Expression and characterization of rat protein phosphatases-1 alpha, -1 gamma 1, -1 gamma 2, and -1 delta

The serine-threonine protein phosphatases that regulate the thiazide-sensitive NaCl cotransporter

The activity of the Na⁺-Cl^- cotransporter (NCC) in the distal convoluted tubule (DCT) is finely tuned by phosphorylation networks involving serine/threonine kinases and phosphatases. While much attention has been paid to the With-No-lysine (K) kinase (WNK)- STE20-related Proline Alanine rich Kinase (SPAK)/Oxidative Stress Responsive kinase 1 (OSR1) signaling pathway, there remain many unanswered questions regarding phosphatase-mediated modulation of NCC and its interactors. The phosphatases shown to regulate NCC's activity, directly or indirectly, are protein phosphatase 1 (PP1), protein phosphatase 2A (PP2A), calcineurin (CN), and protein phosphatase 4 (PP4). PP1 has been suggested to directly dephosphorylate WNK4, SPAK, and NCC. This phosphatase increases its abundance and activity when extracellular K⁺ is increased, which leads to distinct inhibitory mechanisms towards NCC. Inhibitor-1 (I1), oppositely, inhibits PP1 when phosphorylated by protein kinase A (PKA). CN inhibitors, like tacrolimus and cyclosporin A, increase NCC phosphorylation, giving an explanation to the Familial Hyperkalemic Hypertension-like syndrome that affects some patients treated with these drugs. CN inhibitors can prevent high K⁺-induced dephosphorylation of NCC. CN can also dephosphorylate and activate Kelch-like protein 3 (KLHL3), thus decreasing WNK abundance. PP2A and PP4 have been shown in in vitro models to regulate NCC or its upstream activators. However, no studies in native kidneys or tubules have been performed to test their physiological role in NCC regulation. This review focuses on these dephosphorylation mediators and the transduction mechanisms possibly involved in physiological states that require of the modulation of the dephosphorylation rate of NCC.

Regulation of Cardiac PKA Signaling by cAMP and Oxidants

Pathologies, such as cancer, inflammatory and cardiac diseases are commonly associated with long-term increased production and release of reactive oxygen species referred to as oxidative stress. Thereby, protein oxidation conveys protein dysfunction and contributes to disease progression. Importantly, trials to scavenge oxidants by systemic antioxidant therapy failed. This observation supports the notion that oxidants are indispensable physiological signaling molecules that induce oxidative post-translational modifications in target proteins. In cardiac myocytes, the main driver of cardiac contractility is the activation of the β-adrenoceptor-signaling cascade leading to increased cellular cAMP production and activation of its main effector, the cAMP-dependent protein kinase (PKA). PKA-mediated phosphorylation of substrate proteins that are involved in excitation-contraction coupling are responsible for the observed positive inotropic and lusitropic effects. PKA-actions are counteracted by cellular protein phosphatases (PP) that dephosphorylate substrate proteins and thus allow the termination of PKA-signaling. Both, kinase and phosphatase are redox-sensitive and susceptible to oxidation on critical cysteine residues. Thereby, oxidation of the regulatory PKA and PP subunits is considered to regulate subcellular kinase and phosphatase localization, while intradisulfide formation of the catalytic subunits negatively impacts on catalytic activity with direct consequences on substrate (de)phosphorylation and cardiac contractile function. This review article attempts to incorporate the current perception of the functionally relevant regulation of cardiac contractility by classical cAMP-dependent signaling with the contribution of oxidant modification.

Identification and characterization of two distinct PPP1R2 isoforms in human spermatozoa

Background

Protein Ser/Thr Phosphatase PPP1CC2 is an alternatively spliced isoform of PPP1C that is highly enriched in testis and selectively expressed in sperm. Addition of the phosphatase inhibitor toxins okadaic acid or calyculin A to caput and caudal sperm triggers and stimulates motility, respectively. Thus, the endogenous mechanisms of phosphatase inhibition are fundamental for controlling sperm function and should be characterized. Preliminary results have shown a protein phosphatase inhibitor activity resembling PPP1R2 in bovine and primate spermatozoa.

Results

Here we show conclusively, for the first time, that PPP1R2 is present in sperm. In addition, we have also identified a novel protein, PPP1R2P3. The latter was previously thought to be an intron-less pseudogene. We show that the protein corresponding to the pseudogene is expressed. It has PPP1 inhibitory potency similar to PPP1R2. The potential phosphosites in PPP1R2 are substituted by non-phosphorylable residues, T73P and S87R, in PPP1R2P3. We also confirm that PPP1R2/PPP1R2P3 are phosphorylated at Ser121 and Ser122, and report a novel phosphorylation site, Ser127. Subfractionation of sperm structures show that PPP1CC2, PPP1R2/PPP1R2P3 are located in the head and tail structures.

Conclusions

The conclusive identification and localization of sperm PPP1R2 and PPP1R2P3 lays the basis for future studies on their roles in acrosome reaction, sperm motility and hyperactivation. An intriguing possibility is that a switch in PPP1CC2 inhibitory subunits could be the trigger for sperm motility in the epididymis and/or sperm hyperactivation in the female reproductive tract.

