A NOVEL MAMMALIAN PROTEIN, P55CDC, PRESENT IN DIVIDING CELLS IS ASSOCIATED WITH PROTEIN-KINASE ACTIVITY AND HAS HOMOLOGY TO THE SACCHAROMYCES-CEREVISIAE CELL-DIVISION CYCLE PROTEINS CDC20 AND CDC4

A novel protein, p55CDC, has been identified in cycling mammalian cell s. This transcript is readily detectable in all exponentially growing cell lines hut disappears when cells are chemically induced to fall ou t of the cell cycle and differentiate. The p55CDC protein appears to b e essential for cell division, since transfection of antisense p55CDC cDNA into CHO cells resulted in isolation of only those cells which ex hibited a compensatory increase in p55CDC transcripts in the sense ori entation. Immunoprecipitation of p55CDC yielded protein complexes with kinase activity which fluctuated during the cell cycle. Since p55CDC does not have the conserved protein kinase domains, this activity must be due to one or more of the associated proteins in the immune comple x. The highest levels of protein kinase activity were seen with alpha- casein and myelin basic protein as substrates and demonstrated a patte rn of activity distinct from that described for the known cyclin-depen dent cell division kinases. The p55CDC protein was also phosphorylated in dividing cells. The amino acid sequence of p55CDC contains seven r epeats homologous to the beta subunit of G proteins, and the highest d egree of homology in these repeats was found with the Saccharomyces ce revisiae Cdc20 and Cdc4 proteins, which have been proposed to be invol ved in the formation of a functional bipolar mitotic spindle in yeast cells. The GP repeat has been postulated to mediate protein-protein in teractions and, in p55CDC, may modulate its association with a unique cell cycle protein kinase. These findings suggest that p55CDC is a com ponent of the mammalian cell cycle mechanism.