Localization of protein Ser/Thr phosphatase 5 in rat brain

Protein phosphatase 5 is a recently discovered Ser/Thr phosphatase that is structurally related to calcineurin and protein phosphatases 1 and 2. Northern blot and in situ hybridization studies have shown that protein phosphatase 5 mRNA is present at high levels in brain and is localized to discrete regions. In the present study, we used immunocytochemistry and immunoblot analyses to examine the regional and subcellular distribution of this enzyme in brain. Our work demonstrates that protein phosphatase 5 is widely expressed throughout brain, but is not uniformly distributed. The most intense staining occurred in neurons of the cerebellum, cerebral cortex, and the supraoptic nucleus of the hypothalamus. Other areas also contained immunoreactive cell bodies, including the globus pallidus, hippocampus, thalamus, lateral preoptic area of the hypothalamus, substantia nigra and other brainstem nuclei. Staining in these cells was observed primarily in perikarya and proximal processes.

The tetratricopeptide repeat domain and a C-terminal region control the activity of Ser/Thr protein phosphatase 5

Protein Ser/Thr phosphatase 5 is a 58-kDa protein containing a catalytic domain structurally related to the catalytic subunits of protein phosphatases 1, 2A, and 2B and an extended N-terminal domain with three tetratricopeptide repeats. The activity of this enzyme is stimulated 4-14-fold in vitro by polyunsaturated fatty acids and anionic phospholipids. The structural basis for lipid activation of protein phosphatase 5 was examined by limited proteolysis and site-directed mutagenesis. Trypsinolysis removed the tetratricopeptide repeat domain and increased activity to approximately half that of lipid-stimulated, full-length enzyme. Subtilisin removed the tetratricopeptide repeat domain and 10 residues from the C terminus, creating a catalytic fragment with activity that was equal to or greater than that of lipid-stimulated, full-length enzyme. Catalytic fragments generated by proteolysis were no longer stimulated by lipid, and degradation of the tetratricopeptide repeat domain was decreased by association with lipid. A truncated mutant missing 13 C-terminal residues was also insensitive to lipid and was as active as full-length, lipid-stimulated enzyme. These results suggest that the C-terminal and N-terminal domain act in a coordinated manner to suppress the activity of protein phosphatase 5 and mediate its activation by lipid. These regions may be targets for the regulation of protein phosphatase 5 activity in vivo.

Purification of a fatty acid-stimulated protein-serine/threonine phosphatase from bovine brain and its identification as a homolog of protein phosphatase 5

An arachidonic acid-stimulated Ser/Thr phosphatase activity was detected in soluble extracts prepared from rat pituitary clonal GH4C1 cells, rat or bovine brain, and bovine heart. The enzyme activity was purified to homogeneity from bovine brain as a monomer with a Mr of 63,000 and a specific activity of 32 nmol of Pi released per min/mg of protein when assayed in the presence of 10 microM phosphocasein in the absence of lipid. Arachidonic acid stimulated activity 4-14-fold, with half-maximal stimulation at 50-100 microM, when assayed in the presence of a variety of phosphosubstrates including casein, reduced carboxamidomethylated and maleylated lysozyme, myelin basic protein, and histone. Oleic acid, linoleic acid, and palmitoleic acid also stimulated activity; however, saturated fatty acids and alcohol or methyl ester derivatives of fatty acids did not significantly affect activity. The lipid-stimulated phosphatase was identified as the bovine equivalent of protein phosphatase 5 or a closely related homolog by sequence analysis of proteolytic fragments generated from the purified enzyme. When recombinant rat protein phosphatase 5 was expressed as a cleavable glutathione S-transferase fusion protein, the affinity-purified thrombin-cleaved enzyme exhibited a specific activity and sensitivity to arachidonic acid similar to those of the purified bovine brain enzyme. These results suggest that protein phosphatase 5 may be regulated in vivo by a lipid second messenger or another endogenous activator.

Depolarization of rat brain synaptosomes increases phosphorylation of voltage-sensitive sodium channels

Depolarization of rat brain synaptosomes causes an increase in phosphorylation of serine residues 573, 610, 623, and 687 on voltage-sensitive sodium channels. Although these sites have been shown to be phosphorylated by cAMP-dependent protein kinase in vitro and in situ, the depolarization-induced increase in their state of phosphorylation is not due to increased cAMP-dependent protein kinase activity, but requires calcium influx and protein kinase C. Since phosphorylation at this cluster of sites inhibits sodium current and would decrease neuronal excitability, this may be an important negative feedback mechanism whereby calcium influx during prolonged or repetitive depolarization can attenuate neuronal excitability and prevent further calcium accumulation. Phosphorylation of purified channels by protein kinase C decreases dephosphorylation of cAMP-dependent phosphorylation sites by purified calcineurin or protein phosphatase 2A. This suggests that one mechanism by which protein kinase C may increase phosphorylation of cAMP-dependent phosphorylation sites in sodium channels is to inhibit their dephosphorylation. This represents an important new mechanism for convergent regulation of an ion channel by two distinct signal transduction pathways.

