B cell receptor-associated protein alpha4 displays rapamycin-sensitive binding directly to the catalytic subunit of protein phosphatase 2A

Recently, TAP42 was isolated as a high copy suppressor of sit4-, a yeast phosphatase related to protein phosphatase 2A (PP2A). TAP42 is related to the murine alpha4 protein, which was discovered independently by its association with Ig-alpha in the B cell receptor complex. Herein we show that a glutathione S-transferase (GST)-alpha4 fusion protein bound the catalytic subunit (C) of human PP2A from monomeric or multimeric preparations of PP2A in a "pull-down" assay. In an overlay assay, the GST-alpha4 protein bound to the phosphorylated and unphosphorylated forms of C that were separated in two-dimensional gels and immobilized on filters. The results show direct and exclusive binding of alpha4 to C. This is unusual because all known regulatory B subunits, or tumor virus antigens, bind stably only to the AC dimer of PP2A. The alpha4-C form of PP2A had an increased activity ratio compared with the AC form of PP2A when myelin basic protein phosphorylated by mitogen-activated protein kinase and phosphorylase a were used as substrates. Recombinant alpha4 cleaved from GST was phosphorylated by p56(lck) tyrosine kinase and protein kinase C. A FLAG-tagged alpha4 expressed in COS7 cells was recovered as a protein containing phosphoserine and coimmunoprecipitated with the C but not the A subunit of PP2A. Treatment of cells with rapamycin prevented the association of PP2A with FLAG-alpha4. The results reveal a novel heterodimer alpha4-C form of PP2A that may be involved in rapamycin-sensitive signaling pathways in mammalian cells.

Differential localization of myosin and myosin phosphatase subunits in smooth muscle cells and migrating fibroblasts

Myosin II light chains (MLC20) are phosphorylated by a Ca2+/calmodulin-activated kinase and dephosphorylated by a phosphatase that has been purified as a trimer containing the delta isoform of type 1 catalytic subunit (PP1C delta), a myosin-binding 130-kDa subunit (M130) and a 20-kDa subunit. The distribution of M130 and PP1C as well as myosin II was examined in smooth muscle cells and fibroblasts by immunofluorescence microscopy and immunoblotting after differential extraction. Myosin and M130 colocalized with actin stress fibers in permeabilized cells. However, in nonpermeabilized cells the staining for myosin and M130 was different, with myosin mostly at the periphery of the cell and the M130 appearing diffusely throughout the cytoplasm. Accordingly, most M130 was recovered in a soluble fraction during permeabilization of cells, but the conditions used affected the solubility of both M130 and myosin. The PP1C alpha isoform colocalized with M130 and also was in the nucleus, whereas the PP1C delta isoform was localized prominently in the nucleus and in focal adhesions. In migrating cells, M130 concentrated in the tailing edge and was depleted from the leading half of the cell, where double staining showed myosin II was present. Because the tailing edge of migrating cells is known to contain phosphorylated myosin, inhibition of myosin LC20 phosphatase, probably by phosphorylation of the M130 subunit, may be required for cell migration.

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 a protein tyrosine phosphatase and inactivation of Raf-1 by somatostatin

Human somatostatin receptor 3 ('hsstr3') was transiently expressed in NIH 3T3 cells stably transformed with Ha-Ras (G12V). Somatostatin activated a protein tyrosine phosphatase and inactivated the constitutively active, membrane-associated form of the Raf-1 serine kinase present in these cells in vivo and in vitro.

Inactivation of raf-1 by a protein-tyrosine phosphatase stimulated by GTP and reconstituted by Galphai/o subunits

A membrane-associated form of Raf-1 in v-Ras transformed NIH 3T3 cells can be inactivated by protein phosphatases regulated by GTP. Herein, a distinct protein-tyrosine phosphatase (PTPase) in membrane preparations from v-Ras transformed NIH 3T3 cells was found to be activated by guanyl-5'-yl imidodiphosphate (GMPPNP) and was identified as an effector for pertussis toxin (PTx)-sensitive G-protein alpha subunits. PTPase activation was blocked by prior treatment of cells with PTx. PTPase activation by GTP, but not GMPPNP, was transient. A GMPPNP-stimulated PTPase (PTPase-G) co-purified with Galphai/o subunits during Superose 6 and Mono Q chromatography. PTPase-G activity in Superose 6 fractions from GDP-treated membranes was reconstituted by activated Galphai/o, but not G beta gamma, subunits. PTPase-G may contribute to GMPPNP-stimulated inactivation of Raf-1 in v-Ras cell membranes because Raf-1 inactivation was PTx-sensitive and PTPase-G inactivated exogenous Raf-1.

