Identification of cyk, a cyclin B2 kinase, as a novel calcium/calmodulin-dependent protein kinase II and its role during Xenopus laevis oocyte maturation

Recently, we have partially purified and characterized a specific cell cycl e-regulated cyclin B2 kinase (cyk) from prophase oocytes of Xenopus laevis after an ATP-gamma-S activation step (R. Derua, I. Stevens, E. Waelkens, A Fernandez, N. Lamb, W. Merlevede, and J, Goris, 1997, Exp. Cell Res. 230, 3 10-324), In the present paper we describe its purification to homogeneity. We could identify the kinase as a special form of calcium/calmodulin-depend ent protein kinase II (CaMKII), consisting of five isoforms with molecular masses ranging from 52 to 83 kDa. At least three of them could be considere d as novel. Using an in vivo assay with a synthetic peptide (cyktide), an a ctivation of the kinase was shown at about 50% maturation. Further evidence for this observation came from the injection of the calcium chelator BAPTA and the specific cyk/CaMKII inhibitor AIP. A delay of oocyte maturation of at least 1 h was observed. Besides serine 53, a second cyk phosphorylation site in cyclin B2 was identified as threonine 41, Site-directed mutagenesi s of these sites indicated that phosphorylation of these sites in Xenopus c yclin B2 was not required for the hallmark functions Of cyclin B2. (C) 1999 Academic Press.