. Previous results have shown that otu mutants exhibit misregulation of Sxl splicing, resulting in the production of male-specific isoforms in ovaries. Given the largely cytoplasmic localization of Otu, this regulation of Sxl splicing may be indirect. In addition, several studies have shown that Sxl protein regulates the correct splicing of its own transcripts, further complicating potential interpretations of these results. Thus, the question of how Otu regulates Sxl expression and whether this regulation is direct or indirect remains unclear (Mercer, 2025).

This study identified two additional factors needed for Otu stability and function within the female germline. Loss of the gene CG14545, which is refered to as bourbon (bbn), results in a cystic tumor phenotype similar to that caused by disruption of Sxl and otu in the germline. Bbn is a small, evolutionarily divergent protein found in Drosophilids that structurally resembles mammalian c-Myc Binding Protein (MYCBP). Biochemical experiments indicate that Bbn protein interacts with both Otu and the actual Drosophila ortholog of MYCBP. Loss of Mycbp results in the formation of cystic germ cell tumors, mimicking the differentiation defects observed in Sxl, otu, and bbn mutants. Bbn promotes the stability of Otu and fosters physical interactions between Otu and Mycbp. Germ cells from bbn and Mycbp mutants, like otu mutants, display a loss of Sxl protein expression specifically in the germline. Otu physically interacts with a germline-specific isoform of Sxl protein and likely protects it from protein degradation. This function does not appear to require the deubiquitinase activity of Otu. These results provide insights into previously uncharacterized posttranslational mechanism that promotes the protein expression of female Sxl isoforms in the Drosophila germline (Mercer, 2025).

This study reports the identification and characterization of a complex composed of Otu and two structurally related proteins: Bbn and Mycbp. otu and bbn mRNA expression appears specific to germ cells while Mycbp displays broader expression across different tissues. Loss of any of these factors causes female sterility, marked by the appearance of agametic ovarioles and cystic germ cell tumors that fail to differentiate beyond the earliest stages of germ cell development. A regulatory target of the Otu/Bbn/Mycbp complex in adult ovaries is the sex determination factor Sxl and expression of Sxl protein, but not mRNA, is lost upon disruption of any members of the complex (Mercer, 2025).

AlphaFold modeling predicts that Bbn and Mycbp interact with Otu in a region away from its catalytic domain and between Otu's deubiquitinase domain and Tudor domain. Biochemical experiments presented in this study appear to support this model. Moreover, Bbn promotes the stability of Otu and is required for interactions between Otu and Mycbp. The common phenotypes exhibited by null mutations in bbn, Mycbp, and otu further support the idea these three proteins form a complex that acts to promote Sxl protein expression in adult germaria. However, this complex likely has other targets in addition to Sxl. bbn, Mycbp, and otu mutants exhibit a partially penetrant agametic phenotype. At least for bbn mutants, this agametic phenotype does not worsen with age, indicating germ cells are not constantly lost in the absence of this complex. Previous gene expression analysis indicates that bbn and otu are first expressed in germ cells during embryogenesis. Together these data suggest that the Otu/Bbn/Mycbp complex plays a role during early germ cell specification, maintenance, or migration. Further analysis will be required to determine when bbn, Mycbp, and otu mutant phenotypes first begin to manifest during germ cell development (Mercer, 2025).

The Bbn and Otu complex may also function at additional steps during late germ cell differentiation. Weak hypomorphs of otu and the HA::bbn allele both display a dumpless phenotype during late oogenesis. Both allele both display HA::bbn and otu weak hypomorphs have nurse cell nuclei that do not decondense properly after the 5 blob stage reminiscent of a cup phenotype. Previous work in the literature has demonstrated that cup and otu genetically interact. Cup was a top hit in the Otu proteomic analysis and was identified in the Bbn proteomic analysis as well. While this study focused on Sxl in this current study, Cup represents an exciting potential regulatory target for future study (Mercer, 2025).