Flicking the switches: phosphorylation of serine/threonine protein phosphatases

Signal transduction involves protein phosphorylation and dephosphorylation. To produce both substantial and transient changes requires coordinated and reciprocal regulation of kinases and phosphatases. One mechanism to accomplish this is phosphorylation, and there are now reports of phosphorylation of all the major types of protein Ser/Thr phosphatases. Phosphorylation of type-1 and type-2A phosphatases occurs within characteristic C-terminal sequences and results in the loss of phosphatase activity. The phosphatases catalyse intramolecular dephosphorylation, with a restoration of activity. This property probably accounts for the apparent constitutive activity of phosphatases in cell and tissue extracts. Phosphorylation of phosphatases is a way to flick the activity off and on in cells during the growth cycle and in response to stimuli.

Tyrosine phosphorylation of phosphatase inhibitor 2

Inhibitor 2 is a heat-stable protein that complexes with the catalytic subunit of type-1 protein phosphatase. The reversible phosphorylation of Thr 72 of the inhibitor in this complex has been shown to regulate phosphatase activity. Here we show that inhibitor 2 can also be phosphorylated on tyrosine residues. Inhibitor 2 was 32P-labeled by the insulin receptor kinase in vitro, in the presence of polylysine. Phosphorylation of inhibitor 2 was accompanied by decreased electrophoretic mobility. Dephosphorylation of inhibitor 2 by tyrosine phosphatase 1B, restored normal electrophoretic mobility. Phosphotyrosine in inhibitor 2 was detected by immunoblotting with antiphosphotyrosine antibodies and phosphoamino acid analysis. In addition, following tryptic digestion, one predominant phosphopeptide was recovered at the anode. The ability of inhibitor 2 to inhibit type-1 phosphatase activity was diminished with increasing phosphorylation up to a stoichiometry of 1 mole phosphate incorporated/mole of inhibitor 2, where inhibitory activity was completely lost. These data demonstrate that inhibitor 2 can be phosphorylated on tyrosine residues by the insulin receptor kinase, resulting in a molecule with decreased ability to inhibit type-1 phosphatase activity.

Concurrent purification of type-1 and type-2A protein phosphatase catalytic subunits

We describe a simple purification scheme for the active catalytic subunits of both protein phosphatases type-1 and type-2A. The advantage of this procedure over others is that it produces intact proteins with high yield and specific activity and is suitable for either kilograms of tissue or a dish of cells. Type-1 and type-2A phosphatases from rabbit skeletal muscle were resolved on polylysine-agarose and subsequently obtained in homogeneous form. The phosphatases demonstrated characteristic properties. The phosphatase-1 catalytic subunit was inhibited by inhibitor-2 and phosphoinhibitor-1 whereas phosphatase-2A was not. The phosphatase activities were stable for years at -20 degrees C when stored in the presence of Mg2+ and glycerol. Based on the predicted sequence of the carboxyl terminus of each phosphatase, antibodies specific for phosphatases-1 and -2A were produced in rabbits using synthetic peptides as immunogens. Immunoblots showed complete specificity of these antibodies for their respective phosphatases and confirmed that the purified phosphatases has intact carboxyl termini. The purified catalytic subunits and antibodies will be useful for examining the regulation and the physiological roles of these protein phosphatases in cellular physiology.

Regulation of NMDA receptors in cultured hippocampal neurons by protein phosphatases 1 and 2A

Phosphorylation of glutamate receptors is probably an important mechanism for modulating excitatory transmission. However, there is little direct evidence to indicate which protein phosphatases can dephosphorylate glutamate or other ligand-gated channels, although it is known that protein phosphatases 1 and 2A play a major part in modulating voltage and second-messenger-gated channels. Here we report that in cultured hippocampal neurons, the N-methyl-D-aspartate (NMDA) receptor can be regulated by endogenous and exogenous serine/threonine protein phosphatases. Phosphatase inhibitors enhanced NMDA currents recorded using the perforated patch technique or in cell-attached patches, whereas protein phosphatases 1 or 2A decreased the open probability of these channels in inside-out patches.

