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.

The hnRNP and cytoskeletal protein raver1 contributes to synaptic plasticity.

Raver1 is an hnRNP protein that interacts with the ubiquitous splicing regulator PTB and binds to cytoskeletal components like alpha-actinin and vinculin/metavinculin. Cell culture experiments suggested that raver1 functions as corepressor in PTB-regulated splicing reactions and may thereby increase proteome complexity. To determine the role of raver1 in vivo, we inactivated the gene by targeted disruption in the mouse. Here we report that raver1-deficient mice develop regularly to adulthood and show no obvious anatomical or behavioral defects. In keeping with this notion, cells from raver1-null mice were indistinguishable from wild type cells and displayed normal growth, motility, and cytoskeletal architecture in culture. Moreover, alternative splicing of exons, including the model exon 3 of alpha-tropomyosin, was not markedly changed in mutant mice, suggesting that the role of raver1 for PTB-mediated exon repression is not absolutely required to generate splice variants during mouse development. Interestingly however, loss of raver1 caused significantly reduced plasticity of synapses on acute hippocampal slices, as elicited by electrophysiological measurements of markedly lower LTP and LTD in mutant neurons. Our results provide evidence that raver1 may play an important role for the regulation of neuronal synaptic plasticity, possibly by controlling especially the late LTP via posttranscriptional mechanisms.