Oncoprotein 18 (Op18) is a conserved cytosolic protein that is a target for both cell cycle and cell surface receptor-regulated phosphorylation events, The four residues Ser(16), Ser(25), Ser(38), and Ser(63) are all subject to cell cycle-regulated phosphorylation, Ser(25) and Ser(38) are targets for cyclin dependent kinases (CDKs), while Ser(16) and Ser(63) are phosphorylated by an unidentified protein kinase, We have recently shown that induced expression of a CDK target site deficient mutant, Op18-S25A,S38A, blocks human cell lines during G2/M transition, In the present report we show that mitosis is associated with complete phosphorylation of the two Op18 CDK target sites Ser(25) and Ser(38) and that Ser(16) and Ser(63) are also phosphorylated to a high stoichiometry, To evaluate the function of multisite phosphorylation of Op18, we expressed and analyzed the cell cycle phenotype of different kinase target site-deficient mutants. The data showed that induced expression of the S16A,S63A, S25A,S38A, and S16A,S25A,S38A,S63A mutants all resulted in an indistinguishable phenotype, i.e. immediate G2/M block and subsequent endoreduplication, a given fraction of G2 versus M-phase blocked cells, and a characteristic nuclear morphology of M-blocked cells, This result was unexpected; however, a likely explanation was provided by analysis of Op18 phosphoisomers, which revealed that mutations of the CDK sites interfere with phosphorylation of Ser(16) and Ser(63). The simplest interpretation of our results is that phosphorylation of Ser(16) and Ser(63) is essential during G2/M transition and that the phenotype of the S25A,S38A mutant is mediated by the observed block of Ser(16)/Ser(63) phosphorylation.
