The structure and regulation of protein phosphatases

Four major serine/threonine-specific protein phosphatase catalytic subunits are present in the cytoplasm of animal cells. Three of these enzymes, PP-1, PP-2A, and PP-2B, are members of the same gene family, while PP-2C appears to be distinct. PP-1, PP-2A, and PP-2B are complexed to other subunits in vivo, whereas PP-2C has only been isolated as a monomeric protein. PP-1, PP-2A, and PP-2C have broad and overlapping specificities in vitro, and account for virtually all measurable activity in tissue extracts toward a variety of phosphoproteins that regulate metabolism, muscle contractility, and other processes. Their precise functions in vivo are unknown, although important clues to the physiological roles of PP-1 and PP-2A are provided by the effects of okadaic acid and by the subcellular localization of PP-1. The active forms of PP-1 are largely particulate, and their association with subcellular structures is mediated by 'targetting subunits' that direct PP-1 to particular locations, enhance its activity toward certain substrates, and confer important regulatory properties upon it. This concept is best established for the glycogen-bound enzymes in skeletal muscle and liver (PP-1G) and the myofibrillar form (PP-1M) in skeletal muscle. The activities of PP-1 and PP-2B are controlled by the second messengers cyclic AMP and calcium. The activity of PP-2B is dependent on calcium and calmodulin, while PP-1 is controlled in a variety of ways that depend on the form of the enzyme and the tissue. PP-1 can be inhibited by cyclic AMP in a variety of cells through the A-kinase-catalyzed phosphorylation of inhibitor-1 and its isoforms. Phosphorylation of the glycogen-binding subunit of PP-1G by A-kinase promotes translocation of the catalytic subunit from glycogen particles to cytosol in skeletal muscle, inhibiting the dephosphorylation of glycogen-metabolizing enzymes. Allosteric inhibition of hepatic PP-1G by phosphorylase a occrs in response to signals that elevate cyclic AMP or calcium, and prevents the activation of glycogen synthase in liver. PP-1 can also be activated indirectly by calcium through the ability of PP-2B to dephosphorylate inhibitor-1. This control mechanism may operate in dopaminoceptive neurones of the brain and other cells. The inactive cytosolic form of PP-1 (PP-1I) can be activated in vitro through the glycogen synthase kinase-3-catalyzed phosphorylation of its inhibitory subunit (inhibitor-2), but the physiological significance is unclear. PP-2A exists in cells as low-activity forms that have a tremendous potential for activation, but the mechanisms that regulate this enzyme and PP-2C in vivo are unknown.