SERINE THREONINE PHOSPHORYLATION REGULATES BINDING OF C-HNRNP PROTEINS TO PREMESSENGER RNA

The C hnRNP proteins bind to nascent preMRNA and are thought to partic ipate in an early step of the pre-mRNA splicing pathway. We report her e that C hnRNP proteins are phosphorylated by a casein kinase II activ ity in a HeLa cell nuclear extract and that dephosphorylation of C hnR NP proteins is inhibited by the specific protein-serine/threonine-phos phatase 1/2A inhibitor okadaic acid. We further find that dephosphoryl ation of C hnRNP proteins is required for their binding to adenovirus and human beta-globin pre-mRNAs. These results indicate that the parti cipation of C hnRNP proteins in pre-spliceosome assembly is coupled to a dynamic cycle of their phosphorylation and dephosphorylation.