Protein phosphatase 1γ isoforms linked interactions in the brain

Posttranslational protein modifications, in particular reversible protein phosphorylation, are important regulatory mechanisms involved in cellular signaling transduction pathways. Thousands of human proteins are phosphorylatable and the tight regulation of phosphorylation states is crucial for cell maintenance and development. Protein phosphorylation occurs primarily on serine, threonine, and tyrosine residues, through the antagonistic actions of protein kinases and phosphatases. The catalytic subunit of protein phosphatase 1 (PP1), a major Ser/Thr-phosphatase, associates with a large variety of regulatory subunits that define substrate specificity and determine specific cellular pathway responses. PP1 has been shown to bind to different proteins in the brain in order to execute key and differential functions. This work reports the identification of proteins expressed in the human brain that interact with PP1γ1 and PP1γ2 isoforms by the yeast two-hybrid method. An extensive search of PP1-binding motifs was performed for the proteins identified, revealing already known PP1 regulators but also novel interactors. Moreover, our results were integrated with the data of PP1γ interacting proteins from several public web databases, permitting the development of physical maps of the novel interactions. The PP1γ interactome thus obtained allowed for the identification of novel PP1 interacting proteins, supporting novel functions of PP1γ isoforms in the human brain.

The nonconserved N-terminus of protein phosphatases 1 influences its active site

Protein phosphatase 1 consists of a secondary structure arrangement, conserved in the serine/threonine protein phosphatase gene family, flanked by nonconserved N-terminal and C-terminal domains. The deletion mutant of PP1 with the 8 nonconserved N-terminal residues removed was designated PP1-(9-330). PP1-(9-330) had a higher activity and affinity than PP1 when assayed against four different substrates, and it also demonstrated a 6-fold higher sensitivity to the inhibitor okadaic acid. This suggested that the N-terminal domain suppressed the activity of PP1 and interfered with its inhibition by okadaic acid. The ANS fluorescence intensity of PP1-(9-330) was greater than that of PP1, which implies that the hydrophobic groove running from active site in the truncated PP1 was more hydrophobic than in PP1. Our findings provide evidence that the nonconserved N-terminus of PP1 functions as an important regulatory domain that influences the active site and its pertinent properties.

The N-terminal domain influences the structure and property of protein phosphatase 1

The protein phosphatase 1 has conserved cores in PPP gene family flanked by non-conserved N-terminal domains. PP1 with residues 1-8 deleted or substituted by residues 1-42 of calcineurin catalytic subunit were designated PP1-(9-330) and CNA(1-42)-PP1(9-330), respectively. When compared with PP1, PP1-(9-330) had higher and CNA(1-42)-PP1(9-330) had lower activity with three kinds of substrates; PP1-(9-330) has higher and CNA(1-42)-PP1(9-330) has lower sensitivity to okadaic acid. These results imply that the N-terminal residues influence the activity and sensitivity to inhibitors of PP1. PP1-(9-330), PP1, and CNA(1-42)-PP1(9-330) displayed increasing K (m) and decreasing V (max) with three kinds of substrates, which suggest that the N-terminal residues are connected with the substrates affinity and catalytic efficiency of PP1. PP1-(9-330) has higher and CNA(1-42)-PP1(9-330) has lower fluorescence intensity than PP1, and the emission wavelength maximum was blue-shifted from PP1 to PP1-(9-330) and red-shifted from PP1 to CNA(1-42)-PP1(9-330). Our findings provide evidence that the N-terminal domain is an important region influencing the structure and properties of PP1.

Identification of the interaction sites of Inhibitor-3 for protein phosphatase-1

Inhibitor-3 is a potent inhibitor of protein phosphatase-1, with an IC(50) in the nanomolar range for the inhibition of the dephosphorylation of phosphorylase a. Human Inhibitor-3 possesses a putative protein phosphatase-1 binding motif, (39)KKVEW(43). We provide direct evidence that this sequence is involved in PP1 interaction by examining the effects of site-directed mutations of Inhibitor-3 on its ability to inhibit protein phosphatase-1. A second interaction site whose deletion led to loss of inhibitory potency was identified between residues 65 and 77. The existence of two interaction sites is consistent with the high inhibitory potency of Inhibitor-3, and with current models for other inhibitor and targeting proteins that interact with protein phosphatase-1 with high affinity.

Expression of Human Protein Phosphatase-1 in Saccharomyces cerevisiae Highlights the Role of Phosphatase Isoforms in Regulating Eukaryotic Functions

Human (PP1) isoforms, PP1alpha, PP1beta, PP1gamma1, and PP1gamma2, differ in primary sequences at N and C termini that potentially bind cellular regulators and define their physiological functions. The GLC7 gene encodes the PP1 catalytic subunit with >80% sequence identity to human PP1 and is essential for viability of Saccharomyces cerevisiae. In yeast, Glc7p regulates glycogen and protein synthesis, actin cytoskeleton, gene expression, and cell division. We substituted human PP1 for Glc7p in yeast to investigate the ability of individual isoforms to catalyze Glc7p functions. S. cerevisiae expressing human PP1 isoforms were viable. PP1alpha-expressing yeast grew more rapidly than strains expressing other isoforms. On the other hand, PP1alpha-expressing yeast accumulated less glycogen than PP1beta-or PP1gamma1-expressing yeast. Yeast expressing human PP1 were indistinguishable from WT yeast in glucose derepression. However, unlike WT yeast, strains expressing human PP1 failed to sporulate. Analysis of chimeric PP1alpha/beta subunits highlighted a critical role for their unique N termini in defining PP1alpha and PP1beta functions in yeast. Biochemical studies established that the differing association of PP1 isoforms with the yeast glycogen-targeting subunit, Gac1p, accounted for their differences in glycogen synthesis. In contrast to human PP1 expressed in Escherichia coli, enzymes expressed in yeast displayed in vitro biochemical properties closely resembling PP1 from mammalian tissues. Thus, PP1 expression in yeast should facilitate future structure-function studies of this protein serine/threonine phosphatase.

