STRUCTURAL AND FUNCTIONAL-ANALYSIS OF THE MITOTIC ROTAMASE PIN1 SUGGESTS SUBSTRATE RECOGNITION IS PHOSPHORYLATION-DEPENDENT

The human rotamase or peptidyl-prolyl cis-trans isomerase Pin1 is a co nserved mitotic regulator essential for the G2/M transition of the euk aryotic cell cycle. We report the 1.35 Angstrom crystal structure of P in1 complexed with an AlaPro dipeptide and the initial characterizatio n of Pin1's functional properties. The crystallographic structure as w ell as pH titration studies and mutagenesis of an active site cysteine suggest a catalytic mechanism that includes general acid-base and cov alent catalysis during peptide bond isomerization. Pin1 displays a pre ference for an acidic residue N-terminal to the isomerized proline bon d due to interaction of this acidic side chain with a basic cluster. T his raises the possibility of phosphorylation-mediated control of Pin1 -substrate interactions in cell cycle regulation.