Gene name - miranda
Cytological map position - 92C
Function - required for subcellular localization of Prospero
Symbol - miranda
Genetic map position -
Classification - novel
Cellular location - cytoplasmic
|Recent literature||Jia, M., Shan, Z., Yang, Y., Liu, C., Li, J, Luo, Z.G., Zhang, M., Cai, Y., Wen, W. and Wang, W. (2015). The structural basis of Miranda-mediated Staufen localization during Drosophila neuroblast asymmetric division. Nat Commun 6: 8381. PubMed ID: 26423004
During the asymmetric division of Drosophila neuroblasts (NBs), the scaffold Miranda (Mira) coordinates the subcellular distribution of cell-fate determinants including Staufen (Stau) and segregates them into the ganglion mother cells (GMCs). This study shows that the fifth double-stranded RNA (dsRNA)-binding domain (dsRBD5) of Stau is necessary and sufficient for binding to a coiled-coil region of Mira cargo-binding domain (CBD). The crystal structure of Mira514-595/Stau dsRBD5 complex illustrates that Mira forms an elongated parallel coiled-coil dimer, and two dsRBD5 symmetrically bind to the Mira dimer through their exposed β-sheet faces, revealing a previously unrecognized protein interaction mode for dsRBDs. It was further demonstrated that the Mira-Stau dsRBD5 interaction is responsible for the asymmetric localization of Stau during Drosophila NB asymmetric divisions. Finally, it was found that the CBD-mediated dimer assembly is likely a common requirement for Mira to recognize and translocate other cargos including brain tumour (Brat).
|Zhang, F., Huang, Z. X., Bao, H., Cong, F., Wang, H., Chai, P. C., Xi, Y., Ge, W., Somers, W. G., Yang, Y., Cai, Y. and Yang, X. (2015). Phosphotyrosyl phosphatase activator facilitates Miranda localization through dephosphorylation in dividing neuroblasts. Development [Epub ahead of print]. PubMed ID: 26586222
The mechanism for the basal targeting of the Miranda (Mira) complex during the asymmetric division of Drosophila neuroblasts (NBs) is yet to be fully understood. This study has identified conserved Phosphotyrosyl Phosphatase Activator (PTPA) as a novel mediator for the basal localization of the Mira complex in larval brain NBs. In ptpa NBs, Mira remains cytoplasmic during early mitosis where its basal localization is delayed until anaphase. Detailed analyses indicate that PTPA acts independently of, and prior to, aPKC activity to localize Mira. Mechanistically, the data show that the phosphorylation status of the Thr591 (T591) residue determines the subcellular localization of Mira and that PTPA facilitates the dephosphorylation of T591. Furthermore, PTPA associates with the Protein Phosphatase 4 complex to mediate Mira localization. Based on these results, a two-step process for Mira basal localization during NB division is revealed where PTPA/PP4-mediated cortical association followed by apical aPKC-mediated basal restriction.
|Loedige, I., Jakob, L., Treiber, T., Ray, D., Stotz, M., Treiber, N., Hennig, J., Cook, K. B., Morris, Q., Hughes, T. R., Engelmann, J. C., Krahn, M. P. and Meister, G. (2015). The crystal structure of the NHL domain in complex with RNA reveals the molecular basis of Drosophila Brain-tumor-mediated gene regulation. Cell Rep 13: 1206-1220. PubMed ID: 26527002
TRIM-NHL proteins are conserved among metazoans and control cell fate decisions in various stem cell linages. The Drosophila TRIM-NHL protein Brain tumor (Brat) directs differentiation of neuronal stem cells by suppressing self-renewal factors. Brat is an RNA-binding protein and functions as a translational repressor. However, it is unknown which RNAs Brat regulates and how RNA-binding specificity is achieved. Using RNA immunoprecipitation and RNAcompete, this study identified Brat-bound mRNAs in Drosophila embryos and defined consensus binding motifs for Brat as well as a number of additional TRIM-NHL proteins, indicating that TRIM-NHL proteins are conserved, sequence-specific RNA-binding proteins. Brat-mediated repression and direct RNA-binding depend on the identified motif and show that binding of the localization factor Miranda to the Brat-NHL domain inhibits Brat activity. Finally, to unravel the sequence specificity of the NHL domain, the Brat-NHL domain in complex was crystallize with RNA, and a high-resolution protein-RNA structure of this fold is presented.
