A comparative study of the phosphotyrosyl phosphatase specificity of protein phosphatase type 2A and phosphotyrosyl phosphatase type 1B using phosphopeptides and the phosphoproteins p50/HS1, c-Fgr and Lyn

p21-Activated kinase-1 and its role in integrated regulation of cardiac contractility

We review here a novel concept in the regulation of cardiac contractility involving variations in the activity of the multifunctional enzyme, p21-activated kinase 1 (Pak1), a member of a family of proteins in the small G protein-signaling pathway that is activated by Cdc42 and Rac1. There is a large body of evidence from studies in noncardiac tissue that Pak1 activity is key in regulation of a number of cellular functions, including cytoskeletal dynamics, cell motility, growth, and proliferation. Although of significant potential impact, the role of Pak1 in regulation of the heart has been investigated in only a few laboratories. In this review, we discuss the structure of Pak1 and its sites of posttranslational modification and molecular interactions. We assemble an overview of the current data on Pak1 signaling in noncardiac tissues relative to similar signaling pathways in the heart, and we identify potential roles of Pak1 in cardiac regulation. Finally, we discuss the current state of Pak1 research in the heart in regard to regulation of contractility through functional myofilament and Ca(2+)-flux modification. An important aspect of this regulation is the modulation of kinase and phosphatase activity. We have focused on Pak1 regulation of protein phosphatase 2A (PP2A), which is abundant in cardiac muscle, thereby mediating dephosphorylation of sarcomeric proteins and sensitizing the myofilaments to Ca(2+). We present a model for Pak1 signaling that provides a mechanism for specifically affecting cardiac cellular processes in which regulation of protein phosphorylation states by PP2A dephosphorylation predominates.

Structure and mechanism of the phosphotyrosyl phosphatase activator

Phosphotyrosyl phosphatase activator (PTPA), also known as PP2A phosphatase activator, is a conserved protein from yeast to human. Here we report the 1.9 A crystal structure of human PTPA, which reveals a previously unreported fold consisting of three subdomains: core, lid, and linker. Structural analysis uncovers a highly conserved surface patch, which borders the three subdomains, and an associated deep pocket located between the core and the linker subdomains. The conserved surface patch and the deep pocket are responsible for binding to PP2A and ATP, respectively. PTPA and PP2A A-C dimer together constitute a composite ATPase. PTPA binding to PP2A results in a dramatic alteration of substrate specificity, with enhanced phosphotyrosine phosphatase activity and decreased phosphoserine phosphatase activity. This function of PTPA strictly depends on the composite ATPase activity. These observations reveal significant insights into the function and mechanism of PTPA and have important ramifications for understanding PP2A function.

Protein phosphatase 2A: a highly regulated family of serine/threonine phosphatases implicated in cell growth and signalling

Protein phosphatase 2A (PP2A) comprises a family of serine/threonine phosphatases, minimally containing a well conserved catalytic subunit, the activity of which is highly regulated. Regulation is accomplished mainly by members of a family of regulatory subunits, which determine the substrate specificity, (sub)cellular localization and catalytic activity of the PP2A holoenzymes. Moreover, the catalytic subunit is subject to two types of post-translational modification, phosphorylation and methylation, which are also thought to be important regulatory devices. The regulatory ability of PTPA (PTPase activator), originally identified as a protein stimulating the phosphotyrosine phosphatase activity of PP2A, will also be discussed, alongside the other regulatory inputs. The use of specific PP2A inhibitors and molecular genetics in yeast, Drosophila and mice has revealed roles for PP2A in cell cycle regulation, cell morphology and development. PP2A also plays a prominent role in the regulation of specific signal transduction cascades, as witnessed by its presence in a number of macromolecular signalling modules, where it is often found in association with other phosphatases and kinases. Additionally, PP2A interacts with a substantial number of other cellular and viral proteins, which are PP2A substrates, target PP2A to different subcellular compartments or affect enzyme activity. Finally, the de-regulation of PP2A in some specific pathologies will be touched upon.

Identification of protein-tyrosine phosphatase 1B as the major tyrosine phosphatase activity capable of dephosphorylating and activating c-Src in several human breast cancer cell lines

