| Abstract
| - Protein phosphatase-1 (PP1) is regulated by interaction withdifferent subunits, which includeseveral inhibitory proteins. It is also potently inhibited byseveral toxins of diverse origins. Recent workhas identified a region near the C-terminus of PP1 (residues 274−277)whose modification was shown tomoderate okadaic acid binding [Zhang et al. (1994) J. Biol.Chem. 269, 16997−17000]. In thisstudy,the role of this region in toxin binding was explored by site-directedmutagenesis. A residue (Tyr-272)was identified whose mutation had dramatic effects on the spectrum ofinhibitor sensitivity of PP1. TheIC50's of a number of mutants of Tyr-272 toward okadaicacid, tautomycin, calyculin A, microcystin-LR,nodularin, inhibitor-2, and cantharidic acid were determined andcompared to that of the wild-type enzyme.The sensitivity of PP1 toward tautomycin and calyculin A wasmarkedly decreased, by as much as 3orders of magnitude, with lesser effects on okadaic acid and nodularin,and with microcystin-LR andinhibitor-2 being the least affected. These studies show thatTyr-272 is of specific importance for thebinding of these inhibitors and provide strong evidence for thepostulate that these toxins all bind to acommon inhibitor site on PP1. In addition, our studies show thatTyr-272 is not required for catalyticactivity.
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