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 isomeras es (PPlases) has been implicated in the G(2)-M transition of the mammalian cell cycle. Pin1 interacts with a series of mitotic phosphoproteins, includ ing Polo-like kinase-1, Cdc25C, and Cdc27, and is thought to act as a phosp horylation-dependent PPlase for these target molecules. Pin1 recognizes pho sphorylated serine-proline or threonine-proline peptide-bonds in test subst rates up to 1300-fold better than in the respective unphosphorylated peptid es. To test directly whether Pin1 regulates the G(2)-M transition and/or pr ogression through mitosis by catalyzing phosphorylation-dependent prolyl is omerization of essential mitotic targets, we examined the consequences of P in1 depletion, achieved by (a) overexpression of Pin1 antisense RNA, (b) ov erexpression of dominant-negative Pin1, and (c) by a known small-molecule P in1-PPlase inhibitor, juglone, The results of all of the three lines of inv estigation show that the catalytic activity of Pin1 is essential for tumor cell survival and entry into mitosis.