A Protein Phosphatase-1γ1 Isoform Selectivity Determinant in Dendritic Spine-associated Neurabin

Protein phosphatase-1 (PP1) catalytic subunit isoforms interact with diverse proteins, typically containing a canonical (R/K)(V/I)XF motif. Despite sharing approximately 90% amino acid sequence identity, PP1beta and PP1gamma1 have distinct subcellular localizations that may be determined by selective interactions with PP1-binding proteins. Immunoprecipitation studies from brain and muscle extracts demonstrated that PP1gamma1 selectively interacts with spinophilin and neurabin, F-actin-targeting proteins, whereas PP1beta selectively interacted with G(M)/R(GL), the striated-muscle glycogen-targeting subunit. Glutathione S-transferase (GST) fusion proteins containing residues 146-493 of neurabin (GST-Nb-(146-493)) or residues 1-240 of G(M)/R(GL) (GST-G(M)-(1-240)) recapitulated these isoform selectivities in binding and phosphatase activity inhibition assays. Site-directed mutagenesis indicated that this isoform selectivity was not due to sequence differences between the canonical PP1-binding motifs (neurabin, (457)KIKF(460); G(M)/R(GL), (65)RVSF(68)). A chimeric GST fusion protein containing residues 1-64 of G(M)/R(GL) fused to residues 457-493 of neurabin (GST-G(M)/Nb) selectively bound to and inhibited PP1gamma1, whereas a GST-Nb/G(M) chimera containing Nb-(146-460) fused to G(M)-(69-240) selectively interacted with and weakly inhibited PP1beta, implicating domain(s) C-terminal to the (R/K)(V/I)XF motif as determinants of PP1 isoform selectivity. Deletion of Pro(464) and Ile(465) in neurabin (deltaPI) to equally space a conserved cluster of amino acids from the (R/K)(V/I)XF motif as in G(M)/R(GL) severely compromised the ability of neurabin to bind and inhibit both isoforms but did not affect PP1gamma1 selectivity. Further analysis of a series of C-terminal truncated GST-Nb-(146-493) proteins identified residues 473-479 of neurabin as containing a crucial PP1gamma1-selectivity determinant. In combination, these data identify a novel PP1gamma1-selective interaction domain in neurabin that may allow for selective regulation and/or subcellular targeting of PP1 isoforms.

Expression of protein phosphatases during postnatal development of rabbit heart

Protein phosphatases play a major role in the regulation of L-type calcium current (I(Ca)) in heart cells. We previously showed developmental differences in the effects of inhibitors of protein phosphatases (PP's) on the modulation of I(Ca), with greater stimulatory effects on I(Ca) observed in newborn than in adult ventricular cells. We hypothesized that this developmental difference might be due to greater expression and levels of PP 1 and PP 2A in newborn than in adult ventricular cells. We thus determined the mRNA expression of alpha and beta subunits of PP 1 and the a subunit of PP 2A in adult and newborn rabbit ventricles and levels of PP 1 and PP 2A in total homogenates, particulate membranes, and in soluble fraction prepared from isolated ventricular myocytes from adult and newborn rabbits. RT-PCR analysis demonstrated the presence of mRNA of these subunits of PP's in both newborn and adult ventricles. Northern blot analysis using 32P labeled cDNA probes specific for PP 1alpha, PP 1beta and PP 2Aalpha showed that the expression of steady state mRNA levels for PP 1alpha, PP 1beta and PP 2Aalpha were much higher in newborn compared to adult rabbit ventricles. mRNA for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and for sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) in rabbit ventricles were measured as controls. GAPDH did not show significant developmental changes while mRNA for SERCA was higher in adult compared to newborns. Western blot analysis showed that PP 1 and PP 2A protein levels were also much higher in newborn compared to adult rabbit ventricular cells. Immunoblot analysis in particulate membranes and soluble fraction showed that PP1 was mainly membrane bound while PP 2 was present only in soluble fraction. These findings suggest that the two major protein phosphatases (PP 1 and PP 2A) in heart are expressed at much higher levels in newborn and decline to lower levels in adult ventricular myocytes. The presence of high levels of PP's and particularly PP 1 in newborn cells may be responsible for the greater dependence of newborn cells on the inhibition of PP as a mechanism of action of beta-agonist isoproterenol on I(Ca).