|Ramat, A., Hannaford, M. and Januschke, J. (2017). Maintenance of Miranda localization in Drosophila neuroblasts involves interaction with the cognate mRNA. Curr Biol. PubMed ID: 28690114
How cells position their proteins is a key problem in cell biology. Targeting mRNAs to distinct regions of the cytoplasm contributes to protein localization by providing local control over translation. This study reveals that an interdependence of a protein and cognate mRNA maintains asymmetric protein distribution in mitotic Drosophila neural stem cells. Endogenous mRNA or protein products of the gene miranda that is required for fate determination were tagged with GFP. The mRNA was found to localize like the protein it encodes in a basal crescent in mitosis. GFP-specific nanobodies fused to localization domains were used to alter the subcellular distribution of the GFP-tagged mRNA or protein. Altering the localization of the mRNA resulted in mislocalization of the protein and vice versa. Protein localization defects caused by mislocalization of the cognate mRNA were rescued by introducing untagged mRNA coding for mutant non-localizable protein. Therefore, by combining the MS2 system and subcellular nanobody expression, it was uncovered that maintenance of Mira asymmetric localization requires interaction with the cognate mRNA.
|Hannaford, M. R., Ramat, A., Loyer, N. and Januschke, J. (2018). aPKC-mediated displacement and actomyosin-mediated retention polarize Miranda in Drosophila neuroblasts. Elife 7. PubMed ID: 29364113
Cell fate assignment in the nervous system of vertebrates and invertebrates often hinges on the unequal distribution of molecules during progenitor cell division. This study addresses asymmetric fate determinant localization in the developing Drosophila nervous system, specifically the control of the polarized distribution of the cell fate adapter protein Miranda. A step-wise polarization of Miranda occurs in larval neuroblasts, and it was found that Miranda's dynamics and cortical association are differently regulated between interphase and mitosis. In interphase, Miranda binds to the plasma membrane. Then, before nuclear envelope breakdown, Miranda is phosphorylated by aPKC and displaced into the cytoplasm. This clearance is necessary for the subsequent establishment of asymmetric Miranda localization. After nuclear envelope breakdown, actomyosin activity is required to maintain Miranda asymmetry. Therefore, phosphorylation by aPKC and differential binding to the actomyosin network are required at distinct phases of the cell cycle to polarize fate determinant localization in neuroblasts.
Miranda was identified in a search for proteins that interact with Prospero. Prospero was used as bait to screen a Drosophila library of cDNAs using a yeast two-hybrid system (Shen, 1997). In the yeast two-hybrid system two hybrid proteins are used. The first hybrid protein, termed the 'bait,' is a hybrid between the DNA binding domain of yeast transcription factor LexA, and in this case, the Prospero protein. The bait lacks a transcriptional activation domain. The second hybrid protein is derived from a Drosophila cDNA library (containing sequences that may code for proteins interacting with the bait) tagged with a transcriptional activation domain. Protein interaction between the bait and the second hybrid protein gains the ability to activate transcription of a reporter gene. Miranda, the protein identified as interacting with Prospero protein, is named after Prospero's daughter and companion in exile, characters in Shakespeare's play, The Tempest.
Like Prospero and Numb, Miranda protein is asymmetrically localized during mitosis. At this stage, Miranda colocalizes with Prospero to the basal cell membrane of each neuroblast At the end of telophase, both proteins are segregated into the ganglion mother cell (GMC), the basal daughter of the neuroblast. Shortly after cell division in the GMC, Prospero is released from the membrane and translocated into the nucleus, whereas Miranda becomes undetectable. Therefore, the distribution of Miranda may be controlled by rapid delocalization or degradation in a cell-cycle dependent manner (Shen, 1997).
