A 60 kDa polypeptide of skeletal-muscle sarcoplasmic reticulum is a calmodulin-dependent protein kinase that associates with and phosphorylates several membrane proteins

Enhanced capacity for CaMKII signaling mitigates calcium release related contractile fatigue with high intensity exercise

BACKGROUND

We tested whether enhancing the capacity for calcium/calmodulin-dependent protein kinase type II (CaMKII) signaling would delay fatigue of excitation-induced calcium release and improve contractile characteristics of skeletal muscle during fatiguing exercise.

METHODS

Fast and slow type muscle, gastrocnemius medialis (GM) and soleus (SOL), of rats and mouse interosseus (IO) muscle fibers, were transfected with pcDNA3-based plasmids for rat α and β CaMKII or empty controls. Levels of CaMKII, its T287-phosphorylation (pT287-CaMKII), and phosphorylation of components of calcium release and re-uptake, ryanodine receptor 1 (pS2843-RyR1) and phospholamban (pT17-PLN), were quantified biochemically. Sarcoplasmic calcium in transfected muscle fibers was monitored microscopically during trains of electrical excitation based on Fluo-4 FF fluorescence (n = 5-7). Effects of low- (n = 6) and high- (n = 8) intensity exercise on pT287-CaMKII and contractile characteristics were studied in situ.

RESULTS

Co-transfection with αCaMKII-pcDNA3/βCaMKII-pcDNA3 increased α and βCaMKII levels in SOL (+45.8 %, +250.5 %) and GM (+40.4 %, +89.9 %) muscle fibers compared to control transfection. High-intensity exercise increased pT287-βCaMKII and pS2843-RyR1 levels in SOL (+269 %, +151 %) and GM (+354 %, +119 %), but decreased pT287-αCaMKII and p17-PLN levels in GM compared to SOL (-76 % vs. +166 %; 0 % vs. +128 %). α/β CaMKII overexpression attenuated the decline of calcium release in muscle fibers with repeated excitation, and mitigated exercise-induced deterioration of rates in force production, and passive force, in a muscle-dependent manner, in correlation with pS2843-RyR1 and pT17-PLN levels (|r| > 0.7).

CONCLUSION

Enhanced capacity for α/β CaMKII signaling improves fatigue-resistance of active and passive contractile muscle properties in association with RyR1- and PLN-related improvements in sarcoplasmic calcium release.

Redox potential and the response of cardiac ryanodine receptors to CLIC-2, a member of the glutathione S-transferase structural family

The type 2 chloride intracellular channel, CLIC-2, is a member of the glutathione S-transferase structural family and a suppressor of cardiac ryanodine receptor (RyR2) Ca2+ channels located in the membrane of the sarcoplasmic reticulum (SR). Modulators of RyR2 activity can alter cardiac contraction. Since both CLIC-2 and RyR2 are modified by redox reactions, we speculated that the action of CLIC-2 on RyR2 may depend on redox potential. We used a GSH:GSSG buffer system to produce mild changes in redox potential to influence redox sensors in RyR2 and CLIC-2. RyR2 activity was modified only when both luminal and cytoplasmic solutions contained the GSH:GSSG buffer and the effects were reversed by removing the buffer from one of the solutions. Channel activity increased with an oxidizing redox potential and decreased when the potential was more reducing. Addition of cytoplasmic CLIC-2 inhibited RyR2 with oxidizing redox potentials, but activated RyR2 under reducing conditions. The results suggested that both RyR2 and CLIC-2 contain redox sensors. Since cardiac ischemia involves a destructive Ca2+ overload that is partly due to oxidation-induced increase in RyR2 activity, we speculate that the properties of CLIC-2 place it in an ideal position to limit ischemia-induced cellular damage in cardiac muscle.