The dimeric and catalytic subunit forms of protein phosphatase 2A from rat brain are stimulated by C2-ceramide

Protein phosphatase 2A (PP-2A) is a heterotrimeric enzyme consisting of a catalytic (C) subunit and A and B regulatory subunits. PP-2A is activated by ceramide in vitro suggesting that PP-2A may be a target of this putative second messenger in vivo (Dobrowsky, R. T., Kamibayashi, C., Mumby, M. C., and Hannun, Y. A. (1993) J. Biol. Chem. 268, 15523-15530). In this study, sensitivity to ceramide was only observed when the B subunit was present, suggesting that the B subunit was required for ceramide activation. Here we show that dimeric PP-2A, produced from trimeric PP-2A by heparin-agarose-induced dissociation of the B subunit and isolated by preparative native electrophoresis, is activated by ceramide. The catalytic subunit of PP-2A, produced from trimeric PP-2A by freezing and thawing in the presence of 0.2 M beta-mercaptoethanol and isolated by gel filtration, is also activated by ceramide. The trimeric and catalytic subunit forms of PP-2A exhibit a similar dose dependence of activation by ceramide, and are stimulated to a similar extent at ceramide concentrations yielding maximal activation. These findings indicate that neither the A nor the B subunit is required for ceramide stimulation of PP-2A. Together, these results demonstrate that the catalytic subunit contains a ceramide binding site and suggest that efforts to understand the mechanism of activation of PP-2A by ceramide should be focused on this subunit. The discovery that the catalytic subunit contains a ceramide binding site raises the possibility that other members of this serine/threonine phosphatase gene family may contain lipid binding sites and be regulated by ceramide or other lipid second messengers.

Identification of soluble protein phosphatases that dephosphorylate voltage-sensitive sodium channels in rat brain

Rat brain sodium channels are phosphorylated at multiple serine residues by cAMP-dependent protein kinase. We have identified soluble rat brain phosphatases that dephosphorylate purified sodium channels. Five separable forms of sodium channel phosphatase activity were observed. Three forms (two, approximately 234 kDa and one, 192 kDa) are identical or related to phosphatase 2A, since they were 85-100% inhibited by 10 nM okadaic acid and contained a 36-kDa polypeptide recognized by a monoclonal antibody directed against the catalytic subunit of phosphatase 2A. Immunoblots performed using antibodies specific for isoforms of the B subunit of phosphatase 2A indicate that the two major peaks of phosphatase 2A-like activity, A1 and B1, are enriched in either B' or B alpha. The remaining two activities (approximately 100 kDa each) probably represent calcineurin. Each was relatively insensitive to okadaic acid, was active only in the presence of CaCl2 and calmodulin, and contained a 19-kDa polypeptide recognized by a monoclonal antibody raised against the B subunit of calcinerurin. Treatment of synaptosomes with okadaic acid to inhibit phosphatase 2A or cyclosporin A to inhibit calcineurin increased apparent phosphorylation of sodium channels at cAMP-dependent phosphorylation sites, as assayed by back phosphorylation. These results indicate that phosphatase 2A and calcineurin dephosphorylate sodium channels in brain, and thus may counteract the effect of cAMP-dependent phosphorylation on sodium channel activity.

Identification of the sites of selective phosphorylation and dephosphorylation of the rat brain Na+ channel alpha subunit by cAMP-dependent protein kinase and phosphoprotein phosphatases

Voltage-sensitive brain Na+ channels are regulated by cAMP-dependent protein kinase (cA-PK) and protein kinase C. Using synthetic peptides and protein microsequencing, we have determined that the alpha subunit of rat brain Na+ channel is selectively phosphorylated by cA-PK in vitro and in intact cells on 4 serine residues in the intracellular loop connecting homologous domains I and II. Ser-623 was most rapidly and extensively phosphorylated in vitro, whereas Ser-573, Ser-610, and Ser-687 were phosphorylated to lesser extents. In contrast, serine 687 was most extensively phosphorylated in mammalian cells transfected with the alpha subunit of type IIA Na+ channel in response to an increase in intracellular cAMP. Purified protein phosphatases dephosphorylated these sites selectively. Calcineurin rapidly and extensively dephosphorylated Ser-623 and also dephosphorylated Ser-573, Ser-610, and Ser-687 to lesser extents. Phosphatase 2A selectively dephosphorylated Ser-610. Together these results indicate that modulation of neuronal Na+ channel activity and therefore neuronal excitability by cAMP-dependent phosphorylation results from selective phosphorylation and dephosphorylation of four sites in the intracellular loop connecting homologous domains I and II of the alpha subunit.

