The protein Pin1 is a peptidyl-prolyl cis/trans isomerase and is thought to function in control of mitosis. The enzyme specifically acts on phosphorylated Ser- or Thr-Pro bonds, and such phosphporylated motifs occur in various signaling pathways that control cell division and other functions. Zhou et al. propose a new mechanism by which Pin1 might influence signaling through such pathways. They report that the Pro-directed phosphatase protein phosphatase 2A (PP2A) is stereo-specific and dephosphorylates only trans pSer- or pThr-Pro isomers. Thus, Pin1 may function to enhance dephosphorylation of PP2A substrates by promoting conversion to a form that is an appropriate substrate for the phosphatase. Consistent with this possibility, genetic evidence indicates that overexpression of Pin1 can partially overcome defects caused by conditional mutations in PP2A and defects from loss of Pin 1 function are overcome in cells overexpresssing PP2A. Zhou, X.Z., Kops, O., Werner, A., Lu, P.-J., Shen, M., Stoller, G., Küllertz, G., Stark, M., Fischer, G., and Lu, K.P. (2000) Pin1-dependent prolyl isomerization regulates dephosphorylation of Cdc25C and Tau proteins. Mol. Cell 6: 873-883. [Online Journal]