c-Src tyrosine kinase activity is elevated in several types of human cancer, and this has been attributed to elevated c-Src expression levels, increased c-Src specific activity, and activating mutations in c-Src. We have found a number of human breast cancer cell lines with elevated c-Src specific activity that also possess elevated phosphatase activity directed against the carboxyl-terminal negative regulatory domain of Src family kinases. To identify this phosphatase, cell extracts from MDA-MB-435S cells were chromatographed and the fractions were assayed for phosphatase activity. Four peaks of phosphatase activity directed against the nonspecific substrate poly(Glu/Tyr) were detected. One peak also dephosphorylated a peptide modeled against the c-Src carboxyl-terminal negative regulatory domain and intact human c-Src. Immunoblotting and immunodepletion experiments identified the phosphatase as protein-tyrosine phosphatase 1B (PTP1B). Examination of several human breast cancer cell lines with increased c-Src activity showed elevated levels of PTP1B protein relative to normal control breast cells. In vitro c-Src reactivation experiments confirmed the ability of PTP1B to dephosphorylate and activate c-Src. In vivo overexpression of PTP1B in 293 cells caused a 2-fold increase of endogenous c-Src kinase activity. Our findings indicate that PTP1B is the primary protein-tyrosine phosphatase capable of dephosphorylating c-Src in several human breast cancer cell lines and suggests a regulatory role for PTP1B in the control of c-Src kinase activity.

Carboxyl methylation regulates phosphoprotein phosphatase 2A by controlling the association of regulatory B subunits

Phosphoprotein phosphatase 2A (PP2A) is a major phosphoserine/threonine protein phosphatase in all eukaryotes. It has been isolated as a heterotrimeric holoenzyme composed of a 65 kDa A subunit, which serves as a scaffold for the association of the 36 kDa catalytic C subunit, and a variety of B subunits that control phosphatase specificity. The C subunit is reversibly methyl esterified by specific methyltransferase and methylesterase enzymes at a completely conserved C-terminal leucine residue. Here we show that methylation plays an essential role in promoting PP2A holoenzyme assembly and that demethylation has an opposing effect. Changes in methylation indirectly regulate PP2A phosphatase activity by controlling the binding of regulatory B subunits to AC dimers.

The Saccharomyces cerevisiae homologue YPA1 of the mammalian phosphotyrosyl phosphatase activator of protein phosphatase 2A controls progression through the G1 phase of the yeast cell cycle

The Saccharomyces cerevisiae gene YPA1 encodes a protein homologous to the phosphotyrosyl phosphatase activator, PTPA, of the mammalian protein phosphatase type 2A (PP2A). In order to examine the biological role of PTPA, we disrupted YPA1 and characterised the phenotype of the ypa1Delta mutant. Comparison of the growth rate of the wild-type strain and the ypa1Delta mutant on glucose-rich medium after nutrient depletion showed that the ypa1Delta mutant traversed the lag period more rapidly. This accelerated progression through "Start" was also observed after release from alpha-factor-induced G1 arrest as evidenced by a higher number of budding cells, a faster increase in CLN2 mRNA expression and a more rapid reactivation of Cdc28 kinase activity. This phenotype was specific for deletion of YPA1 since it was not observed when YPA2, the second PTPA gene in budding yeast was deleted. Reintroduction of YPA1 or the human PTPA cDNA in the ypa1Delta mutant suppressed this phenotype as opposed to overexpression of YPA2. Disruption of both YPA genes is lethal, since sporulation of heterozygous diploids resulted in at most three viable spores, none of them with a ypa1Delta ypa2Delta genotype. This observation indicates that YPA1 and YPA2 share some essential functions. We compared the ypa1Delta mutant phenotype with a PP2A double deletion mutant and a PP2A temperature-sensitive mutant. The PP2A-deficient yeast strain also showed accelerated progression through the G1 phase. In addition, both PP2A and ypa1Delta mutants show similar aberrant bud morphology. This would support the notion that YPA1 may act as a positive regulator of PP2A in vivo.

Catalytically inactive protein phosphatase 2A can bind to polyomavirus middle tumor antigen and support complex formation with pp60(c-src)

Interaction between the heterodimeric form of protein phosphatase 2A (PP2A) and polyomavirus middle T antigen (MT) is required for the subsequent assembly of a transformation-competent MT complex. To investigate the role of PP2A catalytic activity in MT complex formation, we undertook a mutational analysis of the PP2A 36-kDa catalytic C subunit. Several residues likely to be involved in the dephosphorylation mechanism were identified and mutated. The resultant catalytically inactive C subunit mutants were then analyzed for their ability to associate with a cellular (B subunit) or a viral (MT) B-type subunit. Strikingly, while all of the inactive mutants were severely impaired in their interaction with B subunit, most of these mutants formed complexes with polyomavirus MT. These findings indicate a potential role for these catalytically important residues in complex formation with cellular B subunit, but not in complex formation with MT. Transformation-competent MT is known to associate with, and modulate the activity of, several cellular proteins, including pp60(c-src) family kinases. To determine whether association of MT with an active PP2A A-C heterodimer is necessary for subsequent association with pp60(c-src), catalytically inactive C subunits were examined for their ability to form complexes containing pp60(c-src) in MT-expressing cells. Two catalytically inactive C subunit mutants that efficiently formed complexes with MT also formed complexes that included an active pp60(c-src) kinase, demonstrating that PP2A activity is not essential in cis in MT complexes for subsequent pp60(c-src) association.