The Muscle-specific Calmodulin-dependent Protein Kinase Assembles with the Glycolytic Enzyme Complex at the Sarcoplasmic Reticulum and Modulates the Activity of Glyceraldehyde-3-phosphate Dehydrogenase in a Ca2+/Calmodulin-dependent Manner

The skeletal muscle specific Ca(2)+/calmodulin-dependent protein kinase (CaMKIIbeta(M)) is localized to the sarcoplasmic reticulum (SR) by an anchoring protein, alphaKAP, but its function remains to be defined. Protein interactions of CaMKIIbeta(M) indicated that it exists in complex with enzymes involved in glycolysis at the SR membrane. The kinase was found to complex with glycogen phosphorylase, glycogen debranching enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and creatine kinase in the SR membrane. CaMKIIbeta(M) was also found to assemble with aldolase A, GAPDH, enolase, lactate dehydrogenase, creatine kinase, pyruvate kinase, and phosphorylase b kinase from the cytosolic fraction. The interacting proteins were substrates of CaMKIIbeta(M), and their phosphorylation was enhanced in a Ca(2+)- and calmodulin (CaM)-dependent manner. The CaMKIIbeta(M) could directly phosphorylate GAPDH and markedly increase ( approximately 3.4-fold) its activity in a Ca(2+)/CaM-dependent manner. These data suggest that the muscle CaMKIIbeta(M) isoform may serve to assemble the glycogen-mobilizing and glycolytic enzymes at the SR membrane and specifically modulate the activity of GAPDH in response to calcium signaling. Thus, the activation of CaMKIIbeta(M) in response to calcium signaling would serve to modulate GAPDH and thereby ATP and NADH levels at the SR membrane, which in turn will regulate calcium transport processes.

Calcineurin and intracellular Ca2+-release channels: regulation or association?

The Ca(2+)- and calmodulin-dependent phosphatase calcineurin was reported to interact with the inositol 1,4,5-trisphosphate receptor (IP(3)R) and the ryanodine receptor (RyR) and to modulate their phosphorylation status and activity. However, controversial data on the molecular mechanisms involved and on the functional relevance of calcineurin for these channel-complexes have been described. Hence, we will focus on the functional importance of calcineurin for IP(3)R and RyR function and on the different mechanisms by which Ca(2+)-dependent dephosphorylation can affect the gating of those intracellular Ca(2+)-release channels. Since many studies made use of immunosuppressive drugs that are inhibiting calcineurin activity, we will also have to take the different side effects of these drugs into account for the proper interpretation of the effects of calcineurin on intracellular Ca(2+)-release channels. In addition, it became recently known that various other phosphatases and kinases can associate with these channels, thereby forming macromolecular complexes. The relevance of these enzymes for IP(3)R and RyR functioning will be reviewed since in some cases they could interfere with the effects ascribed to calcineurin. Finally, we will discuss the downstream effects of calcineurin on the regulation of the expression levels of intracellular Ca(2+)-release channels as well as the relation between IP(3)R- and RyR-mediated Ca(2+) release and calcineurin-dependent gene expression.

A role for calcium/calmodulin kinase in insulin stimulated glucose transport

Previous research has shown that the CAMK (calcium/calmodulin dependent protein kinase) inhibitor, KN62, can lead to reductions in insulin stimulated glucose transport. Although controversial, an L-type calcium channel mechanism has also been hypothesized to be involved in insulin stimulated glucose transport. The purpose of this report was to determine if 1) L-type calcium channels and CAMK are involved in a similar signaling pathway in the control of insulin stimulated glucose transport and 2) determine if insulin induces an increase in CAMKII phosphorylation through an L-type calcium channel dependent mechanism. Insulin stimulated glucose transport was significantly (p<0.05) inhibited to a similar extent ( approximately 30%) by both KN62 and nifedipine in rat soleus and epitrochelaris muscles. The new finding of these experiments was that the combined inhibitory effect of these two compounds was not greater than the effect of either inhibitor alone. To more accurately determine the interaction between CAMK and L-type calcium channels, we measured insulin induced changes in CAMKII phosphorylation using Western blot analysis. The novel finding of this set of experiments was that insulin induced an increase in phosphorylated CAMKII ( approximately 40%) in rat soleus muscle that was reversed in the presence of KN62 but not nifedipine. Taken together these results suggest that a CAMK signaling mechanism may be involved in insulin stimulated glucose transport in skeletal muscle through an L-type calcium channel independent mechanism.

Phosphorylation of the triadin cytoplasmic domain by CaM protein kinase in rabbit fast-twitch muscle sarcoplasmic reticulum

Skeletal muscle triadin is a sarcoplasmic reticulum (SR) membrane protein that had been shown to interact structurally and functionally at the cytoplasmic domain (amino acid residues 1-47) with the ryanodine receptor (RyR1), and to undergo phosphorylation by endogenous calmodulin protein kinase (CaM K II) in isolated terminal cisternae from rabbit fast-twitch muscle. Here we show that triadin cytoplasmic domain expressed as glutathione-S-transferase fusion protein, is a substrate of the protein kinase. This finding is corroborated by identification of a specific consensus sequence in the deduced amino sequence between residue 34 and 37 of triadin. Confirming the regulatory features of CaM K II, we show the phosphorylation of triadin cytoplasmic segment by the kinase, when converted to the autonomous form. We propose that triadin modulates RyR1 in a phosphorylation-dependent manner.

