ASSOCIATION OF HUMAN CUL-1 AND UBIQUITIN-CONJUGATING ENZYME CDC34 WITH THE F-BOX PROTEIN P45(SKP2) - EVIDENCE FOR EVOLUTIONARY CONSERVATIONIN THE SUBUNIT COMPOSITION OF THE CDC34-SCF PATHWAY

In normal and transformed cells, the F-box protein p45(SKP2) is requir ed for S phase and forms stable complexes with p19(SKP1) and cyclin A- cyclin-dependent kinase (CDK)2, Here we identify human CUL-1, a member of the cullin family, and the ubiquitin-conjugating enzyme CDC34 as a dditional partners of p45(SKP2) in vivo, CUL-1 also associates with cy clin A and p19(SKP1) in vivo and, with p45(SKP2), they assemble into a large multiprotein complex, In Saccharomyces cerevisiae, a complex of similar molecular composition (an F-box protein, a member of the cull in family and a homolog of p19(SKP1)) forms a functional E3 ubiquitin protein ligase complex, designated SCFCDC4, that facilitates ubiquitin ation of a CDK inhibitor by CDC34, The data presented here imply that the p45(SKP2)-CUL-1-p19(SKP1) complex may be a human representative of an SCF-type E3 ubiquitin protein ligase. We propose that all eukaryot ic cells may use a common ubiquitin conjugation apparatus to promote S phase, Finally, we show that multiprotein complex formation involving p45(SKP2)-CUL-1 and p19(SKP1) is governed, in part, by periodic, S ph ase-specific accumulation of the p45(SKP2) subunit and by the p45(SKP2 )-bound cyclin A-CDK2, The dependency of p45(SKP)-p19(SKP1) complex fo rmation on cyclin A-CDK2 may ensure tight coordination of the activiti es of the cell cycle clock with those of a potential ubiquitin conjuga tion pathway.