Recombinant small subunit of smooth muscle myosin light chain phosphatase. Molecular properties and interactions with the targeting subunit

We expressed the small subunit of smooth muscle myosin light chain phosphatase (MPs) in Escherichia coli, and have studied its molecular properties as well as its interaction with the targeting subunit (MPt). MPs (M(r) = 18,500) has an anomalously low electrophoretic mobility, running with an apparent M(r) of approximately 21,000 in sodium dodecyl sulfate-gel electrophoresis. CD spectroscopy shows that it is approximately 45% alpha-helix and undergoes a cooperative temperature-induced unfolding with a transition midpoint of 73 degrees C. Limited proteolysis rapidly degrades MPs to a stable C-terminal fragment (M(r) = 10,000) that retains most of the helical content. Rotary shadowing electron microscopy reveals that it is an elongated protein with two domains. Sedimentation velocity measurements show that recombinant MPt (M(r) = 107,000), intact MPs, and the 10-kDa MPs fragment are all dimeric, and that MPs and MPt form a complex with a molar mass consistent with a 1:1 heterodimer. Sequence analysis predicts that regions in the C-terminal portions of both MPs and MPt have high probabilities for coiled coil formation. A synthetic peptide from a region of MPs encompassing residues 77-116 was found to be 100% alpha-helical, dimeric, and formed a complex with MPt with a molecular mass corresponding to a heterodimer. Based on these results, we propose that MPs is an elongated molecule with an N-terminal head and a C-terminal stalk domain. It dimerizes via a coiled coil interaction in the stalk domain, and interacts with MPt via heterodimeric coiled coil formation. Since other proteins with known regulatory function toward MP also have predicted coiled coil regions, our results suggest that these regulatory proteins target MP via the same coiled coil strand exchange mechanism with MPt.

Cell-cycle-dependent association of protein phosphatase 1 and focal adhesion kinase

Immunofluorescence studies with protein phosphatase-1 (PP1) isoforms-specific antibodies detected PP1delta, but not alpha or gamma1, at focal adhesions. PP1delta also co-immunoprecipitated with the focal adhesion kinase (FAK) and the alphav-integrin. In the present study glutathione S-transferase (GST)-PP1delta pulled-down FAK from fibroblasts extract and the interaction domain localized between residues 159 and 295 of delta. The association was confirmed by the ability to GST-FAK-related non-kinase (FRNK) to pull-down PP1delta from fibroblasts extract. GST-FRNK also pulled-down purified muscle PP1 catalytic subunit, thus indicating direct interaction between FAK and PP1. FAK displays consensus sequences for phosphorylation by cell division cycle kinase-2-cyclin B, and might be a PP1 substrate. In fact, FAK immunoprecipitated from metabolically-labelled mitotic HeLa cells without tyrosine phosphatase inhibitors was phosphorylated on Ser only and was dephosphorylated in vitro by purified muscle PP1, with loss of phospho-Ser. No PP1 was associated with FAK immunoprecipitated from mitotic HeLa cells. However, progressively more PP1 activity was assayed in FAK-immunoprecipitates obtained from cells released from mitosis. The associated activity was maximal at 2 h from the mitotic release (when 85-90% of the cells remained round) and decreased to basal level by 8 h (when cells were all polygonal). At the same time FAK underwent dephosphorylation, which was completed by 4 h. FAK obtained from cells at 1.5 h was Ser-phosphorylated, and underwent dephosphorylation during in vitro incubation, with loss of phospho-Ser, indicating the presence of active FAK-bound phosphatase. The only FAK-associated PP1 isoform between 1 and 8 h was PP1delta. The results suggest that FAK dephosphorylation by PP1delta occurs in cells released from mitosis, and confirmed the specific association of PP1delta, as detected previously in adherent cells.

Combinatorial control of protein phosphatase-1

The catalytic subunit of the type 1 Ser/Thr protein phosphatases (PP1) can interact with many different regulatory (R) subunits. These R subunits function as activity-modulators, targeting subunits and/or substrates. The specificity of the R subunits can be accounted for by their interaction with specific subsets of binding pockets on the catalytic subunit and by the presence of subcellular targeting sequences. Hormones, growth factors and metabolites control the function of PP1 holoenzymes mainly by modulating the interaction of the subunits.

Cyanobacterial PPP family protein phosphatases possess multifunctional capabilities and are resistant to microcystin-LR

The structural gene for a putative PPP family protein-serine/threonine phosphatase from the microcystin-producing cyanobacterium Microcystis aeruginosa PCC 7820, pp1-cyano1, was cloned. The sequence of the predicted gene product, PP1-cyano1, was 98% identical to that of the predicted product of an open reading frame, pp1-cyano2, from a cyanobacterium that does not produce microcystins, M. aeruginosa UTEX 2063. By contrast, PP1-cyano1 displayed less than 20% identity with other PPP family protein phosphatases from eukaryotic, archaeal, or other bacterial organisms. PP1-cyano1 and PP1-cyano2 were expressed in Escherichia coli and purified to homogeneity. Both enzymes exhibited divalent metal dependent phosphohydrolase activity in vitro toward phosphoserine- and phosphotyrosine-containing proteins and 3-phosphohistidine- and phospholysine-containing amino acid homopolymers. This multifunctional potential also was apparent in samples of PP1-cyano1 and PP1-cyano2 isolated from M. aeruginosa. Catalytic activity was insensitive to okadaic acid or the cyanobacterially produced cyclic heptapeptide, microcystin-LR, both potent inhibitors of mammalian PP1 and PP2A. PP1-cyano1 and PP1-cyano2 displayed diadenosine tetraphosphatase activity in vitro. Diadenosine tetraphosphatases share conserved sequence features with PPP family protein phosphatases. The diadenosine tetraphosphatase activity of PP1-cyano1 and PP1-cyano2 confirms that these enzymes share a common catalytic mechanism.

