The transcription cofactor MAL is regulated by free actin levels and thus by actin dynamics. MAL, together with its DNA-binding partner, SRF, is required for invasive cell migration and in experimental metastasis. Although MAL/SRF has many targets, we provide genetic evidence in both Drosophila and human cellular models that actin is the key target that must be regulated by MAL/SRF for invasive cell migration. By regulating MAL/SRF activity, actin protein feeds back on production of actin mRNA to ensure sufficient supply of actin. This constitutes a dedicated homeostatic feedback system that provides a foundation for cellular actin dynamics.
In drosophila, the preblastodermic syncytial nuclear divisions occur very fast. In this short period of time chromosomes must condense, segregate and decondense, in conditions governed by maternally provided RNAs and proteins. In this report, we show that the Homothorax (Hth) transcription factor is maternally provided and that its function is necessary for the proper assembly of the centric/centromeric heterochromatin during preblastodermic divisions. Embryos lacking the hth maternally-derived transcript, show abnormal localisation of the centromeric CID protein, and aberrant chromosomal segregation. In this syncytial context, Hth presumably acts together with its partner Extradenticle (Exd) and the RNA PolII, to facilitate transcription of satellite repeats. The transcripts derived from these sequences are needed for the correct assembly of the centric heterochromatin.
Xenopus laevis nucleoplasmin is a molecular chaperone that mediates sperm decondensation and nucleosome assembly. Nucleoplasmin has three acidic tracts (A1, A2 and A3) and until recent years the long polyglutamic tract A2 was thought to be the binding site for basic proteins. However, the latest publications in this field show that neither A2 nor A3 is indispensable for histone and sperm-specific protein binding. In this work, we show that the mutation of only four acidic amino acid residues of the small A1 tract drastically reduces nucleoplasmin decondensing activity, pointing out this region as the potential binding site for sperm proteins.
The eyegone (eyg) gene encodes Eyg, a transcription factor of the Pax family with multiple roles during Drosophila development. Although Eyg has been shown to act as a repressor, nothing is known about the mechanism by which it represses its target genes. Here, we show that Eyg forms a protein complex with heterochromatin protein 1a (HP1a). Both proteins bind to the same chromatin regions on polytene chromosomes and act cooperatively to suppress variegation and mediate gene silencing. In addition, Eyg binds to a wingless (wg) enhancer region, recruiting HP1a to assemble a closed, heterochromatin-like conformation that represses transcription of the wg gene. We describe here the evidence that suggests that Eyg, encoded by eyegone (eyg), represses wingless (wg) during eye development by association with HP1a. We show that Eyg forms a protein complex with HP1a and both proteins colocalize on salivary gland polytene chromosomes. Using position effect variegation (PEV) experiments, we demonstrated that eyg has a dose-dependent effect on heterochromatin gene silencing and identified a genetic interaction with HP1a in this process. We further demonstrated that HP1a binds to the same wg enhancer element as Eyg. DNase I sensitivity assays indicated that this enhancer region has a closed heterochromatin-like conformation, which becomes open in eyg mutants. In these mutants, much less HP1a binds to the wg enhancer region, as shown by ChIP experiments. Furthermore, as previously described for Eyg, a reduction in the amount of HP1a in the eye imaginal disc derepresses wg. Together, our results suggest a model in which Eyg specifically binds to the wg enhancer region, recruiting HP1a to that site. The recruitment of HP1a prevents transcription by favoring a closed, heterochromatin-like structure. Thus, for the first time, we show that HP1a plays a direct role in the repression of a developmentally regulated gene, wg, during Drosophila eye development.
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