Alternative transcripts of a polyhomeotic gene homolog are expressed in distinct regions of somites during segmentation of zebrafish embryos

SIAH-1 interacts with mammalian polyhomeotic homologues HPH2 and affects its stability via the ubiquitin-proteasome pathway

Polycomb Group (PcG) genes encode proteins that form large multimeric and chromatin-associated complexes implicated in the stable repression of developmentally essential genes. HPH2, the Homo sapiens polyhomeotic homologue 2, functions as one of the subunits of PcG complex 1. In our study, SIAH-1, an E3 ligase, could directly associate with HPH2 both in vitro and in vivo. Both the cysteine-rich region of SIAH-1 and the PxVxAxP motif of HPH2 were essential for the interaction. HPH2 was co-localized with SIAH-1 in nuclei. Furthermore, SIAH-1 was able to facilitate the ubiquitination and degradation of HPH2 via ubiquitin-proteasome pathway in vivo. The ubiquitination activity was severely impaired in the SIAH-1 mutant that either lost E3 ligase activity or had weakened binding ability with HPH2, strongly suggesting that SIAH-1 was the direct E3 ligase of HPH2. Thus, our results propose a novel role of SIAH-1 in regulating the expression level of HPH2 through the ubiquitin-proteasome pathway.

Zebrafish Polycomb group gene ph2α is required for epiboly and tailbud formation acting downstream of FGF signaling

We analyzed Polycomb group gene ph2alpha functionally in zebrafish embryos by a gene knock-down procedure using morpholino antisense oligos. Inhibition of ph2alpha message translation resulted in abnormal epibolic movements as well as a thick tailbud or incomplete covering of the yolk plug. At the 24hpf stage, morphants had short trunks and tails, phenotypes similar to those with disturbances in FGF signaling. Accordingly, we looked at the effects of ph2alpha expression upstream and downstream of the FGF pathway. Treatment with SU5402, an inhibitor of Fgfrs, or injection of dominant-negative Fgfr1 DNA markedly reduced ph2alpha expression in the tailbud. In addition, cells expressing mRNAs for no tail, spadetail, myoD, and papc, which are involved in FGF-related development of posterior mesoderm, were distributed abnormally. Collectively, the data argue that ph2alpha is required for epiboly and tailbud formation, acting downstream of the FGF signaling pathway.

Maintenance of gene expression patterns

In development, cells pass on established gene expression patterns to daughter cells over multiple rounds of cell division. The cellular memory of the gene expression state is termed maintenance, and the proteins required for this process are termed maintenance proteins. The best characterized are proteins of the Polycomb and trithorax Groups that are required for silencing and maintenance of activation of target loci, respectively. These proteins act through DNA elements termed maintenance elements. Here, we re-examine the genetics and molecular biology of maintenance proteins. We discuss molecular models for the maintenance of activation and silencing, and the establishment of epigenetic marks, and suggest that maintenance proteins may play a role in propagating the mark through DNA synthesis.

pc1 and psc1, zebrafish homologs of Drosophila Polycomb and Posterior sex combs, encode nuclear proteins capable of complex interactions

Drosophila Polycomb group proteins are thought to form multimeric nuclear complexes that are responsible for stable transmission of repressed states of gene expression during the proliferation of differentiating embryos. In this study, we cloned, sequenced, and characterized two Polycomb group homologs, designated pc1 and psc1, in zebrafish. Amino acid sequence analyses determined that pc1 is a structural homolog of Drosophila Polycomb and that psc1 is a homolog of Drosophila Posterior sex combs. Northern blots and whole-mount in situ hybridization revealed that pc1 and psc1 had overlapping expression patterns at successive stages of embryogenesis. Immunocytochemistry localized both Pc1 and Psc1 protein in blastomere nuclei. Pull-down assays and two-hybrid system deletion analyses showed that these proteins were capable of homotypic and heterotypic interactions and identified the regions required for these interactions. The evidence supports the idea that zebrafish Polycomb group proteins, like those of other species, form nuclear complexes with compositions that may vary in a spatio-temporal manner during development.