A conserved peptide motif in Raver2 mediates its interaction with the polypyrimidine tract-binding protein

Raver2 was originally identified as a member of the hnRNP family through database searches revealing three N-terminal RNA recognition motifs (RRMs) bearing highest sequence identity in the RNP sequences to the related hnRNP Raver1. Outside the RRM region, both Raver proteins are quite divergent in sequence except for conserved peptide motifs of the [S/G][I/L]LGxxP consensus sequence. The latter have been implicated in Raver1 binding to the polypyrimidine tract-binding protein (PTB) a regulatory splicing repressor and common ligand of both Raver proteins. In the present study we investigated the association of Raver2 with RNA and PTB in more detail. The isolated RRM domain of Raver2 weakly interacted with ribonucleotides, but the full-length protein failed to directly bind to RNA in vitro. However, trimeric complexes with RNA were formed via binding to PTB. Raver2 harbors two putative PTB binding sequences in the C-terminal half of the protein, whose influence on Raver2-PTB complex formation was analyzed in a mutational approach, replacing critical leucine residues with alanines. While mutation of either sequence motif alone negatively affected Raver2 binding to PTB in vitro, only mutation of the more C-terminally located SLLGEPP motif significantly reduced the recruitment of Raver2 into perinucleolar compartments (PNCs) in HeLa cells. The latter observation was also confirmed for Raver1: out of four sequence motifs matching the PTB binding consensus, mutations in the SLLGEPP motif were the only ones attenuating the recruitment of Raver1 into PNCs. The conserved mode of PTB binding suggests that Raver2, like Raver1, may function as a modulator of PTB activity.     

Sp1-mediated transcriptional activation is repressed by Sp3.

Sp1, Sp3 (SPR-2) and Sp4 (SPR-1) are human sequence-specific DNA binding proteins with very similar structural features. In this report, we have analyzed Sp3 in direct comparison with Sp1. We have raised antibodies against both Sp1 and Sp3, and show that Sp3 protein, like Sp1, is expressed in various cell lines. Co-transfection experiments in different mammalian cell lines reveal that in contrast to Sp1 and Sp4, Sp3 is not able to activate several Sp1 responsive promoters. In addition, Sp3 also fails to activate reporter constructs in Drosophila SL2 cells lacking endogenous Sp factors. Instead, we find that Sp3 represses Sp1-mediated activation in a linear dose-dependent manner. A mutant of Sp3 lacking the DNA binding domain does not affect activation by Sp1, suggesting that the inhibition is most likely due to the competition with Sp1 for their common binding sites. To determine if any structurally similar domain of Sp3 is able to replace partially homologous domains of Sp1, we have generated chimeric proteins and tested their activation characteristics in gene transfer experiments. It appears that neither the glutamine-rich domains A and B nor the D domain of Sp1 can be replaced by the homologous regions of Sp3. Our results suggest that Sp3 is an inhibitory member of the Sp family.