The data support a model in which the Otu/Bbn/Mycbp complex acts downstream of transcription and splicing of Sxl. Sxl splicing defects in bbn mutant ovaries can be rescued by expression of a Sxl cDNA transgene, while the tumorous phenotype of bbn mutants is not. In addition, the localization of Bbn and Otu in the cytoplasm supports the idea that the complex likely promotes Sxl translation or protein stability. Evidence is provided that Sxl is regulated by the proteasome and is potentially ubiquitinated, consistent with previous results. However, Otu's deubiquitinase activity is not required for Sxl stability. Interestingly, proteomic analysis reveals that Otu physically interacts Sxl-PX and Sxl-PY proteins, suggesting that Otu may protect Sxl proteins from degradation in a manner that does not involve deubiquitination. Future experiments should aim to determine how the Otu/Bbn/Mycbp complex carries out this function (Mercer, 2025).

Whether Otu functions as a deubiquitinase has been debated in the literature. Drosophila Otu has a serine in the active site instead of the canonical cysteine. Mutating the D37, S40, and H143 residues all independently result in a loss of deubiquitinase activity in vitro. The current data indicate that the S40 residue is dispensable for early germ cell development and Sxl protein expression. Because the OTU deubiquitinase domain is not required, other domains of Otu may be responsible for promoting Sxl protein stability. Previous structure-function work with Otu demonstrates that Otu's Tudor domain is required for germ cell proliferation and cyst formation in the germarium. Future study of the function of the Otu Tudor domain could bring needed insight into the regulation of Sxl protein and other Bbn, Mycbp, and Otu complex targets (Mercer, 2025).

Last, this study detected interactions between OTUD4, the close human homolog of Drosophila OTU, and MYCBP in human cell lines. There are several plausible distant homologs of Bbn that could be tested, or alternatively, MYCBP could form a dimer in the complex. Regardless, physical interactions between OTUD4 and MYCBP reveal that the association of these proteins has been conserved across evolution. OTUD4 has been shown to function as a deubiquitinase with different substrate specificities. OTUD4 has also been shown to act as a scaffold for other deubiquitinases, thus promoting the protein stability of some targets in a catalytic independent manner. This raises the possibility that there are other interacting partners needed for Sxl protein stability. In addition, OTUD4 also interacts with RNA. Whether Drosophila Otu has similar functions remains to be tested (Mercer, 2025).

MYCBP interacts with Sakura and Otu and is essential for germline stem cell renewal and differentiation and oogenesis

The self-renewal and differentiation of germline stem cells (GSCs) are tightly regulated during oogenesis. The Drosophila female germline provides a powerful model to study these regulatory mechanisms. Sakura (also known as Bourbon/CG14545) has been identified as a crucial factor for maintenance and differentiation of GSCs and oogenesis, and Sakura binds to Ovarian Tumor (Otu), another essential regulator of these processes. This study identified MYCBP (c-Myc binding protein) as an additional essential component of this regulatory network. MYCBP physically associates with itself, Sakura, and Otu, forming binary and ternary complexes including a MYCBP*Sakura*Otu complex. MYCBP is highly expressed in the ovary, and mycbp null mutant females exhibit rudimentary ovaries with germline-less and tumorous ovarioles, fail to produce eggs, and are completely sterile. Germline-specific depletion of mycbp disrupts Dpp/BMP signaling, causing aberrant expression of bag-of-marbles (bam) and leading to defective differentiation and GSC loss. In addition, mycbp is required for female-specific splicing of sex-lethal (sxl), a master regulator of sex identity determination. These phenotypes closely resemble those observed in sakura and otu mutants. Together, the findings reveal that MYCBP functions in concert with Sakura and Otu to coordinate self-renewal and differentiation of GSCs and oogenesis in Drosophila (Azlan, 2025b).

Previous work showed that Sakura and Otu form a protein complex. This study identified MYCBP, encoded by the previously uncharacterized gene CG17202, as a binding partner of both Otu and Sakura. The data support that MYCBP binds with itself, Sakura, and Otu, forming binary and ternary complexes, including the MYCBP-Sakura-Otu ternary complex. Structural predictions suggest that MYCBP and Sakura resemble each other and engage in a pseudo-symmetric interaction. Mutations in mycbp, otu, and sakura result in strikingly similar phenotypes, and all three proteins are highly expressed in germline cells of the ovary, localize to the cytoplasm, and are enriched in developing oocytes. These observations strongly indicate that MYCBP, Sakura, and Otu function cooperatively in the germline during oogenesis. It is proposed that the MYCBP-Sakura-Otu complex regulates Dpp/BMP signaling and the expression of Bam, CycA, and Sxl in GSCs, including playing a crucial role in female-specific sxl splicing, thereby controlling GSC maintenance, proliferation, and differentiation. Additionally, it is proposed that formation of the MYCBP-Sakura-Otu complex is important for the enrichment of Otu in developing oocytes within egg chambers, which is essential for proper oogenesis. MYCBP-Sakura-Otu complex may bind and regulate target RNAs and deubiquitinate target proteins (Azlan, 2025b).

