2 POTENTIALLY ONCOGENIC CYCLINS, CYCLIN-A AND CYCLIN-D1, SHARE COMMONPROPERTIES OF SUBUNIT CONFIGURATION, TYROSINE PHOSPHORYLATION AND PHYSICAL ASSOCIATION WITH THE RB PROTEIN

Originally identified as a 'mitotic cyclin', cyclin A exhibits propert ies of growth factor sensitivity, susceptibility to viral subversion a nd association with a tumor-suppressor protein, properties which are i ndicative of an S-phase-promoting factor (SPF) as well as a candidate proto-oncogene. Other recent studies have identified human cyclin D1 ( PRAD1) as a putative G1 cyclin and candidate proto-oncogene. However, the specific enzymatic activities and, hence, the precise biochemical mechanisms through which cyclins function to govern cell cycle progres sion remain unresolved. In the present study we have investigated the coordinate interactions between these two potentially oncogenic cyclin s, cyclin-dependent protein kinase subunits (cdks) and the Rb tumor-su ppressor protein. The distribution of cyctin D isoforms was modulated by serum factors in primary fetal rat lung epithelial cells. Moreover, cyclin D1 was found to be phosphorylated on tyrosine residues in vivo and, like cyclin A, was readily phosphorylated by pp60c-src in vitro. In synchronized human osteosarcoma cells, cyclin D1 is induced in ear ly G1 and becomes associated with p9Ckshs1, a Cdk-binding subunit. Imm unoprecipitation experiments with human osteosarcoma cells and Ewing's sarcoma cells demonstrated that cyclin D1 is associated with both p34 cdc2 and p33cdk2, and that cyclin D1 immune complexes exhibit apprecia ble histone H1 kinase activity. Immobilized, recombinant cyclins A and D1 were found to associate with cellular proteins in complexes that c ontain the p105Rb protein. This study identifies several common aspect s of cyclin biochemistry, including tyrosine phosphorylation and the p otential to interact directly or indirectly with the Rb protein, that may ultimately relate membrane-mediated signaling events to the regula tion of gene expression.