Overview. Serine/threonine protein phosphatases (PPs) can remove the phosphates from phosphorylated serine and/or threonine residues that have been put on target proteins by serine/threonine protein kinases. In a variety of systems it has been shown that serine/threonine phosphorylation and dephosphorylation figures prominently in ion channel regulation (Armstrong and Eckert, 1987; Haby et al., 1994; Hartzell et al., 1991; Hescheler, et al., 1987; Sperelakis et al, 1994; White et al., 1991). These experiments generally utilize electrophysiological techniques to demonstrate that a protein phosphatase or protein kinase can enzymatically modify and subsequently activate or deactivate an ion channel activity. However, very few of these systems allow for a classical genetic or molecular genetic analysis of this process and most have looked at isolated channel activities. One of the best examples of this approach has been the work done on the role of the cAMP-dependent kinase (PKA) and protein dephosphorylation in the activation of cardiac Ca2+ channels (for a review see Armstrong, 1989). This work has provided good evidence that a protein kinase/protein phosphatase couplet may regulate the activity of this channel in vivo. In addition, it appears that a second protein phosphatase, the Ca2+/calmodulin dependent serine/threonine protein phosphatase (calcineurin/PP2B), may regulate the activity of the protein phosphatase that dephosphorylates the ion channel, although that has not been shown conclusively and may not be true for every system (Victor et al., 1997).

The characterization of ser/thr protein kinase interactions with ion channels has been facilitated by the fact that protein kinases appear to be much more specific in their recognition of targets when compared to protein phosphatases (for a recent review see Shenolikar, 1994). Thus Ser/Thr protein phosphatases have been less well-studied partly because they seem to be less specific (at least in vitro). However, there is evidence that at least in vivo, protein phosphatase regulation of cell activities may be quite specific (Axton et al 1990; Booher and Beach, 1989; Stark et al., 1994).

The Protein Phosphatases. The serine/threonine protein phosphatases have been grouped into two broad classes (type 1 and type 2) on the basis of whether they are inhibited by endogenous inhibitors known as Inhibitors 1 and 2 (for a review see Cohen and Cohen, 1989). The type 2 class of phosphatases have been further subdivided into three additional subclasses (A, B, and C). These classifications have been primarily on the basis of biochemical properties and seem to be an organizing principle for a wide variety of species.

The PP1 class is a highly conserved protein class with about 88-95% identity in its catalytic portion across a range of species (Depaoli-Roach, 1994; Mumby and Walter, 1993). As a class, it has been shown to have a role in regulating mitosis, metabolism, transcription, and ion channel activity (Feng et al, 1991; Tournebize et al, 1997; Zhao et al., 1994; Booher and Beach, 1989; Lebebvre et al., 1995; Klumpp et al., 1990; Armstrong, 1989). It has been postulated that since the catalytic portion of the protein phosphatases is relatively nonspecific that the specificity seen in cell regulatory responses is derived from the targeting of the catalytic subunit by additional proteins that bind and help localize it near target proteins (for a review see Hunter, 1995; Shenolikar, 1994; Pawson and Scott, 1997) or organelles (DePaoli-Roach et al, 1994; Allen et al, 1998). In fact, the PP1 catalytic subunit is usually isolated with other regulatory proteins such as Inhibitors 1 and 2, and the G subunit. These regulate and target the catalytic subunit.

 
 

Theoretically this high degree of relatedness would suggest that cells could survive with just one PP1 gene, and in fact this appears to be true for budding and fission yeast (Booher and Beach, 1989; Ohkura et al., 1989; reviewed in Stark et al., 1994). It has been postulated that if one PP1 isoform is removed, others that are present in the cell can compensate for the missing function in some cases (Ohkura et al., 1989; Axton et al, 1990). However, one emerging observation in higher organisms is the large number of closely related PP1 genes that are seen (Shenolikar, 1994). This suggests that there are either physiological differences between the isoforms or a strong selection for redundancy (as might be expected for a vital function). To address this issue, work in Drosophila has highlighted at least some functional differences between the various isoforms identified in that species (Axton et al., 1990). In addition, Cohen and coworkers have identified that one PP1 isoform (delta) is the preferred catalytic constituent of the glycogen/SR and the myosin-associated phosphatases (Alessi, et al, 1992). This suggests that the different isoforms may have certain specialized physiological functions in addition to some common functions. This divergence of function could derive from the considerable sequence divergence in the carboxy terminal portion of the catalytic subunit or from differences in tissue expression as seen in mammalian brain and plants (da Cruz e Silva, et al., 1995; Suh et al, 1998). Providing strong evidence for this is complicated by the relatedness of the isoforms to each other and the difficulty in doing classical and molecular genetic experiments in those systems where the large number of isoforms are being identified. Nonetheless, determining the role of isoforms in cell physiology is important if we hope to have a detailed molecular picture of these regulatory events.

 
Last updated Wednesday July 18, 2001 Webmaster Dean Fraga.