Voltage-dependent phosphorylation may recruit Ca2+ current facilitation in chromaffin cells

Bovine chromaffin cells have two components of whole-cell Ca2+ current: 'standard' Ca2+ currents that are activated by brief depolarizations, and 'facilitation' Ca2+ currents, which are normally quiescent but can be activated by large pre-depolarizations or by repetitive depolarizations to physiological potentials. The activation of protein kinase A can also stimulate Ca2+ current facilitation, indicating that phosphorylation can play a part in facilitation. Here we investigate the role of protein phosphorylation in the recruitment of facilitation Ca2+ currents by pre-pulses or repetitive depolarizations. We find that recruitment of facilitation by depolarization is a rapid first-order process which is suppressed by inhibitors of protein phosphorylation or by injection of phosphatase 2A into cells. Recruitment of facilitation Ca2+ current by voltage is normally reversible but phosphatase inhibitors render it irreversible. Our results indicate that recruitment of these Ca2+ currents by pre-pulses or repetitive depolarizations involves voltage-dependent phosphorylation of the facilitation Ca2+ channel or a closely associated regulatory protein. Voltage-dependent phosphorylation may therefore be a mechanism by which membrane potential can modulate ion channel activity.

Phosphorylation of the alpha subunit of rat brain sodium channels by cAMP-dependent protein kinase at a new site containing Ser686 and Ser687

The alpha subunit of the rat brain sodium channel is phosphorylated by cAMP-dependent protein kinase in vitro and in situ at multiple sites which yield seven tryptic phosphopeptides. Phosphopeptides 1-4 and 7 are derived from phosphorylation sites between residues 554 and 623 in a single large CNBr fragment from the cytoplasmic segment connecting homologous domains I and II of the alpha subunit (Rossie, S., Gordon, D., and Catterall, W. A. (1987) J. Biol. Chem. 262, 17530-17535). In the present work, antibodies were prepared against a synthetic peptide corresponding to residues 676-692 (AbSP15), which contain one additional potential phosphorylation site at Ser686-Ser687 in a different predicted CNBr fragment of this same intracellular segment. AbSP15 recognizes native and denatured sodium channels specifically and immunoprecipitates phosphorylated CNBr fragments of low molecular mass that contain a new site phosphorylated by cAMP-dependent protein kinase. Comparison of tryptic phosphopeptides derived from intact alpha subunits with those derived from the phosphorylated CNBr fragments isolated by immunoprecipitation with AbSP15 indicates that the two previously unidentified phosphopeptides 5 and 6 derived from the intact alpha subunit arise from phosphorylation of the site containing Ser686-Ser687. These results identify a new cAMP-dependent phosphorylation site and show that the major cAMP-dependent phosphorylation sites of the rat brain sodium channel, which are phosphorylated both in vitro and in intact neurons, are all located in a cluster between residues 554 and 687 in the intracellular segment between domains I and II.

Phosphorylation of the CD20 phosphoprotein in resting B lymphocytes. Regulation by protein kinase C

CD20, a B cell integral membrane protein, regulates B cell activation and is differently phosphorylated in resting and activated cells. We have previously shown that CD20 phosphorylation is increased in activated cells and in phorbol ester-treated resting cells. Phosphorylation is also altered by agents which signal B cell proliferation, such as anti-IgM and a B cell growth factor. The present study was designed to address whether protein kinase C (PKC) or other kinases used CD20 as a substrate. When purified PKC was incubated with isolated CD20, both the 35- and 37-kDa CD20 proteins were phosphorylated in vitro. Intact resting B cells were next incubated with the protein kinase inhibitors H-7, H-8, and W-7. No change in basal CD20 phosphorylation was observed in the presence of W-7 and H-8, indicating that the protein cyclic nucleotide-dependent and calmodulin-dependent kinases were not actively phosphorylating CD20. Surprisingly, the PKC inhibitor H-7 increased CD20 phosphorylation at concentrations above 25-50 microM. To assess whether PKC either activated a phosphatase or inactivated a kinase affecting CD20 phosphorylation, tryptic phosphopeptide mapping of CD20 was performed. These studies revealed that addition of phorbol 12-myristate 13-acetate increased phosphorylation of some peptides differing from those which had increased phosphorylation following addition of H-7. Furthermore, signalling through surface immunoglobulin increased phosphorylation of CD20 peptides distinct from those hyperphosphorylated following addition of phorbol 12-myristate 13-acetate. These results demonstrate that 1) CD20 has multiple phosphorylation sites, as predicted from sequence data, and 2) whereas PKC can use CD20 as substrate, at least one other unidentified kinase phosphorylates CD20 in resting cells. Our data also predict that activation of B cells via the antigen receptor (surface IgM) may activate other protein kinases in addition to PKC.

