CONDITIONAL BINDING TO AND CELL-CYCLE-REGULATED INHIBITION OF CYCLIN-DEPENDENT KINASE COMPLEXES BY P27(KIP1)

Mammalian cultures primarily regulate cell cycle traverse during G(1). For progression through G(1) and commitment to DNA synthesis, the act ivity of a family of proteins, the cyclin-dependent kinases (cdks), is required. There are two primary regulatory portions of G(1): (a) the G(0)-G(1) transition, which allows entry into G1; and (b) the G(1)-S t ransition, promoting entry to DNA synthesis and commitment to cell div ision. In the present manuscript, we provide evidence for cross-talk b etween these two cell cycle transitions. Extracts prepared from quiesc ent mouse mammary epithelial cells are shown to act in a dominant mann er to specifically inhibit the histone H1 kinase activity of preformed /active cdk2, cdk4, cyclin A, or cyclin E complexes from G(1)-S cell e xtracts. The inhibitory activity arises as cells enter quiescence and decreases once cultures are stimulated to begin G(1) traverse and endo genous cdk activity becomes evident. This activity is associated with the regulated binding of the cdk inhibitor p27(Kip1) to cyclin A/cdk2 kinase complexes upon mixing of the extracts. Removal of p27(Kip1) fro m the quiescent cell extract specifically abolishes the inhibitory eff ect. The inhibitory activity and p27(Kip1) binding in vitro depend on incubation of the extracts at physiological temperature or the presenc e of a reducing agent. The results suggest an interplay between the ac quisition of quiescence, cdk activity, and G(1) traverse.