Gene name - lola like
Synonyms - batman
Cytological map position - 55B9
Function - transcription factor
Keywords - Polycomb and trithorax group genes
Symbol - lolal
FlyBase ID: FBgn0022238
Genetic map position - 2-
Classification - BTB/POZ domain
Cellular location - nuclear
|Recent literature||Quijano, J.C., Wisotzkey, R.G., Tran, N.L.,
Huang, Y., Stinchfield, M.J., Haerry, T.E., Shimmi, O. and Newfeld, S.J.
(2016). lolal is an
evolutionarily new epigenetic regulator of dpp transcription during
dorsal-ventral axis formation. Mol Biol Evol [Epub ahead of
print]. PubMed ID: 27401231
Secreted ligands in the Dpp/BMP family drive dorsal-ventral (D/V) axis formation in all Bilaterian species. However, maternal factors regulating Dpp/BMP transcription in this process are largely unknown. This study identified the BTB domain protein longitudinals lacking-like (lolal) as a modifier of decapentaplegic (dpp) mutations. It was shown that Lolal is evolutionarily related to the Trithorax group of chromatin regulators and that lolal interacts genetically with the epigenetic factor Trithorax-like during Dpp D/V signaling. Maternally driven LolalHA is found in oocytes and translocates to zygotic nuclei prior to the point at which dpp transcription begins. lolal maternal and zygotic mutant embryos display significant reductions in dpp, pMad and zerknullt expression, but they are never absent. The data suggest that lolal is required to maintain dpp transcription during D/V patterning. Phylogenetic data reveals that lolal is an evolutionarily new gene present only in insects and crustaceans. The study concludes that Lolal is the first maternal protein with a role in dpp D/V transcriptional maintenance, that Lolal and the epigenetic protein Trithorax-like are essential for Dpp D/V signaling and that the architecture of the Dpp D/V pathway evolved in the arthropod lineage after the separation from vertebrates via the incorporation of new genes such as lolal
Polycomb and trithorax group genes maintain the appropriate repressed or activated state of homeotic gene expression throughout Drosophila development. lola like (lolal), also known as batman (ban), functions in both activation and repression of homeotic genes, including the repression of Sex combs reduced. The 127-amino acid Lolal protein consists almost exclusively of a BTB/POZ domain, an evolutionary conserved protein-protein interaction domain found in a large protein family. This domain is involved in the interaction between Lolal and the DNA binding GAGA factor encoded by the Trithorax-like gene. The GAGA factor and Lolal codistribute on polytene chromosomes, coimmunoprecipitate from nuclear embryonic and larval extracts, and interact in the yeast two-hybrid assay. Lolal, together with the GAGA factor, binds to MHS-70, a 70-bp fragment of the bithoraxoid Polycomb response element. This binding, like that of the GAGA factor, requires the presence of d(GA)n sequences. lolal also interacts with polyhomeotic and, like Trl, both lolal and ph are needed for iab-7 polycomb response element mediated pairing dependent silencing of mini-white transgene (see abdominal-A). lolal was also identified as a strong interactor of GAGA factor in a yeast two-hybrid screen. lolal also interacts geneticially with polyhomeotic and, like Trl, both lolal and ph are needed for iab-7PRE mediated pairing dependent silencing of mini-white transgene. These observations suggest a possible mechanism for how Trl plays a role in maintaining the repressed state of target genes involving Lolal, which may function as a mediator to recruit PcG complexes (Faucheux, 2003; Mishra, 2003).
In Drosophila, segmental identity along the anterior-posterior axis is specified by homeotic genes, whose expression is established early in development by a combination of maternal, pair-rule, and gap gene products. Since these regulators are only transiently expressed during early embryogenesis, a second system of regulation takes over to maintain heritable activation or repression of homeotic gene expression later in development. The maintenance genes have been separated into two groups, the Polycomb group (PcG) of repressors and the trithorax group (trxG) of activators. Loss-of-function mutations of PcG genes lead to posterior segmental transformations in embryos, as a result of the ectopic expression of homeotic genes anterior to their normal expression domain. In contrast, loss-of-function mutations of trxG genes lead to anterior transformations of abdominal segments, as a result of the loss of sustained expression of homeotic genes. Synergistic interactions are observed between mutant alleles of PcG genes, as well as between mutant alleles of trxG genes. In contrast, suppressive interactions between mutations of PcG and trxG genes are usually encountered, consistent with their opposite regulatory functions on homeotic gene expression (Faucheux, 2003 and references therein).
