Cyclic nucleotide-dependent protein kinase activity of adult female locust fat body

Abstract

Locust fat body protein kinase is dependent on, and maximally stimulated by 5 × 10 −6 M cyclic GMP and to a lesser extent by cyclic AMP. It exhibits a pH optimum of 6.9, an unusual requirement for high Mg 2+ (maximum at 100 mM) or an alternative requirement for Mn 2+ (maximum at 5 mM). Maximal stimulation by Mg 2+ is twice that by Mn 2+. Rabbit muscle protein kinase inhibitor uncharacteristically inhibits both cyclic AMP and cyclic GMP stimulated protein kinase activity of locust fat body, the former 5-fold more than the latter, with histone IIA as acceptor. The degree of phosphorylation differs for different acceptor proteins: histone IIA > locust vitellin > arginine-rich histone. Casein is barely phosphorylated and bovine serum albumin (fraction V) not at all. The high threshold for Mg 2+ activation may be related to the high levels of Mg 2+ in locust haemolymph, to the preferential phosphorylation of vitellin, and to the fact that the fat body is the site of vitellogenin synthesis.