Identification of protein phosphatase 2A as the primary target for microcystin-LR in rat liver homogenates

The liver-specific toxin microcystin-LR (MC-LR) is a potent inhibitor of type 1 (PP1) and type 2A (PP2A) protein phosphatases. A tritiated form of the toxin, [3H]dihydromicrocystin-LR ([3H]DMC-LR), was used to identify target proteins in cellular fractions prepared from rat liver homogenates. About 80% of the [3H]DMC-LR bound to proteins was in the cytosolic fraction, which contained essentially all of the PP2A. In contrast, much of the PP1 was found in particulate fractions, each with only a few percent of the total protein-bound [3]HDMC-LR. Protein-bound [3H]DMC-LR in the cytosol co-eluted with PP2A, but not with PP-1 from a DEAE-Sepharose column. Native forms of liver cytoplasmic PP2A and PP1 separated by aminohexyl-Sepharose adsorption showed similar sensitivity to inhibition by MC-LR, and bound [3H]DMC-LR proportional to the amount of phosphatase activity. The results indicate that [3H]DMC-LR can bind both PP2A and PP1 in the liver which must be important for microcystin-induced toxicity, but is recovered mainly bound to PP2A in the cytosol.

Tyrosine phosphorylation of protein phosphatase 2A in response to growth stimulation and v-src transformation of fibroblasts

The catalytic subunit of protein phosphatase 2A (PP2A) is inactivated by in vitro phosphorylation of Tyr307 by receptor and nonreceptor protein tyrosine kinases (Chen, J., Martin, B. L., and Brautigan, D. L. (1992) Science 257, 1261-1264). Here we show the phosphorylation of PP2A in cells under different growth conditions. In lysates of nontransformed murine 10T1/2 fibroblasts, there were two forms of PP2A at 36 kDa detected after two-dimensional gel electrophoresis and immunoblotting with anti-PP2A peptide antibody. These two forms exactly comigrated with unphosphorylated purified PP2A and the PP2A 32P-labeled by in vitro phosphorylation with p60v-src kinase. The phosphorylated form of PP2A recovered from red blood cells or produced by in vitro phosphorylation was eliminated by incubation with tyrosine-specific phosphatase (PTP1B). Transformation of 10T1/2 cells by expression of p60v-src resulted in most of the PP2A in the cells being converted to a phosphorylated form that was reactive with anti-phosphotyrosine antibody. Serum starvation of cells reduced the amount of phosphorylated PP2A, whereas serum stimulation of quiescent cells caused an increase to the same relative amount of phosphorylated PP2A as in src-transformed cells. Addition of epidermal growth factor to quiescent NeoR cells (10T1/2 fibroblasts overexpressing epidermal growth factor receptors) temporarily increased the level of phosphorylation of PP2A, with a peak at 5-15 min and a return to basal level within 60 min. The results show that PP2A is phosphorylated in intact cells, and the extent of this modification is increased by growth factors or cell transformation, providing evidence for a physiological mechanism of PP2A regulation.

Serine phosphorylation of protein tyrosine phosphatase (PTP1B) in HeLa cells in response to analogues of cAMP or diacylglycerol plus okadaic acid

The major intracellular protein tyrosine phosphatase (PTP1B) is a 50kDa protein, localized to the endoplasmic reticulum. This PTP is recovered in the particulate fraction of mammalian cells and can be solubilized as a complex of 150 kDa by extraction with non-ionic detergents. Previous work from this laboratory implicated phosphorylation of serine/threonine residues in the regulation of this PTP. Activity was several-fold higher in cells treated with activators of cAMP-dependent or Ca2+/phospholipid-dependent protein kinases or inhibitors of protein phosphatase 2A. Here we show that these treatments result in more than an 8-fold increase in the phosphorylation of the 50 kDa PTP catalytic subunit within the 150kDa form of the phosphatase in HeLa cells. The phosphorylation occurred exclusively on serine residues, and the same tryptic and cyanogen bromide 32P-phosphopeptides were recovered in the PTP from control and stimulated cells. Either multiple kinases phosphorylate a common site in the PTP1B, or a single kinase is activated 'downstream' of cAMP- and Ca2+/phospholipid-dependent kinases. The results indicate that phosphorylation of a serine residue in the segment 283-364, probably serine 352 in the sequence Lys-Gly-Ser-Pro-Leu, occurs in response to cell stimulation. Phosphorylation in this region of PTP1B, between the N-terminal catalytic domain and the C-terminal membrane localization segment, is proposed to regulate phosphatase activity.

