Identification of a novel cyclin-like protein in human tumor cells.

Cyclins are key regulatory proteins that, in concert with cyclin-dependent protein kinase subunits (cdks), function to govern critical transitions and/or restriction points during the course of cell cycle progression. Recently, a number of putative mammalian G1 cyclins have been characterized at the molecular level; however, the specific activities of the cyclin/cdk complexes and the precise biochemical pathways regulated by the G1 cyclins remain to be elucidated. In the present study we identify a novel cyclin-like protein in pediatric bone and extremity tumors that appears to be related to, but is clearly distinct from, previously identified members of the cyclin D family, as determined by its profile of antibody cross-reactivity, apparent molecular size, chromatographic behavior, physicochemical properties, and pattern of peptide mapping. This 46-kDa cyclin-like protein, tentatively designated p46cyclin X, is first expressed in synchronized MG-63 osteosarcoma cells in mid-G1, well after the induction of p36cyclin D1, yet prior to the induction of cyclins E and A. Northern analysis, utilizing an oligonucleotide probe complementary to an epitope shared by cyclins D1, D2, and X, detected a novel mRNA species, the appearance of which correlates with p46cyclin X expression. The p46cyclin X protein in Ewing's sarcomas and Wilms' tumors is electrophoretically and chromatographically distinct from both p36cyclin D1 and p34cyclin D2. Moreover, the p46cyclin X protein is 1) precipitated by p9Ckshs1-agarose beads, 2) physically associated with p33cdk2, and 3) autophosphorylated in in vitro kinase reactions. Taken together with the biochemical data, the temporal expression of the p46cyclin X/p33cdk2 kinase system is suggestive of a potential role in regulating latter G1 events (i.e. START) in the commitment to S phase.