PcG proteins regulate homeotic genes at the transcriptional level. Characterization of homeotic gene regulatory sequences led to the identification of PcG response elements (PREs) that are required to maintain a spatially restricted pattern of homeotic gene expression in a PcG-dependent manner. Consistent with this, the products of PcG genes were characterized as chromosomal proteins associated to specific regions of larval salivary polytene chromosomes, including already-defined PREs. Although many of these specific regions are common target sites for all the PcG proteins, some are specific to particular PcG proteins. These results led to the proposition that PcG proteins are involved in complexes of overlapping but distinct composition, depending on their targets. Evidence for the participation of PcG proteins to macromolecular complexes comes from in vitro and yeast two-hybrid interaction assays, as well as from coimmunoprecipitation studies. Polycomb (Pc), Extra sex comb (Esc), Enhancer of zeste (Ez), Pleiohomeotic (Pho), and Polyhomeotic (Ph) interact transiently during early embryonic development. In late embryos, these factors are found in two separate complexes, PRC1 (including Ph, Pc, Psc, and Sex comb on midleg [Scm] and another complex involving Esc, Ez, and Pho (Faucheux, 2003 and references therein).
PcG proteins may exert their repressive effect at different levels of transcriptional regulation, including interaction with the transcription machinery, histone modifications, and nucleosomal organization, as well as participating in a higher order chromatin structure. The presence in Pc of a conserved domain called the chromodomain, also found in HP1, a heterochromatin-associated protein, led to the proposal that a higher-order chromatin structure is induced by PcG multimeric complexes. This view is supported by the phenotypic similarities between the silencing of a miniwhite transgene by heterochromatin and that of PRE-miniwhite constructs by PcG proteins, the local spreading of PcG proteins over the PRE, and the reduced accessibility of a locus when repressed by PcG genes. Recent data also point towards a localized effect on nucleosomal organization at the level of the PRE: Pc interacts in vitro with nucleosomes, PcG repression correlates with a modification of histone acetylation, and both the Esc/Ez/Pho and PRC1 complexes contain the histone deacetylase RPD3. In addition, PcG proteins interact physically with general transcription factors and may thereby directly inhibit transcription at bound promoters (Faucheux, 2003 and references therein).
Many trxG genes were recovered as suppressors of Pc loss-of-function mutations or of gain-of-function mutations of the homeotic Antennapedia gene. The trxG proteins, as is the case for PcG proteins, are involved in several separable complexes. One of them, the Brahma complex, contains homologs of the SWI/SNF chromatin remodeling complex necessary for maintaining an active state of transcription in mammals and yeast. At least two other complexes are found in Drosophila, one including Trithorax (Trx) and Ash1 and another containing Ash2 (Faucheux, 2003 and references therein).
The functional antagonism between genes of the PcG and trxG suggests that the remodeling properties of trxG proteins must somehow compete with the properties of PcG proteins at homeotic loci. Molecular characterization of Trx has shown that it contains a SET protein-protein interaction domain that is shared by the PcG protein Ez, which suggests that antagonism of PcG and trxG factors may result from opposite effects on common partners. Furthermore, the binding of PRC1 on a DNA template prevents SWI/SNF ATP-dependent chromatin remodeling. Symmetrically, Trx is necessary to counteract the silencing effect of PcG proteins on a PRE. In addition, Pho binds PcG proteins from the PRC1 complex (Pc and Ph) as well as trxG proteins from the Brahma complex through independent domains. The maintenance of the expression status of their targets by PcG and trxG proteins may thus in part act at the same level, that is, the control of access to the transcription machinery by remodeling of the nucleosomal organization of their target sequences. Along the same line, the GAGA factor, encoded by the trxG gene Trithorax-like (Trl), binds to both active and inactive PREs and interacts with the NURF remodeling complex, with Trx, with Sap18 (a component of the Sin3 corepressor complex), and with Pc, a component of PRC1. In addition, a close association exists between PRE and trxG response elements in homeotic genes. Presumably, these composite elements would switch between activation and repression by PcG or trxG proteins, depending on the preestablished transcriptional state. The vision of PcG and trxG proteins as completely antagonistic factors may not reflect all their regulatory properties as inferred from genetic data. Therefore, they may collectively be addressed as PcG/trxG proteins in order to include all the potential functions of each member (Faucheux, 2003 and references therein).
