ABSTRACT
Several organisms produce protein phosphatase (PP)-inhibiting toxins. Okadaic acid (OA) and calyculin A (both from marine sponges Halichondria okadaii or H. melanodocia), tautomycin (an antibiotic from Streptomyces spiroverticillatus), microcystin and nodularin (both cyanobacterial hepatotoxins), and cantharidin (from blister beetles) are all potent inhibitors of the catalytic subunits of protein phosphatases (see Chapters 13 and 30 in this book). There are four major protein phosphatases identified in the cytosol of eukaryotic cells, the PP1, PP2A, and PP2B group, which belong to the PPP family, and PP2C (calcineurin, a member of the PPM family [Barford, 1996]), and they are responsible for the dephosphorylation of serine and threonine residues in proteins (Cohen and Cohen, 1989). Many other protein phosphatases have been reported (for a review, see Barford, 1996; Burke and Zhang, 1998).
