Identification of nuclear proteins which interact with H1° mRNA.

In developing rat brain the synthesis of H1° histone is mainly regulated at posttranscriptional level and probably depends on RNA-binding proteins (RBPs) (1). We previously identified RBPs apparently specific for this messenger (2) and cloned two novel proteins by screening an expression cDNA library by binding to radiolabeled RNA (3-10). Here we report the use of biotinylated H1° RNA as bait to isolate by chromatography nuclear proteins which interact with H1° mRNA. We identified by mass spectrometry abundant RBPs and molecular chaperones. By western blot we also evidenced, among the RNA-bound proteins, the cold shock domain-containing protein 2 (CSD-C2, also know as PIPPin), a brain-enriched RNA-binding protein cloned in our laboratory. Co-immunoprecipitation assays were performed to confirm interactions. We found that hnRNP K interacts with both hnRNP A1 and Hsc70 whereas there is no interaction between hnRNP A1 and Hsc70. Moreover, CSD-C2 interacts with hnRNP A1 and with the Y box-binding protein 1 (YB1). We also got evidences that CSD-C2 interacts with Hsc70. In conclusion, we characterized a set of interactions which suggest the existence of a ribonucleoprotein particle, that might control H1º histone expression in maturing brain.