Elucidation of the precise mechanisms by which PcG/trxG proteins regulate their diverse targets will require the identification of all the PcG/trxG members and the functional characterization of the interaction network between these proteins. With this aim, lolal/batman has been characterized. ban mutant flies display phenotypes that indicate a function of ban in both activation and repression of homeotic genes, identifying ban as a PcG/trxG member. The Batman protein contains a BTB/POZ domain, and associates to polytene chromosomes at several hundreds sites, including many PcG/trxG sites. The Batman protein appears unique among members of the BTB/POZ family since it is almost reduced to its 117-aa BTB/POZ domain. A Batman partner has been identified as the BTB/POZ-containing GAGA factor, a trxG protein (TRL/GAF), that also has a dual, activating and repressing function in the regulation of homeotic genes (Faucheux, 2003).
Lolal/Batman is a chromatin-associated protein that localizes to more than 300 sites on polytene chromosomes. Therefore, Batman has potentially a wide spectrum of functions outside the regulation of homeotic genes. Since Batman does not contain potential DNA binding domains such as zinc fingers or a Pipsqueak (PSQ) domain found in other nuclear BTB/POZ proteins, specific binding of Batman to its chromosomal targets most likely involves interactions with at least one other protein or a protein complex that can bind DNA. Genetic analyses suggest that PcG and trxG proteins are potential partners of Batman. Among Batman binding sites on polytene chromosomes, only one-sixth correspond to Ph binding sites, suggesting that the interaction of Batman with the PcG protein Ph is not necessary for its binding to chromatin. In contrast, the perfect codistribution of Trl and Batman on chromosomes, in addition to their coimmunoprecipitation from nuclear extracts, their interaction in vivo in a yeast 2-hybrid assay, and their cobinding to GAGA target sites in EMSA, strongly suggests that Batman is recruited to DNA by Trl through heterodimerization between the BTB/POZ domains of the two proteins (Faucheux, 2003).
Batman also interacts with the BTB/POZ transcription factors Broad and Bric-a-brac. This suggests that, in addition to its participation to a Trl complex, Batman performs regulatory functions by interacting with a subgroup of nuclear BTB/POZ proteins, at least some of which can bind DNA. Further characterization of the interaction network between Batman and other BTB/POZ partners in Drosophila will be required in order to better understand how the singular BTB/POZ protein Batman exerts its pleiotropic developmental functions (Faucheux, 2003).
The clear-cut distinction between PcG and trxG antagonistic functions has been questioned in functional assays for suppression of trxG mutations by PcG mutations. Six genes of the PcG were shown to behave unexpectedly as enhancers of trxG mutations, and therefore constitute the ETP (Enhancers of PcG and trxG mutations) group. Symmetrically, a subgroup of the trxG may play a role in repression. Such is the case for Trl, first identified as a transcriptional activator and shown to belong to the trxG. However, genetic data indicate that loss of function alleles of Trl do not suppress, as expected for trxG mutations, but rather enhance the Pc3 extra sexcomb phenotype, suggesting that Trl function is required for the repression of Scr. In addition, Trl is required for the repressing activity of the iab-7 PRE and the Mcp silencer. Other cases of trxG genes involved in repression have been described: the BRG1 gene, a homolog of the trxG gene brahma in humans, and osa and brahma in Drosophila. The Drosophila Osa/Brahma chromatin-remodeling complex has been hypothesized to maintain a chromatin conformation that precludes access of the basal transcriptional machinery to target promoters of Wingless signal transduction in the absence of Wingless signal. A role in transcriptional repression or activation may thus in some cases depend on the target, rather than on intrinsic properties of the PcG or trxG proteins (Faucheux, 2003).