In embryos lacking Miranda, Prospero fails to become localized and is equally distributed to both progenies of the neuroblast. It is concluded that Miranda is required for the asymmetric localization of Prospero. In embryos homozygous for a null allele of inscuteable, both Miranda and Prospero are unable to form crescents or they form crescents that are randomly localized along the cell membrane. Therefore Miranda crescent formation and localization requires inscuteable. Although Numb is found to interact with Miranda, Numb is localized normally in Miranda deficient embryos. It is still possible that Miranda is sufficient for Numb asymmetric localizaton. It is also possible that the maternal contribution of Miranda is sufficient for Numb localization (Shen, 1997).
It is likely that Miranda is degraded in a cell-cycle-dependent manner. There are four potential destruction boxes in the Miranda protein. The destruction box is a 9-amino-acid motif conserved among the N termini of A- and B-type cyclins (King, 1996). The destruction boxes are responsible for the cell-cycle-dependent degradation of A- and B-type cyclins by an ubiquitin-dependent pathway in anaphase during mitosis (Yamamoto, 1996), whereas Miranda staining disappears later, shortly after mitosis (Shen, 1997).
Drosophila neuroblasts (NBs) undergo asymmetric divisions during which cell-fate determinants localize asymmetrically, mitotic spindles orient along the apical-basal axis, and unequal-sized daughter cells appear. This study identified a Drosophila mutant in the Ggamma1 subunit of heterotrimeric G protein, which produces Ggamma1 lacking its membrane anchor site and exhibits phenotypes identical to those of Gß13F, including abnormal spindle asymmetry and spindle orientation in NB divisions. This mutant fails to bind Gß13F to the membrane, indicating an essential role of cortical G1-Gß13F signaling in asymmetric divisions. In Ggamma1 and Gß13F mutant NBs, Pins-Gαi, which normally localize in the apical cortex, no longer distribute asymmetrically. However, the other apical components, Bazooka-atypical PKC-Par6-Inscuteable, still remain polarized and responsible for asymmetric Miranda localization, suggesting their dominant role in localizing cell-fate determinants. Further analysis of Gßgamma and other mutants indicates a predominant role of Partner of Inscuteable-Gi in spindle orientation. It is thus suggested that the two apical signaling pathways have overlapping but different roles in asymmetric NB division (Izumi, 2004).
The question of whether the two apical pathways have redundant functions in aspects of NB division other than cell-size asymmetry has been elusive. In this paper, examination of Ggamma1N159 and G圩F mutant NBs, as well as those overexpressing baz, suggests that the asymmetric localization of Miranda depends solely on polarized Baz activity and not on Pins-Galphai function. Miranda always distributes on the cortical side, opposite the distribution of Baz in these mutants and in the wild-type. This also occurs for sensory precursor cells in the peripheral nervous system: in sensory precursor cell division Insc is not expressed, and Pins and Baz distribute on cortical sides opposite to each other, unlike in NBs; however, both Miranda and Numb localize to the cortex opposite Baz, as seen in NBs (Izumi, 2004).
Phosphorylation of the Lethal (2) giant larvae protein by DaPKC directs the localization of cell-fate determinants to the basal cell cortex. When baz is overexpressed in NBs, ectopically distributed Baz excludes Miranda from the Baz region and DaPKC colocalizes with the ectopic Baz. In contrast, a decrease in Baz activity in the wild-type results in cytoplasmic localization of DaPKC and uniform cortical distribution of Miranda. All these findings suggest that the Baz-directed localization of DaPKC excludes Miranda from the apical cortex via Lethal (2) giant larvae phosphorylation. In the absence of Baz, Miranda is eventually concentrated to the budding GMC during telophase by unknown mechanisms, a phenomenon called 'telophase rescue'. This phenomenon did not occur by depleting both baz activity and Gßgamma signaling, suggesting that telophase rescue involves Gßgamma signaling or asymmetric Pins-Galphai localization (Izumi, 2004).
