PIPPin Is a Brain-specific Protein That Contains a Cold-shock Domain and Binds Specifically to H1° and H3.3 mRNAs

During maturation of mammalian brain, variants of both linker ( i.e. H1°) and core ( i.e. H3.3) histone proteins accumulate in nerve cells. As the concentration of the corresponding transcripts decreases, in postmitotic cells, even if the genes are actively transcribed, it is likely that regulation of variant histone expression has relevant post-transcriptional components and that cellular factors affect histone mRNA stability and/or translation. Here we report that PIPPin, a protein that is highly enriched in the rat brain and contains a cold-shock domain, binds with high specificity to the transcripts that encode H1° and H3.3 histone variants. Both mRNAs are bound through the very end of their 3′-untranslated region that encompasses the polyadenylation signal. Although PIPPin is present both in the cytoplasm and the nucleus of nerve cells, PIPPin-RNA complexes can be obtained only from nuclear extracts. The results of two-dimensional electrophoretic analysis suggest that a relevant proportion of nuclear PIPPin is more acidic than expected, thus suggesting that its RNA binding activity might be modulated by post-translational modifications, such as phosphorylation.