CARBOXYL-TERMINAL DOMAIN OF P27(KIP1) ACTIVATES CDC2

A variant form of p27 was unexpectedly detected in a synchronized cult ure of NIH3T3 cells treated with serum. The expression levels of this form of p27 which lacked its amino (NH2)-terminal region reached maxim um during G(2)/M phase. Since the appearance of the NH2-terminal trunc ated form of p27 coincided with increased expression of Cdc2, we hypot hesized that p27 may play a role in regulating Cdc2 catalytic activity . To test this hypothesis, wild type p27, as well as the aminoterminal (Np27) and carboxyl-terminal (Cp27), were individually expressed, pur ified, and examined for their ability to regulate CDC2 kinase activity in vitro, Our data showed that both p27 and Np27 inhibited CDC2 kinas e activity. However, in marked contrast, Cp27 enhanced the CDC2 kinase activity. In vitro kinase assays showed that Cp27 and p27 were phosph orylated by CDC2, whereas Np27 was not. In addition, we demonstrated t hat deletion of the putative CDC2 phosphorylation site in the carboxyl -terminal domain of Cp27 diminished activation of CDC2 kinase activity otherwise stimulated by Cp27. A similar deletion did not have any eff ect on the inhibitory function of p27. Together these results suggest that the carboxyl-terminal domain of p27 may activate CDC2 kinase acti vity in vivo during G(2)/M and that this effect may be regulated by se rine/threonine phosphorylation.