Fluorescein monophosphates as fluorogenic substrates for protein tyrosine phosphatases

A series of novel fluorescein monophosphates aimed as substrates for protein tyrosine phosphatases (PTPs) were synthesized and evaluated against fluorescein diphosphate (FDP), the currently used fluorescent substrate for PTPs. In contrast to FDP, which is dephosphorylated to monophosphate and then to fluorescein in a sequential reaction, these monophosphates are dephosphorylated in a single step. This eliminates the complication in assaying PTPs due to the cleavage of the second phosphate group. The kinetic studies of these substrates with PTPs were performed and Michaelis-Menten parameters were obtained. These designed substrates have Km 0.03-0. 35 mM, kcat/Km of 3-100 mM-1 s-1 with CD45 and PTP1B. The results showed that the substrates with negative charge groups on the fluorescein have higher affinities for PTP1B, which are consistent with other observations. In this series, fluorescein monosulfate monophosphate (FMSP) was the best substrate observed. Since FMSP showed large increases in both absorption and fluorescence upon dephosphorylation by PTPs at pH>6.0, it is one of the most sensitive, stable and high affinity substrates reported for PTPs.

Mapping the subsite preferences of protein tyrosine phosphatase PTP-1B using combinatorial chemistry approaches

Protein tyrosine phosphatases (PTPases) are important regulators of signal transduction systems, but the specificity of their action is largely unexplored. We have approached this problem by attempting to map the subsite preferences of these enzymes using combinatorial chemistry approaches. Protein-tyrosine peptidomimetics containing nonhydrolyzable phosphotyrosine analogues bind to PTPases with high affinity and act as competitive inhibitors of phosphatase activity. Human PTP-1B, a PTPase implicated to play an important role in the regulation of growth factor signal transduction pathways, was used to screen a synthetic combinatorial library containing malonyltyrosine as a phosphotyrosine mimic. Using two cross-validating combinatorial chemistry screening approaches, one using an iterative method and the other employing library affinity selection-mass spectrometric detection, peptides with high affinity for PTP-1B were identified and subsite preferences were detailed by quantitatively comparing residues of different character. Consistent with previous observations, acidic residues were preferred in subsites X-3 and X-2. In contrast, aromatic substitutions were clearly preferred at the X-1 subsite. This information supports the concept that this class of enzymes may have high substrate specificity as dictated by the sequence proximal to the phosphorylation site. The results are discussed with regards to the use of combinatorial techniques in order to elucidate the interplay between enzyme subsites.

Potent non-peptidyl inhibitors of protein tyrosine phosphatase 1B

The development of inhibitors of protein tyrosine phosphatases (PTPs) has recently been the subject of intensive investigation due to their potential as chemotherapeutics and as tools for studying signal transduction pathways. Here we report the evaluation of a variety of small molecule, non-peptidyl inhibitors of protein tyrosine phosphatase 1B (PTP1B), bearing the alpha, alpha-difluoromethylenephosphonic acid (DFMP) group, a non-hydrolyzable phosphate mimetic. A series of phenyl derivatives bearing a single DFMP group were initially surveyed. In general, these were not significantly more potent inhibitors than the parent compound, alpha, alpha-difluorobenzylphosphonic acid, with the exception being the meta-phenyl substituted species which decreased the IC50 by approximately 17-fold relative to alpha, alpha-difluorobenzylphosphonic acid. However, certain compounds bearing two DFMP moieties were very potent inhibitors. Some of these are among the most potent small molecule inhibitors of any PTP reported to date with the best one exhibiting a Ki of 1.5 microM. The structural basis for these results are discussed. One of the bis-DFMP inhibitors was examined in detail and it was found that the fluorines were essential for potent inhibition. Inhibition was independent of pH between pH 5.5-7.2 suggesting that both the mono and dianionic forms of the individual DFMP groups bind equally well. The trends observed in the inhibitory potency of these compounds with PTP1B were very similar to the trends observed by other workers on the K(m)'s of the analogous phenylphosphate substrates with rat PTP1. This indicates that studies of non-peptidyl substrates with rat PTP1 can be used as a guide for the development of human PTP1B inhibitors.

