A non-cycling mitotic cyclin in the naturally synchronous cell cycle of Physarum polycephalum.

A universal model of the control of the cell cycle in eukaryotic organisms has emerged from the discovery that MPF (maturation or mitosis promoting factor) is a heterodimer consisting of a catalytic subunit (p34cdc2) and a regulatory subunit (mitotic cyclin) encoded by a pair of conserved genes. A prominent feature of the periodic activation of the protein kinase p34cdc2 is the gradual accumulation of cyclin in interphase and its abrupt degradation in mitosis, which is believed to be required for inactivation of MPF and exit from mitosis. Utilizing the precise natural synchrony of mitosis of the plasmodium of the myxomycete Physarum, the high affinity of the p34cdc2/cyclin B complex to p13suc1 Sepharose beads, and immunological reagents including three different anticyclin B antibodies and the anti-PSTAIR antibody, a transient histone H1 kinase activation but not fluctuation in the abundance of cyclin B have been detected during mitosis. It is argued that cyclin degradation may be required for cytokinesis and/or postmitotic controls of cell proliferation in G1 phase and cell-to-cell signaling in development but not for the inactivation of histone H1 kinase in mitosis.