Identification of nuclear proteins which interact with H1° mRNA.

In developing rat brain the synthesis of H1° histone is mainly regulated at posttranscriptionallevel and probably depends on RNA-binding proteins (RBPs) (1). Wepreviously identified RBPs apparently specific for this messenger (2) and cloned twonovel proteins by screening an expression cDNA library by binding to radiolabeledRNA (3-10). Here we report the use of biotinylated H1° RNA as bait to isolate bychromatography nuclear proteins which interact with H1° mRNA. We identified bymass spectrometry abundant RBPs and molecular chaperones. By western blot wealso evidenced, among the RNA-bound proteins, the cold shock domain-containingprotein 2 (CSD-C2, also know as PIPPin), a brain-enriched RNA-binding proteincloned in our laboratory. Co-immunoprecipitation assays were performed to confirminteractions. We found that hnRNP K interacts with both hnRNP A1 and Hsc70whereas there is no interaction between hnRNP A1 and Hsc70. Moreover, CSD-C2interacts with hnRNP A1 and with the Y box-binding protein 1 (YB1). We also gotevidences that CSD-C2 interacts with Hsc70. In conclusion, we characterized a setof interactions which suggest the existence of a ribonucleoprotein particle, that mightcontrol H1º histone expression in maturing brain.