Identification of protein phosphatase-1-binding proteins by microcystin-biotin affinity chromatography

Myosin light chain phosphatase: subunit composition, interactions and regulation

This review has presented some of the recent data on myosin phosphatase from smooth muscle. Although it is not conclusive, it is likely that most of the myosin phosphatase activity is represented by a holoenzyme composed of three subunits. These are: a catalytic subunit of 38 kDa of the type 1 phosphatase, probably the delta isoform (i.e. PP1c delta); a subunit of about 20 kDa whose function is not established; and a larger subunit that is thought to act as a target subunit. This is termed the myosin phosphatase target subunit, MYPT. Various isoforms of MYPT exist and the relatively minor distinctions are in the C-terminal leucine zipper motifs and/or with inserts in the central region. Many regions of the molecule are highly conserved, including the ankyrin repeats in the N-terminal part of the molecule and the sequence around the phosphorylation site. In addition, these isoforms all contain the four residue PP1c-binding motif (Arg/Lys-Val/Ile-Xaa-Phe). MYPT has been detected in a variety of cells and thus is not unique to smooth muscle. With phosphorylated myosin as substrate, the phosphatase activity of PP1c is low and is enhanced on addition of MYPT. It is assumed that MYPT functions as a target subunit and binds to both PP1c and substrate. The N-terminal fragment of MYPT is responsible for the activation of PP1c activity, but how much of the N-terminal sequence is required is not established. An important point is that activation is not a general effect and is specific for myosin. It is not known if other substrates may be targeted to MYPT. There are two binding sites for PP1c on MYPT: a strong site in the N-terminal segment (containing the 4-residue motif) and a weaker site in the ankyrin repeats, possibly in repeats 5, 6 and 7. The location(s) of the myosin-binding sites on MYPT is controversial, and binding of myosin, or light chain, to both N- and C-terminal fragments has been reported. Regulation of myosin phosphatase activity involves changes in subunit interactions, although molecular mechanisms are not defined. There are basically two theories proposed for phosphatase inhibition (i.e. as seen in the agonist-induced increase in Ca2+ sensitivity). One hypothesis is that phosphorylation of Myosin light chain phosphatase MYPT (at residue 654 or 695 of the gizzard MYPT isoforms or an equivalent residue) inhibits the activity of the MP holoenzyme. The kinase involved is not established, but may be an unidentified endogenous kinase or a RhoA-activated kinase. The latter is an attractive possibility because there is convincing evidence that RhoA plays a crucial role in the Ca(2+)-sensitizing process in smooth muscle. A second idea involves arachidonic acid. This is released via phospholipase A2 and could either interact directly with MYPT and cause dissociation of the holoenzyme (thus effectively reducing the phosphatase activity to that of the isolated catalytic subunit), or it could activate a kinase that would phosphorylate MYPT and inhibit the phosphatase. It is possible that MP activity may also be activated, for example, following increases in cAMP and/or cGMP. Evidence in support of this is very limited and under in vivo conditions the phosphorylation of MYPT by the respective kinases has not been demonstrated. There is, however, a tentative hypothesis based on in vitro data that phosphorylation of MYPT by PKA alters its cellular localization. This involves a shuttle between the dephosphorylated membrane-bound and inhibited state (at least towards P-myosin) to a phosphorylated cytosolic or cytoskeletal, and active state. The pathway(s) discussed above originates at the cell membrane and is carried via one or more messengers to the level of the contractile apparatus where it is manifested by regulation of phosphatase activity. Various components of the route have been identified, including RhoA and the atypical PKC isoforms, but more remain to be discovered. It is possible that more than one pathway, or cascade, is

Protein-tyrosine phosphatases: biological function, structural characteristics, and mechanism of catalysis

The protein-tyrosine phosphatases (PTPases) superfamily consists of tyrosine-specific phosphatases, dual specificity phosphatases, and the low-molecular-weight phosphatases. They are modulators of signal transduction pathways that regulate numerous cell functions. Malfunction of PTPases have been linked to a number of oncogenic and metabolic disease states, and PTPases are also employed by microbes and viruses for pathogenicity. There is little sequence similarity among the three subfamilies of phosphatases. Yet, three-dimensional structural data show that they share similar conserved structural elements, namely, the phosphate-binding loop encompassing the PTPase signature motif (H/V)C(X)5R(S/T) and an essential general acid/base Asp residue on a surface loop. Biochemical experiments demonstrate that phosphatases in the PTPase superfamily utilize a common mechanism for catalysis going through a covalent thiophosphate intermediate that involves the nucleophilic Cys residue in the PTPase signature motif. The transition states for phosphoenzyme intermediate formation and hydrolysis are dissociative in nature and are similar to those of the solution phosphate monoester reactions. One strategy used by these phosphatases for transition state stabilization is to neutralize the developing negative charge in the leaving group. A conformational change that is restricted to the movement of a flexible loop occurs during the catalytic cycle of the PTPases. However, the relationship between loop dynamics and enzyme catalysis remains to be established. The nature and identity of the rate-limiting step in the PTPase catalyzed reaction requires further investigation and may be dependent on the specific experimental conditions such as temperature, pH, buffer, and substrate used. In-depth kinetic and structural analysis of a representative number of phosphatases from each group of the PTPase superfamily will most likely continue to yield insightful mechanistic information that may be applicable to the rest of the family members.