Protein phosphatase 1 isoforms in differentiating C2C12 myocytes

In muscle protein phosphatase 1 (PP1) is involved in growth factor signal transduction and metabolic regulations. Three isoforms of the catalytic subunit are found in mammalian cells (PP1alpha, PP1gamma1 and PP1delta), with potentially different functions. We investigated the changes in the PP1 isoforms in differentiating C2C12 myoblasts. Few hours after differentiation induction the soluble PP1 activity was reversibly increased, displaying a peak at 6h. This was due to activation mainly of PP1alpha, with no change in the immunodetected protein. A further indication of PP1alpha involvement came from the observation that electroporation of inactive PP1alpha into myoblasts induced a differentiation delay of at least 24h. Subsequently, starting from 9-12 h, the activities and protein levels of all the three soluble PP1 isoforms decreased, reaching a minimum around 48 h. By this time the cells had undergone morphological changes and myosin became immunodetectable. We conclude that PP1 may be involved in myoblast differentiation, based on: 1) its higher activity in myoblasts than in myocytes, 2) the reversible activation of soluble PP1alpha during the first 6h from differentiation induction, 3) the delay in differentiation onset following electroporation of inactive PP1alpha into myoblasts.

Differential subcellular localization of protein phosphatase-1 alpha, gamma1, and delta isoforms during both interphase and mitosis in mammalian cells

Protein phosphatase-1 (PP-1) is involved in the regulation of numerous metabolic processes in mammalian cells. The major isoforms of PP-1, alpha, gamma1, and delta, have nearly identical catalytic domains, but they vary in sequence at their extreme NH2 and COOH termini. With specific antibodies raised against the unique COOH-terminal sequence of each isoform, we find that the three PP-1 isoforms are each expressed in all mammalian cells tested, but that they localize within these cells in a strikingly distinct and characteristic manner. Each isoform is present both within the cytoplasm and in the nucleus during interphase. Within the nucleus, PP-1 alpha associates with the nuclear matrix, PP-1 gamma1 concentrates in nucleoli in association with RNA, and PP-1 delta localizes to nonnucleolar whole chromatin. During mitosis, PP-1 alpha is localized to the centrosome, PP-1 gamma1 is associated with microtubules of the mitotic spindle, and PP-1 delta strongly associates with chromosomes. We conclude that PP-1 isoforms are targeted to strikingly distinct and independent sites in the cell, permitting unique and independent roles for each of the isoforms in regulating discrete cellular processes.

Protein phosphatase 1 delta is associated with focal adhesions

In all mammalian cells protein phosphatase-1 (PP1) exists in three isoforms, defined as alpha, gamma 1 and delta. Immunofluorescence studies with isoform-specific antibodies indicated that delta, but not alpha or gamma 1, is enriched at focal adhesions in HeLa cells, fibroblasts, endothelial cells and keratinocytes. This was confirmed also by interference reflection microscopy, which indicated that PP1 delta was in areas of tight adhesion of the membrane to the extracellular matrix at sites where the microfilament cytoskeleton is organized. In all the cell types so far considered the PP1 delta in focal adhesions represented only a small aliquot of the total PP1 delta, which was predominantly localized to the nucleus. The association of PP1 delta to focal adhesions was confirmed by the co-immunoprecipitation of PP1 delta with the focal adhesion kinase pp125FAK and with the alpha v integrin. Comparison between the amount of PP1 delta associated with focal adhesion proteins and that of PP1 delta recovered in an anti-PP1 delta immunoprecipitate confirmed that only a minor amount of the enzyme was associated with the focal adhesions. Since some focal adhesion proteins are phosphorylated on Ser/Thr, it is likely that PP1 delta may be involved in the regulation of focal adhesion functions and particularly in the signaling pathway generated by cell-substratum adhesion.

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

Expression in insect cells and characterization of the 110 kDa anchoring subunit of myosin light chain phosphatase

The major myosin light chain phosphatase is composed of three subunits with apparent molecular masses of 130, 38 and 20 kDa, corresponding to the myosin-binding, catalytic and a regulatory subunit of unknown function, respectively. In this work, we have amplified the cDNA coding for each of the three subunits by the polymerase chain reaction, and expressed the 130 kDa subunit in insect cells using the baculovirus expression system. Limited chymotrypsin digestion show that the folding of the expressed protein is similar to that in the native holoenzyme. N-Terminal sequencing reveals that our recombinant protein is authentic. Mass spectrometry shows that the expressed protein is full length. The recombinant protein is capable of binding myosin based on the ELISA assay and myosin affinity chromatography. Finally, rotary shadowing electron microscopy reveals an elongated structure with three globular domains connected by flexible strands. These results pave the way for future biochemical, structural and site-directed mutagenesis studies on the myosin light chain phosphatase. We also found that the cDNA of the 20 kDa subunit may code for a smaller protein with a molecular mass of 18.5 kDa.

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.

Protein phosphatase-1 alpha, gamma 1, and delta: changes in phosphorylation and activity in mitotic HeLa cells and in cells released from the mitotic block

