Replication-independent expression of H1\u2da and H3.3 histone variants is probably regulated by different RNA-binding proteins

DNA in eukaryotes is wrapped around core histones to form nucleosomes, the basic units of chromatin. The linker histones H1 bind DNA where it enters and leaves the nucleosome, thus stabilizing higher order structures. Chromatin is a dynamic complex, modulated by different processes such as DNA-methylation, post-translational modifications of histones, and incorporation of specific histone variants. Throughout rat brain development, expression of H1\ub0 and H3.3 histone variants is mainly regulated at the post-transcriptional level. These proteins are of interest for their possible involvement in the replication-independent chromatin remodelling induced by extracellular stimuli. We previously cloned two cDNAs encoding, respectively, PIPPin (or CSD-C2), a brain-enriched protein able to bind the 3\u2019end of both H1\ub0 and H3.3 mRNAs, and LPI (longer isoform of PEP-19). Both PEP-19 and LPI are brain-specific. By western blot, we found that PIPPin expression in PC12 cells is enhanced by NGF-induced differentiation. We investigated the RNA-binding properties of the three proteins using their 6 histidine-tagged recombinant fusions and found that they all bind H1\ub0 and H3.3 RNAs. Since PEP-19 and LPI are camstatins, we also analyzed whether calmodulin could interfere with RNA-binding, and found that calmodulin competes with H1\ub0 RNA binding to both proteins, while it is not able to bind RNA on its own. This finding suggests that, in the brain, PEP19 and LPI could induce histone mRNA translation in response to calcium. By using biotinylated H1\ub0/H3.3 RNA as fishing molecules, we isolated by affinity chromatography a group of proteins which were analyzed by mass spectrometry. Among them some heterogeneous nuclear ribonucleoproteins (HnRNP K, A1, A2/B1) and the Hsc70 chaperone. We are currently studying the interactions among these proteins by co-immunoprecipitation assays. Castiglia D. et al. (Biochem Biophys Res Commun 218: 390-41996) Scaturro M. et al. (1998) J Biol Chem 273: 22788-91 Nastasi T. et al. (1999) J Biol Chem 274: 24087-93 Sala A et al. (2007) Int J Mol Med 19: 501-9 Saladino P. et al. (2012) Int J Mol Med 29:141-