The regulation of p34cdc2 was investigated by overproducing p34cdc2, cyclin (A and B) and the weel + gene product (p107wee1) using a baculoviral expression system. p34cdc2 formed a functional complex with both cyclins as judged by co-precipitation, phosphorylation of cyclin in vitro, and activation of p34cdc2 histone Hi kinase activity. Co-production of p34cdc2 and p107wl in insect cells resulted in a minor population of p34cdc2 that was phosphorylated on tyrosine and displayed an altered electrophoretic mobility. When p34c c2 and p107w`el were co-produced with cyclin (A or B) in insect cells, there was a dramatic increase in the population of p34cd2 that was phosphorylated on tyrosine and that displayed a shift in electrophoretic mobility. The phosphorylation of p34cdc2 on tyrosine was absolutely dependent upon the presence of kinase-active plO7'e. Tyrosine-specific as well as serine/threonine-specific protein kinase activities co-immunoprecipitated with plrO'. These results suggest that cyclin functions to facilitate tyrosine phosphorylation of p34cdc2 and that p107w`l functions to regulate p34C2, either directly or indirectly, by tyrosine phosphorylation.

The first and last names of the first author, Julien Falk, were transposed. The name is correct as shown in the author list printed here.