THE ESSENTIAL MITOTIC PEPTIDYL-PROLYL ISOMERASE PIN1 BINDS AND REGULATES MITOSIS-SPECIFIC PHOSPHOPROTEINS

Phosphorylation of mitotic proteins on the Ser/Thr-Pro motifs has been shown to play an important role in regulating mitotic progression. Pi n1 is a novel essential peptidyl-prolyl isomerase (PPIase) that inhibi ts entry into mitosis and is also required for proper progression thro ugh mitosis, but its substrate(s) and function(s) remain to be determi ned. Here we report that in both human cells and Xenopus extracts, Pin 1 interacts directly with a subset of mitotic phosphoproteins on phosp horylated Ser/Thr-Pro motifs in a phosphorylation-dependent and mitosi s-specific manner. Many of these Pin1-binding proteins are also recogn ized by the monoclonal antibody MPM-2, and they include the important mitotic regulators Cdc25, Myt1, Wee1, Plk1, and Cdc27. The importance of this Pin1 interaction was tested by constructing two Pin1 active si te point mutants that fail to bind a phosphorylated Ser/Thr-Pro motif in mitotic phosphoproteins. Wild-type, but not mutant, Pin1 inhibits b oth mitotic division in Xenopus embryos and entry into mitosis in Xeno pus extracts. We have examined the interaction between Pin1 and Cdc25 in detail. Pin1 not only binds the mitotic form of Cdc25 on the phosph orylation sites important for its activity in vitro and in vivo, but i t also inhibits its activity, offering one explanation for the ability of Pin1 to inhibit mitotic entry. In a separate paper, we have shown that Pin1 is a phosphorylation-dependent PPIase that can recognize spe cifically the phosphorylated Ser/Thr-Pro bonds present in mitotic phos phoproteins. Thus, Pin1 likely acts as a general regulator of mitotic proteins that have been phosphorylated by Cdc2 and other mitotic kinas es.