ban constitutes a new example of a PcG/trxG candidate that is involved in both the activation and the repression of homeotic genes. Combinations of ban mutant alleles led to the transformation of mesothoracic legs toward prothoracic legs. This extra sexcomb phenotype is enhanced by several mutant alleles of Pc and ph. In addition, the overexpression of ban suppresses the extra sexcomb phenotype of ph410. These data, together with the synergistic interactions between ban and Pc or ph for the Cbx phenotype indicate a function of ban as a repressor of the homeotic genes, Scr and Ubx. However, ban mutant phenotypes also indicate the requirement of ban wild-type function in the activation of homeotic genes. ban enhances the interaction between Trl and Ubx leading to the transformation of halter to wing, which suggests that ban wild-type function is required for the activation of Ubx by Trl. Together, these results provide further support for the idea that at least a subset of PcG/trxG genes, including ban, exerts both positive and negative effects on the regulation of homeotic genes (Faucheux, 2003).
Several hypotheses may explain the dual function of ban. It is not possible to exclude the fact that ban mutations may have opposite effects on distinct homeotic genes indirectly through the transcriptional regulation of both activators and repressors of these genes. Alternatively, the dual function of ban may come from its interaction with Trl, whose function in both activation and repression of homeotic genes has already been documented. This hypothesis is further supported by results demonstrating the partnership between Batman and Trl (Faucheux, 2003).
Several lines of evidence suggest a close association between Batman and Trl. In 0- to 18-h embryos, increasing the dose of Batman through the use of the Gal4/UAS system increases the formation of the Batman- and Trl-containing complexes on the MHS-70 Ubx PRE fragment, which are fully displaced by both anti-Trl and anti-Batman antibodies. This result suggests that Batman may be a Trl cofactor that modulates its binding to MHS-70. Consistent with this, lowering the dose of ban has the same effect as lowering the dose of Trl in at least two regulatory pathways: the repression of Scr, and pairing-sensitive silencing of a white reporter gene next to an AbdB PRE. In addition, ban function is necessary for the activity of Trl in the activation of Ubx. Finally, the increased lethality of Trl13c mutants when the dose of ban is reduced provides additional evidence for the functional significance of the interaction of ban with Trl (Faucheux, 2003).
Batman is thus the second BTB/POZ protein that has been shown to participate in a Trl-containing complex involved in homeotic gene regulation, the other being Psq. Psq, like Batman, binds to the bxd MHS-70 PRE fragment. Psq colocalizes and coimmunoprecipitates with Trl, an interaction that depends on the BTB/POZ domains of the two proteins. In addition, Psq shares functions with Trl and Batman, such as its requirement for the activation of Ubx, as well as for the repression of Scr. However, among the three proteins, Batman appears to display unique functional features since it does not contain a DNA binding domain and since ban mutant phenotypes include the extra sexcomb phenotype that is characteristic of PcG proteins. It is thus likely that understanding the possible function of d(GA)n binding complexes in tethering PcG or trxG complexes to PREs will require deciphering of the triangular interactions of the three BTB/POZ proteins Trl, Psq, and Batman (Faucheux, 2003).
Comparison of the EST sequence of a transcript encoding a 127 amino acid protein (ORF127) with databank sequences reveals the presence of a BTB/POZ domain (Broad Complex, Tramtrack and Bric-à-brac proteins, Poxvirus and Zinc finger) located from aa 6 to 117. The BTB/POZ domain, of this protein, termed Lola like, or alternatively, Batman, shows conservation over its entire length with other fly BTB/POZ domains from the Tramtrack subfamily (Faucheux, 2003).
date revised: 20 December 2003
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