Since there are multiple protein association states that can be possibly formed among MYCBP, Sakura, and Otu, including MYCBP alone, Sakura alone, Otu alone, MYCBP-MYCBP, Sakura-Sakura, MYCBP-Sakura, MYCBP-Otu, MYCBP-MYCBP-Otu, Sakura-Otu, Sakura-Sakura-Otu, and MYCBP-Sakura-Otu, the relative expression levels among the three proteins and potential regulatory mechanisms for their interaction may determine the relative abundance of these multiple protein complexes, which could be critically important for GSC maintenance and differentiation and oogenesis (Azlan, 2025b).

Both tumorous and germless ovarioles were observed in the mycbp, otu, and sakura mutant ovaries. While upregulation of Bam and CycA and activation of the apoptotic pathway may underlie the germless phenotypes, the mechanism leading to tumorous ovarioles remains unclear. The relative balance of the multiple protein complexes formed by MYCBP, Sakura, and Otu may be differently disrupted among ovarioles within the same mutant, resulting in both tumorous and germless phenotypes. Furthermore, tumorous ovarioles may eventually become germless due to germline apoptosis. The precise mechanism by which these two distinct phenotypes arise within the same mutant ovaries warrants future investigation (Azlan, 2025b).

In mycbp^null germline clone cells, Sakura protein levels were severely reduced, and Otu lost its localization to developing oocytes, despite unchanged Otu protein levels and normal posterior localization of Orb. These results indicate that MYCBP is required for Sakura protein expression and/or stability, as well as for proper Otu localization. Similarly, in sakura^null germline clones, MYCBP levels were reduced, and both MYCBP and Otu lost their posterior localization, again without affecting Otu levels or Orb localization, indicating that Sakura is crucial for MYCBP protein expression and/or stability, as well as for proper MYCBP and Otu localization to developing oocytes. These mutual dependencies of protein expression/stability and oocyte localization among MYCBP, Sakura, and Otu further support the model that they function as protein complexes (Azlan, 2025b).

Although MYCBP and Sakura did not directly affect Otu's deubiquitinase activity in vitro using Ub-Rhodamine 110 as a model substrate, this does not rule out the possibility that they influence Otu's enzymatic activity in vivo. For instance, they may modulate Otu's substrate specificity. Previous work has shown that Otu also interacts with Bam-primarily through its Otu domain-to form a deubiquitinase complex that deubiquitinates and thereby stabilizes CycA, promoting GSC differentiation. It is possible that MYCBP and Sakura regulate the interaction between Otu and Bam and/or modulate the enzymatic activity of the Otu-Bam complex. For example, binding of MYCBP and/or Sukura to Otu may be mutually exclusive with Bam binding. Alternatively, MYCBP, Sakura, Otu, and Bam might form a ternary complex, while this study found that MYCBP does not bind Bam. Further studies are required to elucidate whether and how MYCBP and Sakura influence Otu's protein interactions and enzymatic function (Azlan, 2025b).

Otu also functions as an RNA-binding protein, and its deubiquitinase activity is enhanced by RNA binding. Sxl controls both alternative mRNA splicing and translation of downstream targets, and promotes its own expression via a positive autoregulatory loop. Female-specific splicing of sxl mRNA is disrupted in mycbp, sakura and otu mutant ovaries, leading to production of the male-specific isoform. Consistent with these findings, Sxl protein expression was found in germline cells in germaria including GSCs depends on MYCBP and Sakura. However precise mechanism how MYCBP, Sakura, and Otu play an essential role in Sxl expression remains unknown. Bam, together with Bgcn, Mei-P26, and Sxl, binds nanos mRNA-a key stem cell maintenance-and represses its translation after germ cells exit the niche. Identifying the RNA targets and deubiquitinase substrates of Otu beyond CycA and how MYCBP and Sakura regulate these Otu's activities will be critical to understanding their roles in oogenesis and other developmental processes. MYCBP-Sakura-Otu complex may bind directly to RNAs and regulate post-transcriptional processes such as sxl alternative splicing and translational control of oogenic RNAs (Azlan, 2025b).