Asymmetric cell divisions generate cell fate diversity during both invertebrate and vertebrate development. Drosophila neural progenitors or neuroblasts (NBs) each divide asymmetrically to produce a larger neuroblast and a smaller ganglion mother cell (GMC). The asymmetric localisation of neural cell fate determinants and their adapter proteins to the neuroblast cortex during mitosis facilitates their preferential segregation to the GMC upon cytokinesis. In this study a novel role is reported for the anaphase-promoting complex/cyclosome (APC/C) during this process. Attenuation of APC/C activity disrupts the asymmetric localisation of the adapter protein Miranda and its associated cargo proteins Staufen, Prospero and Brat, but not other components of the asymmetric division machinery. Miranda is ubiquitylated via its C-terminal domain; removal of this domain disrupts Miranda localisation and replacement of this domain with a ubiquitin moiety restores normal asymmetric Miranda localisation. These results demonstrate that APC/C activity and ubiquitylation of Miranda, in a proteasomal-independent process, are required for the asymmetric localisation of Miranda and its cargo proteins to the NB cortex (Slack, 2007).
A recently published study reported the isolated a novel allele of imaginal discs arrested (ida), homozygotes of which survive until early pupal stages of development and fail to properly localise Miranda to the basal cortex of mitotic neuroblasts (Slack, 2006). Sequence analysis of this allele, idaPL17, revealed a single nucleotide transversion within the ida coding sequence, resulting in a premature stop codon at aa 334 (Q334-->stop), the first residue of a putative tetratricopeptide (TPR) motif. In mutant larvae, only 58% of prophase NBs properly localised Miranda to the cortex compared with 100% of wild-type prophase NBs. A total of 41% of idaPL17 metaphase NBs showed Miranda accumulation in a pericentrosomal compartment at the expense of cortically localised protein, with 8% of mutant NBs showing a complete loss of cortically localised protein, whereas 91% of wild-type metaphase NBs showed cortically localised protein only. At low frequency, anaphase cells were observed, as defined by separated chromosome populations displaced towards opposite poles of the cell, confirming previous observations that ida mutant neuroblasts are not arrested at metaphase. Miranda was still mislocalised to pericentrosomal regions in these anaphase neuroblasts. No phenotypic defects were detected in idaPL17 homozygous mutant embryos, presumably due to the perdurance of maternally provided protein, and NB clones induced at early larval stages did not show any obvious mitotic or Miranda-localisation defects, again indicative of protein stability. Attempts to induce maternal germline clones homozygous for idaPL17 did not yield any surviving embryos, suggesting an essential requirement for ida function during oogenesis. The idaPL17 mutant appears to be a genetic null, because the penetrance of the Miranda localisation phenotype did not increase in hemizygotes over a small deficiency that removes the entire ida locus or in transheterozygotes over the mRNA null allele, idaD14. The defects in Miranda localisation in ida mutant NBs could not be rescued by expressing a GFP::Ida fusion protein, which, at all stages of the cell cycle, was localised throughout the cytoplasm (Slack, 2007).
ida encodes the Drosophila homologue of human APC5, a subunit of the APC/C multiprotein complex. In order to determine whether the defects in Miranda localisation are a specific consequence of ida loss of function or are caused by a more general disruption to APC/C activity, the effect was analyzed of loss-of-function mutations in genes encoding other APC/C subunits. A significant number of mitotic NBs with pericentrosomal Miranda accumulation was observed in animals homozygous for mutations in either cdc27 or morula (APC2), suggesting that Miranda asymmetric cortical localisation is disrupted when APC/C activity is attenuated. However, no defects in Miranda localisation were observed in homozygous mutants for strong loss-of-function alleles for the two APC/C activators, cdc20 (fizzy) or cdh1 (retina aberrant in pattern/fizzy related), indicating that Miranda targeting might occur independently of these two proteins. Loss of ida function causes several mitotic defects, including an increased mitotic index, loss of cyclin B degradation and hypercondensed chromosomes. However, a strong hypomorphic allele of polo that shows similar mitotic defects had normal cortical Miranda localisation. Furthermore, colchicine treatment of wild-type NBs to depolymerise microtubules and induce metaphase arrest did not disrupt Miranda localisation, therefore suggesting that the ida mutant phenotype is not a secondary consequence of a delay or block in mitosis (Slack, 2007).
