Characterization of a basic polypeptide-activated protein kinase in the eggs of the silkworm, Bombyx mori

Abstract

A basic polypeptide-activated protein kinase (designated BA-kinase), which requires Mn2+ rather than Mg2+ for its activity, has been highly purified from a 1.5 M KCl extract of fertilized and unfertilized eggs of the silkworm, Bombyx mori. It was found that (i) the molecular weight of the purified protein kinase was approx. 260,000; (ii) the kinase phosphorylated vitellin, protamine and casein in a cAMP-, cGMP- and Ca2(+)-independent manner; (iii) the kinase phosphorylated only threonine residues when casein was used as a phosphate acceptor; (iv) protein phosphorylation by the kinase was remarkably stimulated by basic polypeptides, such as histone, polyArg and polyLys; and (v) the purified kinase had an affinity with DNA, but not with RNAs. Moreover, the observations that (a) the BA-kinase activity and phosphorylation of polypeptides by the kinase in the 1.5 M KCl egg extract greatly increased after fertilization, and (b) polypeptides, from this extract, of 16.5-21 kDa, which were highly phosphorylated by the kinase but not by A- or G-kinases, appeared within 4 h after fertilization, suggest that a basic polypeptide activated protein kinase (BA-kinase) may play an important role in the progression of embryogenesis through specific phosphorylation of yolk proteins by the kinase, in the presence of nucleic basic proteins.