Protein phosphatase-1 is phosphorylated "in vitro" by cdc2-cyclin B (E. Villa-Moruzzi, FEBS Lett. 304, 211-215, 1992). In the present study the phosphatase-1 isoforms alpha, gamma 1, and delta were analyzed in mitotic (nocodazole-blocked) HeLa cells. Phosphorylation on threonine increased in gamma 1 and delta at mitosis. alpha was phosphorylated only in mitotic cells and mainly on serine. Exposure of permeabilized mitotic cells to a peptide that inhibits cdc2 decreased the phosphorylation of the isoforms. Cell fractionation indicated that phosphatase-1 was over 90% inactivated and phosphorylated in the soluble, but not in the chromosomal fraction of mitotic cells. Immunoprecipitation from the mitotic soluble fraction indicated that only gamma 1 and delta, but not alpha, were inactivated. Altogether the data pointed to a correlation between phosphatase-1 inactivation and phosphorylation in mitotic cells. cdc2-cyclin B might be the kinase (or one of the kinases) that phosphorylates phosphatase-1. In cells released from the mitotic block, the phosphatase-1 activity in the soluble, but not in the nuclear fraction, increased progressively, reaching control values by 16 h. Immunoprecipitation indicated that the increase in activity was due to alpha and delta only. On the other hand, the activity of gamma 1 remained low, and this was also the only isoform that remained phosphorylated, though less than in mitotic cells. Also in the case of the cells released from mitosis, a correlation may exist between phosphorylation and inactivation of phosphatase-1. However, the regulation of phosphatase-1 is complex and may involve also regulatory subunits that are still unknown. Altogether, the results indicated the differential regulation of the phosphatase-1 isoforms both at mitosis and in G1 cells.

Enhanced expression of PP1 gamma 1, a catalytic subunit isoform of protein phosphatase type 1, in invasive ductal carcinoma of the breast

Breast cancer is one of the most common malignancies of women. Assessing the biological parameters of malignant tumors may facilitate predictions of clinical outcome. The expression of the three catalytic subunits of protein phosphatase (PP) type 1, PP1 alpha, PP1 gamma 1 and PP1 delta, as well as the one catalytic subunit of PP type 2, PP2AC, were examined in ten cases of mammary dysplasia, ten cases of fibroadenoma and 12 cases of invasive ductal carcinoma, using immunohistochemical analysis. Moreover, we measured the S-phase fraction of the cell cycle for use as a marker value of cell growth, using flow cytometric analysis. The percentage of proliferating cells that stained positive with antisera against PP1 gamma 1 was significantly higher in invasive ductal carcinoma than in mammary dysplasia and fibroadenoma. Furthermore, invasive ductal carcinoma showed a markedly high number of tumor cells in the S-phase of the cell cycle, as compared to mammary dysplasia and fibroadenoma. Our results indicate that PP1 gamma 1 may be involved in the accelerated growth of malignant cells in breast tumors.

Protein phosphatase type-1 and glycogen bind to a domain in the skeletal muscle regulatory subunit containing conserved hydrophobic sequence motif

This study identifies a 100-residue domain within the rabbit skeletal muscle regulatory subunit (PP1G) that binds both type-1 protein phosphatase (PP1C) and glycogen. An N-terminal portion of PP1G was cloned by RT-PCR, and different sized fragments were expressed in bacteria as glutathione S-transferase (GST) fusion proteins. A GST-PP1G fusion containing residues 51-240 bound both PPIC and glycogen, whereas GST alone or fusions containing residues 51-140 or 241-360 bound neither PP1C nor glycogen. The PPIC in whole cell lysates or partially purified PP1C from skeletal muscle, or a complex of PP1C-MCLR-biotin, all bound more effectively than Mn(2+)-activated, recombinant PP1C purified from bacteria. Binding was enhanced by increasing the ionic strength and was disrupted by ethylene glycol, consistent with hydrophobic interactions being critical for stable association. Phosphorylation of the GST-PP1G fusion by cAMP-dependent protein kinase prevented completely association of PP1C. This domain of PP1G, from residues 141-240, contains two sequence motifs of hydrophobic residues: Gx8FEKx10W and DxFxFxIxL, that are conserved among the known glycogen-binding PP1 regulatory subunits. These segments are predicted to form an alpha helix and a beta sheet, and we propose that they are the sites for association with PP1C and glycogen, respectively.

Activation of protein phosphatase 1. Formation of a metalloenzyme

The recombinant catalytic subunit of protein phosphatase 1 is produced as an inactive enzyme which can be activated by Mn2+ (Zhang, Z., Bai, G., Deans-Zirattu, S., Browner, M. F., and Lee, E. Y. C. (1992) J. Biol. Chem. 267, 1484-1490). In this report, we have investigated the effects of divalent cations on the activity of recombinant catalytic subunit of protein phosphatase 1. Latent phosphatase 1 can be activated by Co2+ or Mn2+, whereas other metal ions tested including Fe2+, Zn2+, Mg2+, Ca2+, Cu2+, or Ni2+ were not effective or were only weakly effective in activating the enzyme. The Mn(2+)-stimulated activity was susceptible to inactivation by EDTA; however, the Co(2+)-activated phosphatase was stable after dilution and chelation of the Co2+ with excess EDTA. After stable activation of phosphatase 1 using 57Co2+, a stoichiometric amount of 57Co2+ was shown to be tightly bound to phosphatase 1. These findings demonstrate for the first time the generation of a stable metalloenzyme form of phosphatase 1. Fe2+ reversibly deactivated the Co(2+)-stimulated activity, but did not displace the bound Co2+. Interestingly, treatment of the enzyme with a combination of Fe2+ and Zn2+ (but not the individual metal ions) significantly activated phosphatase 1. These results suggest that at least two metal binding sites exist on the enzyme and that protein phosphatase 1 may be an iron/zinc metalloprotein in vivo.

