Mutation of the toxin binding site of PP-1c: Comparison with PP-2B

The catalytic cores of PP-1c and PP-BB (calcineurin) are structurally conse rved. However, PP-BB is resistant to inhibition by toxins of the okadaic ac id and cyclic peptide classes, while PP-lc is potently inhibited. Molecular docking of the structure of microcystin-LR onto the catalytic core of PP-P B identified residues that may be responsible for blocking access of toxins to the catalytic site. Amino acids in PP-lc were substituted with these PP -BB residues to investigate their contribution to PP-2B toxin resistance. M utants of PP-lc were also produced to test the importance of hydrophobic in teractions to toxin binding. Our results suggest that different classes of toxin inhibitors interact with the same hydrophobic side chains of PP-le th rough different mechanisms. Substitution of amino acids in PP-1c with PP-BB residues demonstrated no highly significant changes in toxin inhibition, W e hypothesize that an interaction outside the catalytic core causing the L7 loop of PP-BB to block the catalytic site may be responsible for PP-SB res istance to toxins. (C) 2000 Academic Press.