In C. elegans, APC/C function during embryonic anteroposterior axis formation promotes the association of the paternal pronucleus/centrosome with the embryonic cortex. The pericentrosomal accumulation of Miranda in APC/C mutant NBs led to an investigation of a possible requirement for centrosomal function during Miranda localisation. Loss-of-function mutants for centrosomin (cnn), which encodes a core component of the centrosome in Drosophila and is required for proper centrosome assembly, were examined and found that Miranda localisation to the NB cortex was normal in these mutants. Furthermore, cnn; ida double mutants still accumulated pericentrosomal Miranda, suggesting that the ida mutant phenotype is not dependent on intact centrosomal function. It was also noted that complete colocalisation of Miranda with Centrosomin was not seen in ida mutant NBs, suggesting that Miranda is localised in a separate compartment that itself localises to a region near to the centrosome. Accumulation of Miranda in ida mutants was insensitive to colchicine treatment to depolymerise microtubules, suggesting that the ida mutant phenotype is not dependent on the mitotic spindle, although the compartment in which Miranda localises separates from the region of the centrosome upon colchicine treatment. Pericentrosomal accumulation of Miranda was also observed in ida mutants after latrunculin treatment, suggesting that the ida mutant phenotype occurs independently of intact actin filaments (Slack, 2007).
So far this pericentrosomal compartment has not been identified, although colocalisation was not observed with either Rab11 or Nuclear fallout (Nuf), both of which are markers for recycling endosomes, suggesting that pericentrosomal Miranda accumulation occurs independently of the recycling endosomal machinery. Miranda has been shown to localise to the centrosome both in biochemical and immunohistochemical studies, but, under the conditions used for the current experiments, significant levels of centrosomal Miranda localisation was not observed in wild-type NBs (Slack, 2007).
The localisation of Miranda to the NB basal cortex requires the correct localisation of the apical protein complex, which includes Inscuteable, Pins and aPKC. In ida mutant NBs, Inscuteable localises normally to the apical cortex during early prophase and is maintained as an apical cortical crescent during metaphase. Similar results were obtained using anti-aPKC and anti-Pins, suggesting that the defects in Miranda localisation in ida mutant NBs is not caused by a disruption to the apical complex and that the APC/C functions downstream of or in parallel to the apical components (Slack, 2007).
Miranda acts as an adapter protein for the cell fate determinant Prospero, for the prospero mRNA adapter Staufen and for the translational repressor Brat. In ida mutant NBs, Prospero, Staufen and Brat all lose their cortical localisation and colocalise with Miranda pericentrosomally, suggesting that they are still able to complex with Miranda. By contrast, the cortical localisation of both Numb and PON was unaffected by loss of ida function, and both Numb and PON formed normal cortical crescents in ida mutant NBs that showed strong Miranda mislocalisation defects, suggesting that ida function is required specifically for the localisation of Miranda and its associated cargo proteins to the NB basal cortex (Slack, 2007).
The ida mutant phenotype resembles that seen in mutants for the tumour suppressor lgl, in which the targeting of Miranda and other basally localised molecules to the NB cortex is disrupted so that Miranda no longer forms a cortical crescent at metaphase but is instead mislocalised to the centrosomes and mitotic spindle. It was reasoned that, if the ida mutant phenotype resulted from a reduction of Lgl function, then further reducing Lgl activity should enhance the Miranda mislocalisation phenotype. However, removing one copy of lgl using a null allele had no effect on the ida mutant phenotype and the number of NBs with pericentrosomal Miranda was comparable to mutants for ida alone (49% of NBs with pericentrosomal Miranda), suggesting that Lgl is not a downstream effector of Ida activity. By contrast, removing one copy of miranda using a deficiency that removes the entire miranda locus [Df(3R)ora19] in the idaPL17 mutant background strongly suppressed the Miranda mislocalisation phenotype, suggesting that Miranda itself might be a target for APC/C activity (Slack, 2007).