Activation of protein phosphatase-1 isoforms and glycogen synthase kinase-3 beta in muscle from mdx mice

Three Protein Phosphatase-1 (PP1) isoforms (PP1 alpha, PP1 gamma-1 and PP1 delta) are found in skeletal muscle. These are bound to regulatory subunits, such as inhibitor 2 (I2) in the cytosol and G in the glycogen and microsomal fractions. In vitro, the PP1-12 complex is activated by Glycogen Synthase Kinase-3 (GSK-3 or FA). We investigated the activities and protein levels of the three PP1 isoforms and of GSK-3 in muscle of mdx dystrophic mice. PP1 was assayed as phosphorylase phosphatase, in the presence of 5 nM okadaic acid (which inhibits PP2A). Peptide antibodies were produced and used to investigate PP1 alpha, PP1 gamma-1 and PP1 delta. GSK-3 was assayed using a previously described peptide. This was synthesized in a pre-phosphorylated from, which avoids the additional use of Casein Kinase II. Higher PP1 activity was assayed in the cytosol from mdx rather than from control muscles. Immunoprecipitation indicated that only PP1 alpha and PP1 gamma-1 were more active. This was most likely due to enzyme activation, since the immunodetected proteins were unchanged. On the other hand, the immunodetected PP1 delta was lower in the glycogen and microsomal fractions from mdx muscle. GSK-3 was more active in the mdx extract Selective immunoprecipitation of GSK-3 alpha and GSK-3 beta indicated that both isoforms were activated. In the case of GSK-3 beta, the immunodetected protein was also increased. The changes described herein may be related to the pathological events occurring in the mdx muscle. These include increased protein degradation and turnover, and fibre regeneration. In fact, the decreased PP1 delta may be due to protein degradation and the increased GSK-3 may be the consequence of increased protein turnover or regeneration. The apparent correlation between the increased PP1 alpha and PP1 gamma-1 activities and the increased GSK-3 may agree with the hypothesis that GSK-3 activates the newly synthesized PP1.

Subunit structure and regulation of protein phosphatase-1 in rat liver nuclei

The activity of protein phosphatase-1 in rat liver nuclei (PP-1N) was decreased by up to 97% by associated inhibitory polypeptides, depending on the assay and extraction conditions. These inhibitors were rapidly degraded by endogenous proteases, resulting in the accumulation of active heat-stable intermediates. Two major species of PP-1N could be differentiated by fractionation of a nuclear extract. PP-1NR111 contained, besides the delta-isoform of the catalytic subunit, an inhibitory polypeptide of 111 kDa. PP-1NR41 was found to be an inactive heterodimer between the delta-isoform of the catalytic subunit and NIPP-1, a nuclear inhibitor of PP-1, which in its undegraded form is heat labile and migrates during SDS-polyacrylamide gel electrophoresis as a polypeptide of 41 kDa. Native hepatic NIPP-1 displayed a reduced affinity for the catalytic subunit after phosphorylation by protein kinase A in vitro and after glucagon-induced phosphorylation in vivo.

Molecular cloning of the cDNA for rat phosphatase inhibitor-2 and its wide gene expression in the central nervous system

Inhibitor-2 (I-2) is the cytosolic regulatory subunit of type 1 protein phosphatase. We cloned and sequenced a cDNA for rat I-2 from rat brain cDNA library. By in situ hybridization histochemistry, I-2 mRNA was expressed throughout the adult rat brain (on the postnatal 7th week) at various hybridization intensities: high expression signals were detected in the hippocampal formation, olfactory neuronal layers, choroid plexus and cerebellar Purkinje and granule cell layers. Moderate expression signals were detected homogeneously through the layers II-VI of the neocortex. On the embryonic day 15, 1-2 mRNA was expressed moderately in both ventricular and mantle zones of the fore-, mid-, hindbrain and spinal cord, suggesting the possible involvement of I-2 together with type 1 protein phosphatase (PP-1) in the regulation of various early neuronal events including mitosis, migration and differentiation.

Enhanced expression of catalytic subunit isoform PP1 gamma 1 of protein phosphatase type 1 associated with malignancy of osteogenic tumor

The expressions of the three catalytic subunits of protein phosphatase (PP) type 1 and 2A, PP1 alpha, PP1 gamma 1, and PP2AC, were examined in 14 cases of three types of osteogenic tumor using immunohistochemical analysis. The percentage of tumor cells stained positively with antiserum against PP1 catalytic subunit-isoform PP1 gamma 1 was significantly higher in malignant osteogenic tumors than in benign osteogenic tumors. Furthermore, malignant osteogenic tumor showed markedly high S-phase fraction in the cell cycle of tumor cells, as compared to benign osteogenic tumors. These results suggest that PP1 gamma 1 is involved in the accelerated growth of malignant cells in osteogenic tumors.

Characterization of the PP2A alpha gene mutation in okadaic acid-resistant variants of CHO-K1 cells

Okadaic acid (OA)-resistant variants of Chinese hamster ovary cells, clones CHO/OAR6-6 and CHO/OAR2-3, were isolated from a CHO-K1 culture. These variant cells were 17- to 26-fold more resistant to OA than the parental cells. The phosphorylase phosphatase activity of the variant cell extracts was 2- to 4-fold more resistant to OA than that of the parental cells in the presence of inhibitor 2, a specific inhibitor of type 1 protein serine/threonine phosphatase (PP1). Nucleotide sequencing of PP2A alpha (an isotype of PP2A catalytic subunit) cDNA demonstrated that both variants have a T-->G transversion at the first base of codon 269 (805 nt), which results in substitution of glycine for cysteine. We expressed in COS-1 cells a mutant PP2A alpha tagged with the influenza hemagglutinin epitope. The recombinant mutant PP2A alpha protein immunoprecipitated with an anti-influenza hemagglutinin antibody was more resistant than the wild type to OA, their IC50 values being 0.65 nM and 0.15 nM, and their IC80 values being 4.0 nM and 0.45 nM, respectively. The cysteine at residue 269 present only in highly OA-sensitive protein serine/threonine phosphatase catalytic subunit isozymes, PP2A alpha, PP2A beta, and PPX, is suggested to be involved in the binding of OA. CHO/OAR6-6 and CHO/OAR2-3 cells also overexpressed the P-glycoprotein, and the efflux of OA was more rapid. It is suggested that the PP2A alpha mutation in cooperation with a high level of P-glycoprotein makes the CHO-K1 variants highly resistant to OA.

