Protein kinase activities associated with ribosomes of developing rat brain. Identification of eukaryotic initiation factor 2 kinases.

Protein kinases associated with ribosomes in the brains of suckling (4-10 days) and adult (2 months) rats were extracted from ribosomal fraction with 0.5 M KCl. The different protein kinase activities were characterized by their ability to phosphorylate three exogenous substrates: casein, histone IIs and histone IIIs in the presence of different modulators. Ribosomal salt wash fractions contain a high casein kinase activity which was partially inhibited by heparin and stimulated by calmodulin in the presence of Ca2+, indicating the presence of casein kinase I and II and calcium/calmodulin-dependent kinases. Cyclic AMP and cyclic GMP-dependent kinases and protein kinase C (calcium/phospholipids-dependent kinase) were also present. No differences were found in the casein kinase activities of suckling and adult animals, but histone kinase activities were higher in adult than in suckling animals. To identify initiation factor 2 kinases, purified factor from adult brains was used as a protein marker. In addition to the phosphorylation of both factor subunits alpha and beta by casein kinase I or II, an increased phosphorylation was detected of alpha subunit in the presence of cyclic AMP, and beta subunit, in the presence of Ca2+/calmodulin or Ca2+/phospholipids. Present results reinforce our hypothesis that, as occurs in other eukaryotic cells, the decreased rate of protein synthesis during brain development may be regulated by phosphorylation of initiation factor 2.