Cytoskeletal integrity in interphase cells requires protein phosphatase activity

Phosphorylation by protein kinases has been established as a key factor in the regulation of cytoskeletal structure. However, little is known about the role of protein phosphatases in cytoskeletal regulation. To assess the possible functions of protein phosphatases in this respect, we studied the effects of the phosphatase inhibitors calyculin A, okadaic acid, and dinophysistoxin 1 (35-methylokadaic acid) on BHK-21 fibroblasts. Within minutes of incubation with these inhibitors, changes are seen in the structural organization of intermediate filaments, followed by a loss of microtubules, as assayed by immunofluorescence. These changes in cytoskeletal structure are accompanied by a rapid and selective increase in vimentin phosphorylation on interphase-specific sites, and they are fully reversible after removal of calyculin A. The results indicate that there is a rapid phosphate turnover on cytoskeletal intermediate filaments and further suggest that protein phosphatases are essential for the maintenance and structural integrity of two major cytoskeletal components.

Protein phosphatases modulate the apparent agonist affinity of the light-regulated ion channel in retinal rods

Ion channels directly activated by cGMP mediate the light response in retinal rods. Several components of the enzyme cascade controlling cGMP concentration are regulated, but there are no accepted mechanisms for modulation of the response of the channel to cGMP. Here we report evidence that in excised patches an endogenous protein phosphatase converts the channel from a state with low cGMP sensitivity to a state with almost 3 orders of magnitude higher sensitivity in the predicted physiological range of cGMP concentration. The action of this endogenous phosphatase was blocked by specific serine/threonine phosphatase inhibitors (microcystin-LR, okadaic acid, and calyculin A). An increase in apparent agonist affinity also was produced by addition of purified protein phosphatase 1. In contrast, protein phosphatase 2A decreased apparent agonist affinity, suggesting that two phosphorylation sites may regulate the agonist sensitivity of the channel in a reciprocal manner. This regulation may be involved in fine-tuning the light response or in light or dark adaptation.

Regulation of protein serine-threonine phosphatase type-2A by tyrosine phosphorylation

Extracellular signals that promote cell growth activate cascades of protein kinases. The kinases are dephosphorylated and deactivated by a single type-2A protein phosphatase. The catalytic subunit of type-2A protein phosphatase was phosphorylated by tyrosine-specific protein kinases. Phosphorylation was enhanced in the presence of the phosphatase inhibitor okadaic acid, consistent with an autodephosphorylation reaction. More than 90% of the activity of phosphatase 2A was lost when thioadenosine triphosphate was used to produce a thiophosphorylated protein resistant to autodephosphorylation. Phosphorylation in vitro occurred exclusively on Tyr307. Phosphorylation was catalyzed by p60v-src, p56lck, epidermal growth factor receptors, and insulin receptors. Transient deactivation of phosphatase 2A might enhance transmission of cellular signals through kinase cascades within cells.

Raf-1 activates MAP kinase-kinase

The normal cellular homologue of the acutely transforming oncogene v-raf is c-raf-1, which encodes a serine/threonine protein kinase that is activated by many extracellular stimuli. The physiological substrates of the protein c-Raf-1 are unknown. The mitogen-activated protein (MAP) kinases Erk1 and 2 are also activated by mitogens through phosphorylation of Erk tyrosine and threonine residues catalysed by a protein kinase of relative molecular mass 50,000, MAP kinase-kinase (MAPK-K). Here we report that MAPK-K as well as Erk1 and 2 are constitutively active in v-raf-transformed cells. MAPK-K partially purified from v-raf-transformed cells or from mitogen-treated cells can be deactivated by phosphatase 2A. c-Raf-1 purified after mitogen stimulation can reactivate the phosphatase 2A-inactivated MAPK-K over 30-fold in vitro. c-Raf-1 reactivation of MAPK-K coincides with the selective phosphorylation at serine/threonine residues of a polypeptide with M(r) 50,000 which coelutes precisely on cation-exchange chromatography with the MAPK-K activatable by c-Raf-1. These results indicate that c-Raf-1 is an immediate upstream activator of MAPK-K in vivo. To our knowledge, MAPK-K is the first physiological substrate of the c-raf-1 protooncogene product to be identified.

