Identification of casein kinase II and phosphorylated proteins associated with messenger ribonucleoproteins particles from reticulocytes.

Messenger ribonucleoprotein (mRNP) particles, isolated from reticulocyte polysomes and purified by buoyant density centrifugation in metrizamide, contained an endogenous protein kinase activity. The cyclic-nucleotide-independent protein kinase phosphorylated casein using either ATP or GTP as the phosphoryl donor and had properties similar to casein kinase II, an enzyme previously purified and characterized from the post-ribosomal supernate of reticulocytes. Antibody prepared to casein kinase II was shown to inhibit the protein kinase activity in the mRNP particles. The endogenous enzyme phosphorylated four peptides (Mr 125 000, 107 000, 76 000 and 63 000) in the mRNP particle. Three of the four peptides, plus another (Mr 175 000), were phosphorylated by purified casein kinase II while two peptides (Mr 95 000 and Mr 76 000) were phosphorylated with casein kinase I. The mRNP particles were not substrates for the cAMP-dependent protein kinases.