Protein serine/threonine phosphatases: structure, regulation, and functions in cell growth.

It is clear that much remains to be discovered regarding the roles of protein phosphatases in mitogenic signaling pathways. The ability of okadaic acid to activate MAPK/ERKs demonstrates that alteration in serine/threonine dephosphorylation can have significant effects on common steps in growth stimulation induced by different types of mitogens. As in the case of cell cycle control, protein serine/threonine phosphatase plays a central role in the reentry of quiescent cells into the cycle. Because the only known targets of okadaic acid are the catalytic subunits PP1 and PP2A, these enzymes are crucial components of two basic functions carried out by cells: growth and division. Important and obligatory roles for PP2B, PP2C, and newly discovered serine/threonine phosphatases are also likely. However, the limited tissue distribution, unique regulatory properties, and limited substrate specificities of these forms suggest more specialized functions in restricted cell types. The available information on the specific functions of different forms of protein serine/threonine phosphatases, let alone their individual isoforms and different multimeric holoenzymes, is still severely limited. Years of biochemical characterization and cDNA cloning have left us with far more forms than functions. This has led to the gratifying situation, at least for the biochemists, in which genetics and cell biology identify protein phosphatases for which a wealth of biochemical information is already available. The appreciation of the importance of these enzymes in the coming years can only increase as the functions for individual forms are discovered.