Phosphorylation of anchoring protein by calmodulin protein kinase associated to the sarcoplasmic reticulum of rabbit fast-twitch muscle

Regulatory phosphorylation of phospholamban and of SR Ca(2+)-ATPase SERCA2a isoform by endogenous CaM-K II in slow-twitch skeletal and cardiac sarcoplasmic reticulum (SR) is well documented, but much less is known of the exact functional role of CaM K II in fast-twitch muscle SR. Recently, it was shown that RNA splicing of brain-specific alpha CaM K II, gives rise to a truncated protein (alpha KAP), consisting mainly of the association domain, serving to anchor CaM K II to SR membrane in rat skeletal muscle [Bayer, K.-U., et al. (1998) EMBO J. 19, 5598-5605]. In the present study, we searched for the presence of alpha KAP in sucrose-density purified SR membrane fractions from representative fast-twitch and slow-twitch limb muscles, both of the rabbit and the rat, using immunoblot techniques and antibody directed against the association domain of alpha CaM K II. Putative alpha KAP was immunodetected as a 23-kDa electrophoretic component on SDS-PAGE of the isolated SR from fast-twitch but not from slow-twitch muscle, and was further identified as a specific substrate of endogenous CaM K II, in the rabbit. Immunodetected, (32)P-labeled, non-calmodulin binding protein, behaved as a single 23-kDa protein species under several electrophoretic conditions. The 23-kDa protein, with defined properties, was isolated as a complex with 60-kDa delta CaM K II isoform, by sucrose-density sedimentation analysis. Moreover, we show here that putative alphaKAP, in spite of its inability to bind CaM in ligand blot overlay, co-eluted with delta CaM K II from CaM-affinity columns. That raises the question of whether CaM K II-mediated phosphorylation of alpha KAP and triadin together might be involved in a molecular signaling pathway important for SR Ca(2+)-release in fast-twitch muscle SR.

Coordinate expression of Ca2+-ATPase slow-twitch isoform and of beta calmodulin-dependent protein kinase in phospholamban-deficient sarcoplasmic reticulum of rabbit masseter muscle

Modulation of sarcoplasmic reticulum (SR) Ca(2+) transport by endogenous calmodulin-dependent protein kinase II (CaM K II) involves covalent changes of regulatory protein phospholamban (PLB), as a common, but not the only mechanism, in limb slow-twitch muscles of certain mammalian species, such as the rabbit. Here, using immunofluorescent techniques in situ, and biochemical and immunological methods on the isolated SR, we have demonstrated that rabbit masseter, a muscle with a distinct embryological origin, lacks PLB. Accommodating embryological heterogeneity in the paradigm of neural-dependent expression of specific isogenes in skeletal muscle fibers, our results provide novel evidence for the differential expression in the SR of 72 kDa beta components of CaM K II, together with the expression of a slow-twitch sarcoendoplasmic reticulum Ca(2+)-ATPase isoform, both in limb muscle and in the masseter.

Protein-protein interactions in intracellular Ca2+-release channel function

Release of Ca2+ ions from intracellular stores can occur via two classes of Ca2+-release channel (CRC) protein, the inositol 1,4, 5-trisphosphate receptors (InsP3Rs) and the ryanodine receptors (RyRs). Multiple isoforms and subtypes of each CRC class display distinct but overlapping distributions within mammalian tissues. InsP3Rs and RyRs interact with a plethora of accessory proteins which modulate the activity of their intrinsic channels. Although many aspects of CRC structure and function have been reviewed in recent years, the properties of proteins with which they interact has not been comprehensively surveyed, despite extensive current research on the roles of these modulators. The aim of this article is to review the regulation of CRC activity by accessory proteins and, wherever possible, to outline the structural details of such interactions. The CRCs are large transmembrane proteins, with the bulk of their structure located cytoplasmically. Intra- and inter-complex protein-protein interactions between these cytoplasmic domains also regulate CRC function. Some accessory proteins modulate channel activity of all CRC subtypes characterized, whereas other have class- or even isoform-specific effects. Certain accessory proteins exert both direct and indirect forms of regulation on CRCs, occasionally with opposing effects. Others are themselves modulated by changes in Ca2+ concentration, thereby participating in feedback mechanisms acting on InsP3R and RyR activity. CRCs are therefore capable of integrating numerous signalling events within a cell by virtue of such protein-protein interactions. Consequently, the functional properties of InsP3Rs and RyRs within particular cells and subcellular domains are 'customized' by the accessory proteins present.