Identification and localization of myosin phosphatase in human platelets

Type 1 (PP1) and type 2A (PP2A) phosphatase activity was measured in three subcellular fractions of human platelets. About 80% of the activity was in the high-speed supernatant. Western blots showed that the catalytic subunit of PP1 (PP1c), including alpha- and delta-isoforms, was present in each fraction, but the level of the catalytic subunit of PP2A was very low in the low-speed pellet (cytoskeletal fraction). Various antibodies detected a subunit similar to the 130 kDa subunit (M130) of myosin phosphatase (MP) of smooth muscle in the low- and the high-speed pellets of human platelets. PP1c and associated proteins were isolated by microcystin-Sepharose. Many proteins were separated from each fraction, including myosin, actin and PP1c. M130 was separated only from the low-speed and the high-speed pellets. Kinase activities were detected in the unbound fractions, and fractions from the low- and high-speed pellets phosphorylated M130 and myosin respectively. Treatment of platelets with calyculin A increased the phosphorylation level of many proteins, including myosin heavy- and light-chains, and caused association of cytoskeletal proteins with the low-speed pellet. No marked change in the distribution of PP1c and M130 was detected. These results suggest that the MP in human platelets is composed of PP1c plus a subunit similar to M130 of the smooth muscle phosphatase.

Isolation and characterization of PNUTS, a putative protein phosphatase 1 nuclear targeting subunit

Protein phosphatase 1 (PP1) is found in the cell nucleus and has been implicated in several aspects of nuclear function. We report here the cloning and initial characterization of a novel protein approximately named phosphatase 1 nuclear targeting subunit (PNUTS). This protein interacts with PP1 in a yeast two-hybrid assay, is found in a stable complex with PP1 in mammalian cell lysates, and exhibits a potent modulation of PP1 catalytic activity toward exogenous substrate in vitro. PNUTS is a ubiquitously expressed protein that exhibits a discreet nuclear compartmentalization and is colocalized with chromatin at distinct phases during mitosis. The subcellular localization of PP1 and the activity toward substrates involved in many aspects of cell physiology have previously been shown to be regulated by association with noncatalytic targeting subunits. The properties of PNUTS are consistent with its role as a targeting subunit for the regulation of nuclear PP1 function.

The regulation of glycogen synthase by protein phosphatase 1 in 3T3-L1 adipocytes. Evidence for a potential role for DARPP-32 in insulin action

The stimulation of glycogen-targeted protein phosphatase 1 (PP1), glycogen synthase, and glycogen synthesis by insulin was examined during the differentiation of 3T3-L1 fibroblasts into adipocytes. Insulin treatment barely changed the low levels of glycogen synthesis measured in fibroblasts. Following differentiation into adipocytes, insulin increased glycogen synthesis up to 40-fold. After further culturing of the adipocytes for a week, insulin stimulated glycogen accumulation 700-fold. Differentiation of 3T3-L1 cells also resulted in the increased expression of glycogen synthase and in increases in both total glycogen synthase activity and -fold stimulation by insulin. While the levels of PP1 protein were unchanged by differentiation, PP1 specific activity decreased over 60%, although sensitivity to insulin treatment was augmented. Concurrently, levels of the PP1 inhibitor protein DARPP-32 were dramatically induced upon 3T3-L1 adipogenesis. DARPP-32 in both 3T3-L1 and primary rat adipocytes was exclusively localized to the particulate fractions, including the glycogen-enriched pellet. PP1 activity from 3T3-L1 adipocytes exhibited a kinetic lag in vitro, which was not present in fibroblast extracts. Insulin pretreatment of the adipocyte cells overcame the in vitro lag in PP1 activity, resulting in up to 5-fold stimulation of PP1 activity being measured at early assay time points. These results suggest that in 3T3-L1 adipocytes, DARPP-32 may maintain glycogen-targeted PP1 activity in a low basal state, priming the phosphatase for stimulation by insulin.

Identification of protein phosphatase 1 as a mitotic lamin phosphatase

At the onset of mitosis, the nuclear lamins are hyperphosphorylated leading to nuclear lamina disassembly, a process required for nuclear envelope breakdown and entry into mitosis. Multiple lamin kinases have been identified, including protein kinase C, that mediate mitotic lamin phosphorylation and mitotic nuclear lamina disassembly. Conversely, lamin dephosphorylation is required for nuclear lamina reassembly at the completion of mitosis. However, the protein phosphatase(s) responsible for the removal of mitotic phosphates from the lamins is unknown. In this study, we use human lamin B phosphorylated at mitosis-specific sites as a substrate to identify and characterize a lamin phosphatase activity from mitotic human cells. Several lines of evidence demonstrate that the mitotic lamin phosphatase corresponds to type 1 protein phosphatase (PP1). First, mitotic lamin phosphatase activity is inhibited by high nanomolar concentrations of okadaic acid and the specific PP1 peptide inhibitor, inhibitor-2. Second, mitotic lamin phosphatase activity cofractionates with PP1 after ion exchange chromatography. Third, microcystin-agarose depletes mitotic extracts of both PP1 and lamin phosphatase activity. Our results demonstrate that PP1 is the major mitotic lamin phosphatase responsible for removal of mitotic phosphates from lamin B, a process required for nuclear lamina reassembly.

