MRNP3 AND MRNP4 ARE PHOSPHORYLATABLE BY CASEIN KINASE-II IN XENOPUS OOCYTES, BUT PHOSPHORYLATION DOES NOT MODIFY RNA-BINDING AFFINITY

mRNP3 and mRNP4 (also called FRGY2) are two mRNA-binding proteins whic h are major constituents of the maternal RNA storage particles of Xeno pus laevis oocytes, The phosphorylation of mRNP3-4 has been implicated in the regulation of mRNA masking, In this study, we have investigate d their phosphorylation by casein kinase II and its consequence on the ir affinity for RNA, Comparison of the phosphopeptide map of mRNP3-4 p hosphorylated in vivo with that obtained after phosphorylation in vitr o by purified Xenopus laevis casein kinase II strongly suggests that c asein kinase II is responsible for the in vivo phosphorylation of mRNP 3-4 in oocytes, The phosphorylation occurs on a serine residue in a ce ntral domain of the proteins, The affinity of mRNP3-4 for RNA substrat es remained unchanged after the treatment with casein kinase II or cal f intestine phosphatase in vitro, This suggests that phosphorylation o f these proteins does not regulate their interaction with RNA but rath er controls their interactions with other proteins. (C) 1997 Federatio n of European Biochemical Societies.