A latent form of protein phosphatase 1 alpha associated with bovine heart myofibrils

The catalytic subunit of the major protein phosphatase associated with bovine cardiac myofibrils was purified to homogeneity. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of the enzyme revealed only one band with an apparent molecular weight of 37,000. On gel filtration chromatography, the phosphatase activity and the protein co-eluted as a single peak with an apparent molecular weight of 37,000. The purified enzyme was identified as the catalytic subunit of protein phosphatase 1, as determined by sensitivity to inhibitor 1, inhibitor 2, okadaic acid and by specific immunostaining. Evidence obtained with specific antipeptide antibodies demonstrated that this myofibril protein phosphatase was predominantly the alpha isoform of protein phosphatase 1. The purified catalytic subunit was completely inactive. It was activated by pretreatment with Co2+/trypsin in the presence of high ionic strength. Treatment with trypsin alone did not activate the latent enzyme. The enzyme was also activated by Co2+ or Mn2+ alone but not by Ca2+, Mg2+, Ni2+, Cu2+ or Zn2+. Activation of the enzyme was not reversed by removal of Co2+, but Mn(2+)-activated phosphatase activity was partially reversed when Mn2+ was removed. The catalytic subunit could form a 1:1 complex with inhibitor 2 in vitro. The resulting holoenzyme was also activated by pretreatment with Co2+. Since phosphatase 1 alpha is the major phosphatase associated with cardiac myofibril, it is suggested that it is responsible for the dephosphorylation of myosin and other myofibril phosphoproteins.

PP1 gamma 2, a testis-specific protein-serine/threonine-phosphatase type 1 catalytic subunit, is associated with a protein having high sequence homology with the 78-kDa glucose-regulated protein, a member of the 70-kDa heat shock protein family

Protein phosphatase 1 gamma 2 (PP1 gamma 2) is a testis-specific isotype of the protein-serine/threonine-phosphatase type 1 catalytic subunit. Three native forms of PP1 gamma 2 were detected in a crude fraction of rat testis by electrophoresis in a nondenaturing polyacrylamide gel. We purified a major native form of PP1 gamma 2 to homogeneity by successive column chromatography on Mono Q-Sepharose, EAH-agarose, protamine-agarose, and G3000SW and by electrophoresis in a nondenaturing polyacrylamide gel. The G3000SW-purified PP1 gamma 2 native form had an apparent molecular mass of 170 kDa. The purified holoenzyme from nondenaturing polyacrylamide gel was composed of the catalytic subunit and two noncatalytic subunits, of 78 kDa and 55 kDa. Partial amino acid sequence analysis of the 78-kDa protein suggested that it is the 78-kDa glucose-regulated protein, a member of the 70-kDa heat shock protein family. The 78-kDa protein may possibly function as a chaperone or by confining substrate specificity of PP1 gamma 2.

Selective increases in isoform PP1 alpha of type-1 protein phosphatase in ascites hepatoma cells

The amounts of four isoforms of the catalytic subunit of type-1 protein phosphatases, PP1 alpha, PP1 gamma 1, PP1 gamma 2, and PP1 delta, have been determined in extracts of rat ascites hepatomas, AH131A, AH13, AH13NMOR, AH143A, and Yoshida sarcoma, and compared to those of rat liver by Western blot analysis. The amount of PP1 alpha was increased over three times in all five hepatomas. The amount of PP1 gamma 1 was increased over two times in AH13, AH13NMOR, and AH143A. The amount of PP1 delta was selectively increased about 4 times in AH131A and AH143A. The PP1 gamma 2 protein was undetectable in both liver and hepatomas. There was good parallelism between the general increase in only PP1 alpha protein in the hepatomas and the previous data demonstrating the general increase in PP1 alpha mRNA in numerous ascites hepatomas. These results suggest that PP1 alpha plays important roles in the expression of malignant phenotype, that its amount is under strict control at the transcription level, and that PP1 gamma 1 and PP1 delta play different roles in the expression of some phenotype(s) of the ascites hepatomas.

Mutagenesis of the catalytic subunit of rabbit muscle protein phosphatase-1

We have generated site-directed mutants of the catalytic subunit of rabbit muscle ppase-1. Since it is known that ppase-1 and ppase-2A are highly susceptible to inactivation by sulfhydryl reagents, we have mutagenized the six cysteine residues conserved between these two enzymes to serines. The six mutants were purified to near homogeneity by affinity chromatography on inhibitor-2-Sepharose and characterized. All six exhibited enzymatic activity. These results indicate that the catalytic mechanism of ppase-1 is different from that of the protein tyrosine phosphatases which involve a cysteinyl phosphate intermediate.