CDC2 KINASE-MEDIATED PHOSPHORYLATION OF SPLICING FACTOR SF2 ASF/

SR proteins are a family of splicing factors which are important compo nents of spliceosomes. Recent studies suggested that phosphorylation o f SR proteins might be a key event for the regulation of pre-mRNA spli cing and is prevalent in metaphase cells. To investigate the role of c dc2 kinase in cell cycle-dependent phosphorylation of SR protein, we e xamined its phosphorylation of SFS/ASF, a representative SR protein. S F2/ASF was phosphorylated both by recombinant cdc2 kinase, a cdc2 cycl in B complex, and by cdc2 kinase immunoprecipitated from G(2)/m phase HeLa cells. In vitro phosphorylation and phosphopeptide mapping of sev eral mutant proteins revealed that cdc2 kinase specifically phosphoryl ates the RS domain of SFS/ASF with serines 227, 238 and presumably 199 as major phosphorylation sites. These findings suggest the possibilit y that cdc2 kinase takes part in the cell cycle-dependent phosphorylat ion of SR proteins which regulates the function of spliceosomes. (C) 1 998 Academic Press.