Src homology-2 domains protect phosphotyrosyl residues against enzymatic dephosphorylation

The SH2 domain of c-Fgr (class 1A) has been expressed in E. coli as GST fusion protein and tested for its ability to prevent the dephosphorylation of a variety of phosphotyrosyl (poly)peptides by three distinct protein tyrosine phosphatases (TC-PTPase, YOP, and Low Mr PTPase). Dephosphorylation of HS1 protein and of a derived phosphopeptide, HS1 (388-402), exhibiting the motif selected by class 1A SH2 domains is inhibited in a dose dependent manner with full inhibition promoted by a 2- to 3-molar excess of GST/SH2 domain irrespective of either the nature or the amount of phosphatase used. The IC50 values for inhibition of these and other phosphotyrosyl substrates roughly correlates with their expected affinity for class 1A SH2 domain. Inhibition is partially reversed by the addition of D-myo-inositol 1,4,5-triphosphate, which competes for the binding to the SH2 domains. Our data on one side show that additional mechanism(s) besides mere competition must assist PTPases to dissociate SH2-PTyr complexes and on the other suggest a role for SH2 domains in protecting phosphotyrosyl residues from premature dephosphorylation.

Naphthalenebis[alpha,alpha-difluoromethylenephosphonates] as potent inhibitors of protein tyrosine phosphatases

A series of naphthalenebis(difluoromethylenephosphonates) were prepared and compared to their monosubstituted counterparts as inhibitors of the protein phosphatases, PTP1B, CD45 and PP2A. In general, the bissubstituted compounds were better inhibitors than the mono derivatives and some of these are among the most potent, nonpeptidyl inhibitors of protein tyrosine phosphatases, PTP1B and CD45, reported to date.

Structure and function of the low Mr phosphotyrosine protein phosphatases

Phosphotyrosine protein phosphatases (PTPases) catalyse the hydrolysis of phosphotyrosine residues in proteins and are hence implicated in the complex mechanism of the control of cell proliferation and differentiation. The low Mr PTPases are a group of soluble PTPases displaying a reduced molecular mass; in addition, a group of low molecular mass dual specificity (ds)PTPases which hydrolyse phosphotyrosine and phosphoserine/threonine residues in proteins are known. The enzymes belonging to the two groups are unrelated to each other and to other PTPase classes except for the presence of a CXXXXXRS/T sequence motif containing some of the catalytic residues (active site signature) and for the common catalytic mechanism, clearly indicating convergent evolution. The low Mr PTPases have a long evolutionary history since microbial (prokaryotic and eukaryotic) counterparts of both tyrosine-specific and dsPTPases have been described. Despite the relevant number of data reported on the structural and catalytic features of a number of low Mr PTPases, only limited information is presently available on the substrate specificity and the true biological roles of these enzymes, in prokaryotic, yeast and eukaryotic cells.

Sequence specificity of C-terminal Src kinase (CSK)--a comparison with Src-related kinases c-Fgr and Lyn

An eicosapeptide encompassing the C-terminal tail of c-Src (Tyr527) which is conserved in most Src-related protein kinases, is phosphorylated by C-terminal Src kinase (CSK) and by the two Src-related protein kinases c-Fgr and Lyn, with similar kinetic constants. Two related peptides reproducing the C-terminal segments of c-Src mutants defective in CSK phosphorylation [MacAuley, A., Okada, M., Nada, S., Nakagawa, H. & Cooper, J. A. (1993) Oncogene 8, 117-124] AFLEDSCTGTEPLYQRGENL (mutant number 28) and AFLEDNFTGTKPQYHPGENL (mutant number 29), proved a better and a much worse substrates, respectively than the wild-type peptide, with either CSK or the two Src kinases. By changing individual residues in the best peptide substrate, it was shown that the main element responsible for its improved phosphorylation is leucine at position -1 (instead of glutamine), while lysine at position -3 (instead of glutamate) has a detrimental effect, possibly accounting for the negligible phosphorylation of peptide derived from mutant number 29. By contrast to most peptide substrates, including the Src C-terminal peptides, which exhibit relatively high K(m) values, a polyoma-virus-middle-T-antigen-(mT)-derived peptide with tyrosine embedded in a highly hydrophobic sequence (EEEPQFEEIPIYLELLP) exhibits with CSK a quite low K(m) value (63 microM). Consistent with this, the optimal sequence selected by CSK in an oriented peptide library is XXXIYMFFF. This is different from sequences selected by Lyn (DEEIYEELX) and c-Fgr (XEEIYGIFF), although they all share a high selection for a hydrophobic residue at n-1. In sharp contrast, TPKIIB/p38syk, related to the catalytic domain of p72syk, selects acidic residues at nearly all positions, n-1 included. These data support the notion that the features determining the specific phosphorylation of the C-terminal tyrosine residue of Src do not reside in the primary structure surrounding the target tyrosine. They also show that this site does not entirely fulfil the optimal consensus sequence recognized by CSK, disclosing the possibility that as yet unrecognized CSK targets structurally unrelated to the C-terminal tyrosine residue of Src kinases may exist.