Phosphorylation-dependent proline isomerization catalyzed by Pin1 is essential for tumor cell survival and entry into mitosis.

Pin1, a member of the parvulin family of peptidyl-prolyl cis-trans isomerases (PPIases) has been implicated in the G2-M transition of the mammalian cell cycle. Pin1 interacts with a series of mitotic phosphoproteins, including Polo-like kinase-1, Cdc25C, and Cdc27, and is thought to act as a phosphorylation-dependent PPIase for these target molecules. Pin1 recognizes phosphorylated serine-proline or threonine-proline peptide-bonds in test substrates up to 1300-fold better than in the respective unphosphorylated peptides. To test directly whether Pin1 regulates the G2-M transition and/or progression through mitosis by catalyzing phosphorylation-dependent prolyl isomerization of essential mitotic targets, we examined the consequences of Pin1 depletion, achieved by (a) overexpression of Pin1 antisense RNA, (b) overexpression of dominant-negative Pin1, and (c) by a known small-molecule Pin1-PPIase inhibitor, juglone. The results of all of the three lines of investigation show that the catalytic activity of Pin1 is essential for tumor cell survival and entry into mitosis.