These data demonstrate that the correct localisation of Miranda to the basal NB cortex requires APC/C. Because the APC/C normally functions as an E3 ubiquitin ligase, whether this role of the APC/C could be mediating the effects on Miranda localisation was examined. In order to determine whether Miranda can be ubiquitylated, immunoprecipitations were performed on protein extracts of Drosophila S2 cells in which FLAG-tagged Miranda and expressed HA-tagged ubiquitin were constitutively expressed under the control of a heat-shock promoter. After immunoprecipitation using anti-HA antibody, FLAG-Miranda was detected in the immune complex in extracts only from cells in which HA-ubiquitin expression had been induced by heat shock. Immunoprecipitations were performed on protein extracts both from S2 cells and larval brains expressing only FLAG-Miranda without expressing exogenous HA-tagged ubiquitin. In both cases, ubiquitylated Miranda was detected in the immune complex after immunoprecipitation using anti-FLAG antibodies. These results using brain and S2 extracts clearly demonstrate that Miranda can be ubiquitylated both in vivo and in S2 cells. Although the antibody used recognises both mono- and poly-ubiquitin conjugates, only a single band was observed on the western blots probed for ubiquitylated Miranda. The absence of higher molecular weight Miranda species, even in the presence of proteasome inhibitors, suggests that Miranda might be mono- rather than poly-ubiquitylated (Slack, 2007).
The C-terminal region of Miranda contains a putative APC/C-recognition motif (amino acids 811 to 814: GKEN) that shows homology to the KEN box, a motif required for Cdh1-dependent APC/C-mediated ubiquitylation of substrate proteins. To examine the effects of the removal of this motif on Miranda localisation, the mirandaRR127 allele was used; in this allele the C-terminal 103 amino acids of the encoded protein, including the GKEN motif, are replaced by an unrelated stretch of 112 amino acids. The localisation of this truncated form of Miranda was examined in mitotic larval NBs by generating somatic clones using the MARCM system, which allows the generation of homozygous-mutant NB clones that express membrane-bound CD8::GFP in an otherwise heterozygous background. In mirandaRR127 mutant NBs, Miranda localisation was similar to that seen in ida mutant NBs: the truncated protein was exclusively cytoplasmic during early prophase and accumulated in a pericentrosomal compartment at the expense of cortical protein during metaphase. As in ida mutant NBs, accumulation of Miranda in mirandaRR127 mutant NBs was insensitive to colchicine treatment to depolymerise microtubules, and accumulation of pericentrosomal Prospero was also observed in these mutant NBs. Removal of this C-terminal domain prevented ubiquitylation of Miranda in S2 cells. Furthermore, NBs that overexpress this truncated protein showed a similar mislocalisation of the expressed protein and replacement of this C-terminal domain with ubiquitin restored normal localisation. Interestingly, mutation of the GKEN motif itself did not prevent Miranda ubiquitylation and had no effect on the localisation of the protein, suggesting that mutation of this site alone is insufficient to disrupt ubiquitylation of Miranda (Slack, 2007).
A high proportion of embryonic mirandaRR127 mutant NBs was observed in which Miranda was localised pericentrosomally at the expense of cortical protein. The observations that both embryonic and larval NBs mutant for the mirandaRR127 allele showed some cortical localisation although at a much reduced level and the presence of low levels of cortical Miranda in embryos derived from mirandaRR127 germline clones suggest that the mutant protein produced by the mirandaRR127 allele retains some ability to localise to the cortex. Ubiquitylation is clearly an important aspect of the Miranda localisation process, but in its absence a proportion of the Miranda present in the NB can be localised at the basal cortex by other mechanisms. Hence, the inability to ubiquitylate Miranda in the mirandaRR127 mutant causes only an incomplete loss of localisation (Slack, 2007).
