Cyclic nucleotide-independent phosphorylation of Vitellin by casein kinase II purified from Rhodnius prolixus oocytes

Abstract

In this study we show that Vitellin (VT) phosphorylation in chorionated oocytes of Rhodnius prolixus is completely inhibited by heparin (10 μg/ml), a classical casein kinase II (CK II) inhibitor. VT phosphorylation is not affected by modulators of cyclic nucleotide-dependent protein kinases such as c-AMP (10 μM), H-8 (1 μM) and H-89 (0.1 μM). We have obtained a 3000-fold VT-free enriched preparation of CK II. Autophosphorylation of this enzyme preparation in the presence of 32P-ATP demonstrated that it lacks any endogenous substrates. Rhodnius CK II is strongly inhibited by heparin (Ki = 9 nM) and uses ATP (Km = 36 μM) or GTP (Km = 86 μM) as phosphate donors. Incubation of VT with purified Rhodnius CK II and 32P-ATP led to the incorporation of 2 mols of phosphate/mol VT. However, the total number of phosphorylation sites available can be altered by previous incubation of VT with alkaline phosphatase. These data show that an insect yolk protein contain phosphorylation sites for a cyclic nucleotide-independent protein kinase such as CK II.