Sakura is exclusively expressed in female germline cells including GSCs, and MYCBP is also highly expressed in these cells. However, MYCBP is additionally expressed at lower levels in somatic follicle cells in egg chambers and in other tissues, including testes, and Otu is broadly expressed in various tissues such as the gut and testis as well as in female germline cells in ovaries. These differential expression patterns suggest that Otu may have tissue-specific functions depending on the presence or absence of MYCBP and Sakura (Azlan, 2025b).

Transposons pose significant threat to genomic stability by inducing DNA damage if not properly silenced. piRNAs suppress transposons through transcriptional and post-transcriptional silencing mechanisms, thus preserving genome integrity. Loss of piRNA function results in transposon derepression, increased DNA damage, germ cell apoptosis, arrested oogenesis, and sterility. Because damaged germ cells can transmit harmful mutations to the next generation, selective elimination of defective germ cells is critical for maintaining germline integrity of a species.This study found that loss of function of mycbp, sakura or otu impairs piRNA-mediated transposon silencing and causes apoptosis. Thus, the germless phenotypes may arise, at least in part, from activation of a transposon-induced apoptotic elimination program. It will be important to investigate whether MYCBP, Sakura, and Otu's have any direct roles in the piRNA pathway (Azlan, 2025b).

MYCBP and Otu are conserved through human (human MYCBP, also known as AMY-1, and OTUD4) while Sakura is not. Human MYCBP was suggested to bind via its C-termina region to the N-terminal region of C-MYC and stimulate the activation of E-box-dependent transcription by C-MYC. However, this study showed that Drosophila MYCBP does not bind Drosophila ortholog of MYC (dMyc). Large scale protein interaction studies indicated that human MYCBP and OTUD4 associate. Alphafold suggested that human MYCBP and OTUD4 form complexes, MYCBP-OTUD4 and/or MYCBP-MYCBP-OTUD4, via the N-termina region of OTUD4, suggesting the evolutionary conserved interaction between MYCBP ortholog and Otu ortholog (Azlan, 2025b).

After these studies were published in which Sakura was identified, characterized by Buszczak group 2025, referring to Sakura as Bourbon. Consistent with both previous and current findings from this lab, the Buszczak group concluded that Sakura, MYCBP, and Otu form a ternary complex and function together to regulate germline differentiation, including promoting Sxl expression. While the overall conclusions align, there are notable differences as well. Mercer proposed that Sakura stabilizes Otu and facilitates physical interactions between Otu and MYCBP. Their conclusions were based on observations that Otu-GFP levels in the germarium driven by nos-Gal4 > UASp-otu::GFP were markedly reduced in sakura mutants, and that MYCBP and Otu failed to in S2 cells unless Sakura was co-expressed. While observed a reduction of endogenous Otu was also observed in whole ovary lysates of sakura and mycbp null mutants and NGT-Gal4 driven RNAi by Western blot, interpretation of these data is complicated by the severe ovarian degeneration in these mutants. In contrast, the current mosaic clone analyses-using Otu-EGFP transgene expressed from the otu promoter-clearly demonstrated that Otu protein levels were not reduced in mycbp^null or sakura^null germline cells within germaria or egg chambers, although Otu enrichment in developing oocytes was lost. Furthermore, this study showed that MYCBP and Otu physically associate in S2 cells without Sakura, unlike the findings reported in Mercer. This discrepancy may reflect differences in epitope-tagging strategies. The current results indicate that neither Sakura nor MYCBP is required for Otu stability of for the physical interaction between Otu and MYCBP or Sakura (Azlan, 2025b).

In summary, this study identifies and characterizes evolutionary conserved MYCBP as a novel and essential regulator of Drosophila oogenesis. Together with Sakura and Otu, MYCBP likely orchestrates germline cell fate decision, maintenance, and differentiation (Azlan, 2025b).

