SSX2 is a novel DNA-binding protein that antagonizes polycomb group body formation and gene repression

Polycomb group (PcG) complexes regulate cellular identity through epigenetic programming of chromatin. Here, we show that SSX2, a germline-specific protein ectopically expressed in melanoma and other types of human cancers, is a chromatin-associated protein that antagonizes BMI1 and EZH2 PcG body formation and derepresses PcG target genes. SSX2 further negatively regulates the level of the PcG-associated histone mark H3K27me3 in melanoma cells, and there is a clear inverse correlation between SSX2/3 expression and H3K27me3 in spermatogenesis. However, SSX2 does not affect the overall composition and stability of PcG complexes, and there is no direct concordance between SSX2 and BMI1/H3K27me3 presence at regulated genes. This suggests that SSX2 antagonizes PcG function through an indirect mechanism, such as modulation of chromatin structure. SSX2 binds double-stranded DNA in a sequence non-specific manner in agreement with the observed widespread association with chromatin. Our results implicate SSX2 in regulation of chromatin structure and function.     

The C-terminal nsP1a protein of human astrovirus is a phosphoprotein that interacts with the viral polymerase

Human astrovirus nonstructural C-terminal nsP1a protein (nsP1a/4) colocalizes with the endoplasmic reticulum and viral RNA. It has been suggested that nsP1a/4 protein is involved in the RNA replication process in endoplasmic reticulum-derived intracellular membranes. A hypervariable region (HVR) is contained in the nsP1a/4 protein, and different replicative patterns can be distinguished depending on its variability. In the present work, both the astrovirus RNA-dependent RNA polymerase and four types (IV, V, VI, and XII) of nsP1a/4 proteins have been cloned and expressed in the baculovirus system to analyze their interactions. Different isoforms of each of the nsP1a/4 proteins exist: a nonphosphorylated isoform and different phosphorylated isoforms. While the polymerase accumulates as a monomer, the nsP1a/4 proteins accumulate as oligomers. The oligomerization domain of nsP1a/4-V is mapped between residues 176 and 209. For all studied genotypes, oligomers mainly contain the nonphosphorylated isoform. When RNA polymerase is coexpressed with nsP1a/4 proteins, they interact, likely forming heterodimers. The polymerase binding region has been mapped in the nsP1a/4-V protein between residues 88 and 176. Phosphorylated isoforms of nsP1a/4 type VI show a stronger interactive pattern with the polymerase than the nonphosphorylated isoform. This difference is not observed in genotypes IV and V, suggesting a role of the HVR in modulating the interaction of the nsP1a/4 protein with the polymerase through phosphorylation/dephosphorylation of some critical residues.