Identification of an intracellular domain of the sodium channel having multiple cAMP-dependent phosphorylation sites

Cyclic AMP-dependent protein kinase catalyzes the incorporation of 3-4 mol of phosphate into the alpha subunit of rat brain sodium channels in vitro or in situ. Digestion of phosphorylated sodium channels with CNBr yielded three major phosphorylated fragments of 25, 31, and 33 kDa. These fragments were specifically immunoprecipitated with site-directed antisera establishing their location within an intracellular loop between the first and second homologous domains containing residues 448 to 630 of sodium channel RI or residues 450-639 of sodium channel RII. Five of the seven major tryptic phosphopeptides generated from intact sodium channel alpha subunits were contained in each of the 25-, 31-, and 33-kDa CNBr fragments, indicating that most cAMP-dependent phosphorylation sites are in this domain. Since CNBr digestion of sodium channels which had been metabolically labeled with 32P in intact neurons yielded the same phosphorylated fragments, the phosphorylated region we have identified is the major location of phosphorylation in situ. Only serine residues were phosphorylated by cAMP-dependent protein kinase in vitro, while approximately 16% of the phosphorylation in intact neurons was on threonine residues that must lie outside the domain we have identified. Since this domain is phosphorylated in intact neurons, our results show that it is located on the intracellular side of the plasma membrane. These results are considered with respect to models for the transmembrane orientation of the alpha subunit.

Cyclic-AMP-dependent phosphorylation of voltage-sensitive sodium channels in primary cultures of rat brain neurons

We have studied cAMP-dependent phosphorylation of sodium channels in rat brain neurons maintained in primary culture. In back phosphorylation studies, cells were treated with drugs to increase intracellular cAMP and sodium channels were solubilized and isolated by immunoprecipitation. Surface and intracellular pools of sodium channels were isolated separately. Purified channels were then phosphorylated with [gamma-32P]ATP by the catalytic subunit of cAMP-dependent protein kinase to incorporate 32P into available cAMP-dependent phosphorylation sites. The amount of 32P incorporated in vitro is inversely proportional to the extent of endogenous phosphorylation. Incubation of cells with forskolin (0.1-100 microM), 8-Br-cAMP (0.1-10 mM), or isobutylmethylxanthine (0.01-1.0 mM) inhibited subsequent incorporation of 32P into isolated sodium channels by 70-80%, indicating that treatment of cells with these drugs had increased endogenous phosphorylation to nearly maximum levels. The phosphopeptides phosphorylated in vivo and in vitro were identical. To examine the magnitude of basal phosphorylation and the extent of stimulated phosphorylation, the amount of 32P incorporated into sodium channels from control and stimulated cells was compared to that from matched samples which had been dephosphorylated with calcineurin. Sodium channels from control cells incorporated approximately 2-fold more 32P after dephosphorylation, indicating that cAMP-dependent sites on the channel are at least 47% phosphorylated in the basal state. Sodium channels from forskolin-treated cells incorporated 7-8-fold more 32P after dephosphorylation, indicating that cAMP-dependent phosphorylation sites are 80-90% phosphorylated after stimulation. Cell surface and intracellular pools of sodium channels were phosphorylated similarly. In cells metabolically labeled with 32P, cell surface sodium channels incorporated 2.7 mol of phosphate/mol of channel. Forskolin stimulated 32P incorporation into sodium channels 1.3-fold, consistent with the results obtained by back phosphorylation. We conclude that the rat brain sodium channel is substantially phosphorylated in both the cell surface and intracellular pools in vivo in unstimulated rat brain neurons, and the extent of phosphorylation is increased to 80-90% of maximum phosphorylation by agents that elevate intracellular cAMP.