Ubiquitylation by the APC/C normally targets proteins for degradation via the 26S proteasome. Although it is possible that a proportion of ubiquitylated Miranda is targeted for degradation, disruption to proteasome function caused markedly different phenotypes than those observed when APC/C activity was attenuated. Although Miranda was observed accumulating in the region of the centrosomes in NBs mutant for the proteasome regulatory subunits Rpn6 or Tbp-1, this process was microtubule dependent, whereas in both ida mutant NBs and NBs mutant for the mirandaRR127 allele, accumulation of Miranda was observed even after microtubule depolymerisation with colchicine. In addition, pericentrosomal accumulation of either Prospero or Staufen was not seen in proteasome-mutant NBs. Furthermore, no significant differences by western blot were observed in Miranda protein levels between wild-type and ida mutant brain extracts, suggesting that the pericentrosomal localisation of Miranda in ida mutants is not caused by excessive Miranda accumulation and therefore reflects the disruption of a process other than proteasomal degradation. Several proteasome-independent processes regulated by ubiquitylation have been identified, including protein kinase activation, vesicle trafficking, DNA repair and gene silencing (Slack, 2007).
The asymmetric localisation of cell fate determinants during NB division is tightly coordinated with changes in the cell cycle. The formation of an apical complex of proteins during early prophase not only directs the correct orientation of the mitotic spindle during metaphase but is also required for the formation of a basal crescent of cell fate determinants and their adapter molecules during late prophase/metaphase. It thus appears likely that multiple components of the cell cycle machinery that coordinate cell cycle transitions might also be involved in the regulation of basal protein localisation, as has been shown for cdc2. This study has shown that the efficient localisation of the adapter protein Miranda to the NB basal cortex requires the activity of the APC/C mitotic regulator. Mutations in several APC/C core subunits showed reduced cortically localised Miranda, with cytosolic accumulation of Miranda in an as yet unidentified pericentrosomal compartment. By contrast, apical complex formation was unaffected in these mutant NBs, showing that the APC/C acts downstream of or in parallel to the apical complex to ensure proper basal protein localisation. Furthermore, the basal localisation of PON/Numb were also unaffected by loss of APC/C activity, suggesting that Miranda itself might be a specific target for APC/C activity in mitotic NBs. This is further supported by the observation that the ida mutant phenotype can be partially rescued by specifically reducing Miranda protein levels (Slack, 2007).
The APC/C functions as an E3 ubiquitin ligase, and this study has shown that Miranda is a ubiquitylated protein in both cultured cells and larval NBs. Extensive attempts to demonstrate that Miranda ubiquitylation is APC/C-dependent have proved inconclusive. Therefore, the possibility cannot be ruled out that the effects of loss of APC/C activity on Miranda localisation might be indirect. However, the phenotypes observed in APC/C mutants are recapitulated in the mirandaRR127 allele encoding a C-terminal truncation of the protein. Not only are both Miranda localisation and ubiquitylation dependent on this region of protein, but replacement of this domain with ubiquitin is able to restore wild-type protein localisation. Although the C-terminal domain of Miranda is clearly required for ubiquitylation and cortical localisation of the protein, the precise ubiquitylation sites in Miranda are as yet unknown and will be an interesting area for further study. It is therefore speculated that the pericentrosomal accumulation of Miranda and the reduction of basally localised protein in APC/C mutants might reflect a loss of Miranda ubiquitylation. Recently, it has been shown that the ubiquitin moiety itself can function as a protein-protein interaction domain. Ubiquitylation of Miranda could function as a signal to regulate its transport to the basal cell cortex perhaps by influencing its association with motor proteins that mediate basal protein targeting. Alternatively, ubiquitylation of Miranda could regulate the retention of Miranda at the basal cell cortex by influencing its association with anchoring or scaffolding molecules. Efficient localisation and/or retention of Miranda to the basal cortex clearly requires APC/C activity, but the presence of Miranda protein at the basal cortex in APC/C mutants, albeit at a much reduced level compared with wild-type, indicates that other processes and molecules might also be involved (Slack, 2007).
