MEMBERS OF THE NAP SET FAMILY OF PROTEINS INTERACT SPECIFICALLY WITH B-TYPE CYCLINS/

Cyclin-dependent kinase complexes that contain the same catalytic subu nit are able to induce different events at different times during the cell cycle, but the mechanisms by which they do so remain largely unkn own. To address this problem, we have used affinity chromatography to identify proteins that bind specifically to mitotic cyclins, with the goal of finding proteins that interact with mitotic cyclins to carry o ut the events of mitosis. This approach has led to the identification of a 60-kD protein called NAP1 that interacts specifically with member s of the cyclin B family. This interaction has been highly conserved d uring evolution: NAP1 in the Xenopus embryo interacts with cyclins B1 and B2, but not with cyclin A, and the S. cerevisiae homolog of NAP1 i nteracts with Clb2 but not with Clb3. Genetic experiments in budding y east indicate that NAP1 plays an important role in the function of Clb 2, while biochemical experiments demonstrate that purified NAP1 can be phosphorylated by cyclin B/p34(cdc2) kinase complexes, but not by cyc lin A/p34(cdc2) kinase complexes. These results suggest that NAP1 is a protein involved in the specific functions of cyclin B/p34(cdc2) kina se complexes. In addition to NAP1, we found a 43-kD protein in Xenopus that is homologous to NAP1 and also interacts specifically with B-typ e cyclins. This protein is the Xenopus homolog of the human SET protei n, which was previously identified as part of a putative oncogenic fus ion protein (Von Lindern et al., 1992).