GENE STRUCTURE

Sequence analysis of otu cDNAs suggests that the two Otu proteins (apparent molecular masses of 98 and 104 kD) are generated by alternative splicing of a 126-bp exon (6a) between the sixth and seventh exon of the smaller transcript (Steinhauer, 1992)

Transcript length - 4.1 kb for the 104 kDa isoform and 3.2 kb for the 98 kDa isoform

Bases in 5' UTR - 1332

Exons - 8

Bases in 3' UTR - 505+

PROTEIN STRUCTURE

Amino Acids - 811 and 853 for the 98 and 104 kD proteins respectively (Steinhauer, 1992)

Structural Domains

Otu is a proline-rich, hydrophilic novel protein (Steinhauer, 1989).

Otu protein C-terminal sequence contains two domains with a statistically significant homology to regions of human, rat and mouse microtubule associated proteins (MAP2s) (Tirronen, 1995).

Three potential structural motifs have been identified by analysis of the Otu sequence: an N-terminal cysteine protease domain, a central Tudor domain, and proline-rich motifs in the C-terminal region. While the amino acids that are unique to the large Otu isoform are part of an important functional domain, the nature of the domain and its boundaries have been unknown. This study has established that the region that encodes the putative Tudor domain, which was identified solely by sequence analysis, corresponds to the domain that is required for the early Otu function. Of the otu transgenes analyzed, those encoding Otu polypeptides that contain the Tudor domain homology region promote normal egg chamber differentiation through the early stages, whereas polypeptides that lack a portion or all of these sequences are defective in the early Otu function. Thus, the Tudor domain homology region is a functional domain of Otu (Glenn, 2001).

The biochemical function of Tudor domains is unknown. Ten Tudor domain repeats are found in the D. melanogaster Tudor protein, which is required for pole cell formation and normal abdominal development. One copy of this domain is found in the protein encoded by the D. melanogaster homeless gene, which appears to function in RNA transport and localization during oogenesis, and Tudor domains are also found in other proteins that interact with RNA. However, the Tudor domain located in the central region of SMN, a snRNP assembly factor required for survival of motor neurons, appears to mediate protein/protein rather than protein/RNA interactions. The Tudor domain of Otu does not appear to be required for the Otu/mRNP interaction because truncated Otu polypeptides, missing this domain, were affinity-selected with oligo-dT. However, oligo-dT selection experiments were performed in the presence of wild-type Otu, and it is possible that Otu oligomerizes via a domain in the N-terminal region. If so, the truncated proteins may have been oligo-dT selected due to an association with endogenous Otu, and full-length Otu, but not the truncated proteins, could be responsible for the observed mRNP interaction (Glenn, 2001).

As yet, the relevant functional domain(s) in the N-terminal and C-terminal regions of Otu have not been delineated. The only discernable feature of the C-terminal Otu region is that it is rich in proline residues. It is plausible that these proline-rich motifs comprise a functional domain because the deletions in otu¹⁴, otu-423, and otu-627, all of which are defective in the late Otu function, remove some or all of these motifs. The N-terminal region contains the cysteine protease domain. However, Otu may not be an active protease because an essential cysteine residue, found in the catalytic site of other members of the cysteine protease family, is replaced by a serine in the homologous region of Otu. If it lacks protease activity, the cysteine protease homology region in Otu may be a protein/protein interaction domain. The N-terminal region is also responsible for the Otu/mRNP association. Given that no known RNA binding motifs are found in Otu, the mRNP association of Otu is most likely indirect. For example, it is possible that Otu interacts with an RNA binding protein via amino acids in the N-terminal region, using the cysteine protease domain or a separate domain (Glenn, 2001).

The modification of cellular proteins by ubiquitin (Ub) is an important event that underlies protein stability and function in eukaryotes. Protein ubiquitylation is a dynamic and reversible process; attached Ub can be removed by deubiquitylating enzymes (DUBs), a heterogeneous group of cysteine proteases that cleave proteins precisely at the Ub-protein bond. Two families of DUBs have been identified previously. This study describe new, highly specific Ub iso-peptidases, that have no sequence homology to known DUBs, but which belong to the OTU (ovarian tumour) superfamily of proteins. Two novel proteins were isolated from HeLa cells by affinity purification using the DUB-specific inhibitor, Ub aldehyde (Ubal). These proteins were named otubain 1 and otubain 2, for OTU-domain Ubal-binding protein. Functional analysis of otubains shows that the OTU domain contains an active cysteine protease site (Balakirev, 2003).

date revised: 11 January 2026

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