Miranda is a multidomain adaptor protein involved in neuroblast asymmetric division in Drosophila. The central domain of Miranda is necessary for cargo binding of the neural transcription factor Prospero, the Prospero-mRNA carrier Staufen, and the tumor suppressor Brat. This study reports the first solution structure of Miranda central 'cargo-binding' domain (residues 460-660) using small-angle X-ray scattering. Ab initio modeling of the scattering data yields an elongated 'rod-like' molecule with a maximum linear dimension of approximately 22 nm. Moreover, circular dichroism and cross-linking experiments indicate that the cargo-binding domain is predominantly helical and forms a parallel coiled-coil homodimer in solution. Based on the results, the full-length Miranda protein was modeled as a double-headed, double-tailed homodimer with a long central coiled-coil region. The cargo-binding capacity of the central domain is discussed, and a structure-based mechanism is proposed for cargo release and timely degradation of Miranda in developing neuroblasts (Yousef, 2008).
Based on the primary sequence analysis, Miranda full-length protein is predicted to contain a long central domain of coiled coils (residues 150-700) flanked by non-coiled-coil N and C termini. This study has shown that Miranda CBD (residues 460-660) dimerizes into an elongated parallel coiled-coil structure. Because the CBD is part of the long central domain, the same structural trend, i.e., the shape, orientation, and oligomerization, is likely to continue throughout the central coiled-coil region of Miranda. Therefore, the central domain of Miranda is expected to serve as a dimerization motif for the entire protein via parallel coiled-coil self-association. A model is suggesed for the entire protein, in which Miranda forms a double-headed and double-tailed homodimer with a long central coiled-coil region (Yousef, 2008).
Miranda is not detectable in cells after cytokinesis, suggesting that Miranda degradation is a prerequisite for the release of the cargos. Miranda contains four potential destruction boxes, a 9 amino acid motif originally identified in A and B cyclins, that are involved in their cell-cycle-dependent degradation. Two of these destruction boxes are located within the CBD coiled-coil region (residues 544-552 and 572-580), whereas the other two lie N-terminal to the CBD (residues 356-364 and 431-439). The coiled-coil structure of the CBD, along with the cargo binding, is expected to shield the destruction boxes, rendering them inaccessible to potential degradation pathways (Yousef, 2008).
In contrast, the extreme C-terminal 103 amino acids (residues 727-830) were found to be essential for cargo release and the timely degradation of Miranda. The C-terminal region of Miranda contains multiple consensus protein kinase C phosphorylation sites, raising the possibility of phosphorylation-dependent degradation. Based on the model of full-length Miranda, phosphorylation at the C termini could lead to unfavorable electrostatic interactions between and/or structural changes of the identical double-tails of the Miranda dimer. These changes could destabilize (unzip) the nearby coiled coil and the CBD. When unzipped, the CBD would not only release the bound cargos but also expose the destruction boxes. That is consistent with the observation that replacing the extreme C terminus of Miranda, which contains the consensus phosphorylation sites, with a segment of unrelated (non-phosphorylatable) residues prevents cargo release. The proposed mechanism implies that the degradation of Miranda is a consequence rather than the cause of cargo release (Yousef, 2008).
Additionally, the first 290 residues of Miranda protein are known to be sufficient for its membrane association. About half of the residues (residues 150-290) can form coiled coil. Although it is not known exactly which molecule(s) tether Miranda to the cell cortex, the dimerization of the two identical N-terminal domains forming the double-heads of the Miranda dimer might be required to increase avidity for specific binding partners (Yousef, 2008).
In summary, These data suggest that the central coiled-coil domain of Miranda protein forms an extended dimerization motif for the entire protein. It is proposed that the stability and cargo-binding capacity of the central domain could be regulated by phosphorylation at the C terminus (Yousef, 2008).
The short transcript has a 90-nucleotide internal deletion and codes for a protein of 800 amino acids (Shen, 1997).
Bases in 5' UTR - 323
Bases in 3' UTR - 161
The Miranda protein shows no significant homology with any known protein in computer databases. The central portion of the Miranda protein is predicted to contain several coiled-coil structures, which have been implicated in mediating protein-protein interactions. There are four potential destruction boxes in the Miranda protein and therefore, Miranda may be degraded in a cell-cycle dependent fashion (Shen, 1997).
date revised: 10 February 2008
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