P19(SKP1) AND P45(SKP2) ARE ESSENTIAL ELEMENTS OF THE CYCLIN A-CDK2 S-PHASE KINASE

In normal human fibroblasts, cyclin A-CDK2 exists in a quaternary comp lex that contains p21 and PCNA. In many transformed cells, p21 disappe ars, and a substantial fraction of cyclin A-CDK2 complexes with p9(CKS 1/CKS2), p19 and p45. To investigate the significance of these rearran gements, we have isolated cDNAs encoding pig and p45. In vitro reconst itution demonstrated that binding of pie to cyclin A-CDK2 requires p45 . Addition of these proteins to the kinase had no substantial effect o n the kinase activity in vitro. Interference with p45 function in vivo by microinjection of antibodies or antisense oligonucleotides prevent ed entry into S phase in both normal and transformed cells. Cyclin A-C DKS has previously been identified as a kinase whose activity is essen tial for S phase. Our results identify p45 as an essential element of this activity. The abundance of p45 is greatly increased in many trans formed cells. This could result in changes in cell cycle control that contribute to the process of cellular transformation.