Small-molecule inhibitors of ser/thr protein phosphatases: specificity, use and common forms of abuse

Natural product extracts have proven to be a rich source of small molecules that potently inhibit the catalytic activity of certain PPP-family ser/thr protein phosphatases. To date, the list of inhibitors includes okadaic acid (produced by marine dinoflagelates, Prorocentrum sp. and Dinophysis sp.), calyculin A, dragmacidins (isolated from marine sponges), microcystins, nodularins (cyanobacteria, Microcystis sp. and Nodularia sp.), tautomycin, tautomycetin, cytostatins, phospholine, leustroducsins, phoslactomycins, fostriecin (soil bacteria, Streptomyces sp.), and cantharidin (blister beetles, approx 1500 species). Many of these compounds share structural similarities, and several have become readily available for research purposes. Here we will review the specificity of available inhibitors and present methods for their use in studying sensitive phosphatases. Common mistakes in the employment of these compounds will also be addressed briefly, notably the widespread misconception that they only inhibit the activity of PP1 and PP2A. Inhibitors of PP2B (calcineurin) will only be mentioned in passing, except to state that, in our hands, cypermethrin, deltamethrin, and fenvalerate, which are sold as potent inhibitors of PP2B, do not inhibit the catalytic activity of PP2B.