AN INHIBITOR OF P34(CDC2) CYCLIN-B THAT REGULATES THE G(2) M TRANSITION IN XENOPUS EXTRACTS/

The activity of maturation-promoting factor (MPF), a protein kinase co mplex composed of p34(cdc2) and cyclin B, is undetectable during inter phase but rises abruptly at the G(2)/M transition to induce mitosis, A fter the synthesis of cyclin B, the suppression of MPF activity before mitosis has been attributed to the phosphorylation of p34(cdc2) On si tes (threonine-14 and tyrosine-15) that inhibit its catalytic activity , We previously showed that the activity of the mitotic p34(cdc2)/cycl in B complex is rapidly suppressed when added to interphase Xenopus ex tracts that lack endogenous cyclin B, Here we show that a mutant of p3 4(cdc2) that cannot be inhibited by phosphorylation (threonine-14 --> alanine, tyrosine-15 --> phenylalanine) is also susceptible to inactiv ation, demonstrating that inhihitory mechanisms independent of threoni ne-14 and tyrosine-15 phosphorylation must exist, We have partially ch aracterized this inhibitory pathway as one involving a reversible bind ing inhibitor of p34(cdc2)/cyclin B that is tightly associated with ce ll membranes, Kinetic analysis suggests that this inhibitor, in conjun ction with the kinases that mediate the inhibitory phosphorylations on p34(cdc2), maintains the interphase state in Xenopus; it may play an important role in the exact timing of the G(2)/M transition.