Identification of Casein Kinase II and Phosphorylated Proteins Associated with Messenger Ribonucleoproteins Particles from Reticulocytes

Abstract

Messenger ribonucleoprotein (mRNP) particles, isolated from [rabbit] 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 inhibited the protein kinase activity in the mRNP particles. The endogenous enzyme phosphorylated 4 peptides (MW 125,000, 107,000, 76,000 and 63,000) in the mRNP particle. Three of the 4 peptides, plus another (MW 175,000), were phosphorylated by purified casein kinase II while 2 peptides (MW 95,000 and 76,000) were phosphorylated with casein kinase I. The mRNP particles were not substrates for the cAMP-dependent protein kinases.