A protein phosphatase-1-binding motif identified by the panning of a random peptide display library

An unusually large number of regulatory or targeting proteins that bind to the catalytic subunit of protein phosphatase-1 have been recently reported. This can be explained by their possession of a common protein motif that interacts with a binding site on protein phosphatase-1. The existence of such a motif was established by the panning of a random peptide library in which peptide sequences are displayed on the Escherichia coli bacterial flagellin protein for bacteria that bound to protein phosphatase-1. There were 79 isolates containing 46 unique sequences with the conserved motif VXF or VXW, where X was most frequently His or Arg. In addition, this sequence was commonly preceded by 2-5 basic residues and followed by 1 acidic residue. This study demonstrates that binding to protein phosphatase-1 can be conferred to a protein by the presentation of a peptide motif on a surface loop. This binding motif is found in a number of protein phosphatase-1-binding proteins.

Differential inhibition and posttranslational modification of protein phosphatase 1 and 2A in MCF7 cells treated with calyculin-A, okadaic acid, and tautomycin

Calyculin-A (CA), okadaic acid (OA), and tautomycin (TAU) are potent inhibitors of protein phosphatases 1 (PP1) and 2A (PP2A) and are widely used on cells in culture. Despite their well characterized selectivity in vitro, their exact intracellular effects on PP1 and PP2A cannot be directly deduced from their extracellular concentration because their cell permeation properties are not known. Here we demonstrate that, due to the tight binding of the inhibitors to PP1 and/or PP2A, their cell penetration could be monitored by measuring PP1 and PP2A activities in cell-free extracts. Treatment of MCF7 cells with 10 nM CA for 2 h simultaneously inhibited PP1 and PP2A activities by more than 50%. A concentration of 1 microM OA was required to obtain a similar time course of PP2A inhibition in MCF7 cells to that observed with 10 nM CA, whereas PP1 activity was unaffected. PP1 was predominantly inhibited in MCF7 cells treated with TAU but even at 10 microM TAU PP1 inhibition was much slower than that observed with 10 nM CA. Furthermore, binding of inhibitors to PP2Ac and/or PP1c in MCF7 cells led to differential posttranslational modifications of the carboxyl termini of the proteins as demonstrated by Western blotting. OA and CA, in contrast to TAU, induced demethylation of the carboxyl-terminal Leu309 residue of PP2Ac. On the other hand, CA and TAU, in contrast to OA, elicited a marked decrease in immunoreactivity of the carboxyl terminus of the alpha-isoform of PP1c, probably reflecting proteolysis of the protein. These results suggest that in MCF7 cells OA selectively inhibits PP2A and TAU predominantly affects PP1, a conclusion supported by their differential effects on cytokeratins in this cell line.

Characterization of the interaction between DARPP-32 and protein phosphatase 1 (PP-1): DARPP-32 peptides antagonize the interaction of PP-1 with binding proteins

The catalytic subunit of PP-1 (PP-1C) is potently inhibited (IC50, approximately 1 nM) by DARPP-32 (dopamine- and cAMP-regulated phosphoprotein, M(r) 32,000), inhibitor-1, and inhibitor-2. The NH2-terminal 50 amino acid residues of DARPP-32 and inhibitor-1 are similar, and phosphorylation of a common threonine residue (Thr-34/Thr-35) is necessary for inhibition of PP-1C. We have characterized further the interaction between DARPP-32 and PP-1C. Using synthetic peptides derived from the NH2-terminal region of DARPP-32, residues 6-11, RKKIQF, have been shown to be required for inhibition of PP-1C. Peptides containing this motif were able to antagonize the inhibition of PP-1C by phospho-DARPP-32 and phosphoinhibitor-1. The inhibition of PP-1C by inhibitor-2, but not by okadaic acid, microcystin, or calyculin A, was also attentuated by these antagonist peptides. These results together with results from other studies support a model in which two subdomains of phospho-DARPP-32 interact with PP-1C. The region encompassing phospho-Thr-34 appears to interact with the active site of the enzyme blocking enzyme activity. The region encompassing the RKKIQF motif binds to a domain of PP-1C removed from the active site. Amino acid sequence analysis indicates that basic and hydrophobic features of the RKKIQF motif are conserved in the binding domains of certain PP-1C targeting proteins, suggesting that interaction of inhibitor proteins and targeting proteins may be mutually exclusive.