Inhibition of membrane fusion in vitro via cyclin B but not cyclin A

It is now clear that complexes of cdc2 kinase with 'mitotic' cyclins regulate the transition between the G2 phase of the cell cycle and mitosis and that membrane traffic in mammalian cells is arrested during mitosis. Using a cell-free assay, we have previously reported that the fusion of early endosomes is, in fact, inhibited via the cdc2 kinase (Tuomikoski, T., Felix, M.-A., Doree, M., and Gruenberg, J. (1989) Nature 342, 942-945). In the present paper, we show that this in vitro inhibition occurs efficiently only when the kinase activity is specifically evoked by a cyclin of the B-type but not by cyclins of the A-type. In addition, high resolution two-dimensional gel analysis revealed that the kinases associated with A- and B-type cyclins exhibit different substrate preferences. These data suggest that the complexes of the cdc2 kinase with different cyclins may control specific events of the cell cycle.