Differentiation-induced changes in protein-tyrosine phosphatase activity and commensurate expression of CD45 in human leukemia cell lines

Pharmacologic differentiation of the promyelocytic leukemia HL60 is associated with an increase in cellular tyrosine phosphatase activity. We asked (a) if this increase might, at least in part, be due to changes in a transmembranous protein-tyrosine phosphatase, CD45; and (b) if CD45 changes similarly in other differentiating leukemias. Differentiation of HL60, several chronic myelogenous leukemias, a monocytic leukemia (THP-1), and a monoblastoid leukemia (U-937) could be induced by phorbol ester, 1,25-dihydroxy vitamin D3, dimethyl sulfoxide, or cyclic AMP analogues. This differentiation was associated with a marked increase in (a) total cellular tyrosine phosphatase activity (2-4-fold as measured by the ability to dephosphorylate a tyrosine-phosphorylated peptide); (b) CD45-specific tyrosine phosphatase activity (2-4-fold); (c) CD45 cell surface expression by flow cytometry (2-5-fold); (d) synthesis of both exon B-dependent M(r) 205,000 and exon ABC- M(r) 185,000 CD45 proteins, as revealed by immunoprecipitation with antisera specific for CD45 isoforms. Both isoforms have enhanced electrophoretic mobility when isolated from the differentiated cells. This enhanced mobility did not appear to be due to decreased stoichiometry of CD45 phosphorylation on serine/threonine residues. Interestingly, 12-O-tetradecanoylphorbol-13-acetate transiently reduced CD45 protein-tyrosine phosphatase activity in the chronic myelogenous leukemia cell RWLeu4 without altering the CD45 amount (as measured by cell surface immunofluorescence). Modulation of CD45 tyrosine phosphatase activity (and protein levels) may play a role in differentiation or in maintaining cells in a nonproliferative state or may represent a phenotypic marker of differentiation.

Ca(2+)-calmodulin-dependent protein kinases Ia and Ib from rat brain. II. Enzymatic characteristics and regulation of activities by phosphorylation and dephosphorylation

In addition to physical properties (DeRemer, M. F., Saeli, R. J., and Edelman, A. M. (1992) J. Biol. Chem. 267, 13460-13465), enzymatic and regulatory characteristics indicate that calmodulin (CaM) kinase Ia and CaM kinase Ib are distinct entities. The Km values for ATP and site 1 peptide were similar between the two kinases, however, CaM kinase Ib is approximately 20-fold more sensitive to CaM than is CaM kinase Ia. The kinases also displayed differential sensitivities to divalent metal ions. For both kinases, site 1 peptide, synapsin I, and syntide-2 were highly preferred substrates relative to others tested. A 72-kDa protein from a heat-treated extract of rat pancreas was phosphorylated by CaM kinase Ib but not by CaM kinase Ia. CaM kinase Ia activity displayed a pronounced lag in its time course suggesting enzyme activation over time. Preincubation of CaM kinase Ia in the combined presence of Ca(2+)-CaM and MgATP led to a time-dependent increase in its site 1 peptide kinase activity of up to 15-fold. The extent of activation of CaM kinase Ia correlated with the extent of autophosphorylation. The enzyme retained full Ca(2+)-CaM dependence in the activated state which was rapidly reversible by treatment with protein phosphatase 2A catalytic subunit. Thus, the activation of CaM kinase Ia is a result of its Ca(2+)-CaM-dependent autophosphorylation. CaM kinase Ib was not activated by preincubation under autophosphorylating conditions yet lost approximately 90% of its activity toward either an exogenous substrate (site 1 peptide) or itself (autophosphorylation) after incubation with protein phosphatase 2A catalytic subunit. The deactivated state was not reversed by subsequent incubations under autophosphorylating conditions. Thus, CaM kinase Ib activity is dependent upon phosphorylation by a regulating kinase(s) which is resolved from CaM kinase Ib during purification of the latter.

Regulation of an epithelial chloride channel by direct phosphorylation and dephosphorylation

A native chloride channel in Necturus gallbladder epithelial cells is opened by a theophylline-induced rise in cellular cyclic AMP and is closed by removal of theophylline or by addition of specific antibody; however, it does not close if okadaic acid, an inhibitor of protein phosphatases 1 and 2A, is added. The purified channel reconstituted into lipid bilayers closes upon the addition of protein phosphatase 2A and is reopened by the addition of Mg-ATP and the catalytic subunit of cyclic AMP-dependent protein kinase. These results indicate that the channel protein is purified in a phosphorylated state and that its functional characteristics are at least partly controlled by direct phosphorylation and dephosphorylation.