Immunohistochemical localization of Ca2+/Calmodulin-dependent protein kinase IV in outer hair cells

A smooth membrane system consisting of subsurface cisternae (SSC) underlies the lateral plasmalemma of auditory outer hair cells (OHCs). The SSC contain Ca-ATPase and are regarded as an intracellular Ca2+ reservoir like the sarcoplasmic reticulum of myocytes. Recently, it has been demonstrated that Ca-ATPase activity in sarcoplasmic reticulum is regulated by Ca2+/calmodulin-dependent protein kinases (CaM kinases). Here we investigated the presence of CaM kinases in OHCs and their possible association with the SSC. Inner ears collected from adult gerbils and from neonates at 2-day intervals between 0 and 20 days after birth were immunostained with antibodies specific for different CaM kinases. A polyclonal antiserum against CaM kinase IV yielded a strong immunostaining reaction along the lateral wall of OHCs. The staining appeared after the tenth postnatal day and continued into adulthood. No other site in the inner ear, including cochlear inner hair cells and vestibular hair cells, was reactive. The kinase's apparent association with the SSC strongly supports its involvement in intracellular Ca2+ homeostasis and suggests a role in regulating the OHCs' slow motile responses.

The structure, function, and cellular regulation of ryanodine-sensitive Ca2+ release channels

The fundamental biological process of Ca2+ signaling is known to be important in most eukaryotic cells, and inositol 1,2,5-trisphosphate and ryanodine receptors, intracellular Ca2+ release channels encoded by two distantly related gene families, are central to this phenomenon. Ryanodine receptors in the sarcoplasmic reticulum of skeletal and cardiac muscle have a predominant role in excitation-contraction coupling, but the channels are also present in the endoplasmic reticulum of noncontractile tissues including the central nervous system and the immune system. In all, three highly homologous ryanodine receptor isoforms have been identified, all very large proteins which assemble as (homo)tetramers of approximately 2 MDa. They contain large cytoplasmically disposed regulatory domains and are always associated with other structural or regulatory proteins, including calmodulin and immunophilins, which can have marked effects on channel function. The type 1 isoform in skeletal muscle is electromechanically coupled to surface membrane voltage sensors, whereas the remaining isoforms appear to be activated solely by endogenous cytoplasmic second messengers or other ligands, including Ca2+ itself ("Ca(2+)-induced Ca2+ release"). This review concentrates on ryanodine receptor structure-function relationships as probed by a variety of methods and on the molecular mechanisms of channel modulation at the cellular level (including evidence for the regulation of gene expression and transcription). It also touches on the relevance of ryanodine receptors to complex cellular functions and disease.

Functional behaviour of the ryanodine receptor/Ca(2+)-release channel in vesiculated derivatives of the junctional membrane of terminal cisternae of rabbit fast muscle sarcoplasmic reticulum

We have devised a novel procedure, employing Chaps rather than Triton [Costello B., Chadwick C., Saito A., Chu A., Maurer A., Fleischer S. J Cell Biol 1986; 103: 741-753], for obtaining vesiculated derivatives of the junctional face membrane (JFM) domain of isolated terminal cisternae (TC) from fast skeletal muscle of the rabbit. Enriched JFM is minimally contaminated with junctional transverse tubules. The characteristic ultrastructural features and the most essential features of TC function relating to this membrane domain-i.e. both the Ca(2+)-release system and the Ca2+ and calmodulin (CaM)-dependent protein kinase (CaM I PK) system-appear to be retained in enriched JFM. We show that our isolation procedure, yielding up to a 2.5-fold enrichment in ryanodine receptor (RyR) protein and in the maximum number of high affinity [3H]-ryanodine binding sites, does not alter the assembly for integral proteins associated with the receptor in its native membrane environment, i.e. FKBP-12, triadin and the structurally related protein junction [Jones L.R., Zhang L., Sanborn K., Jorgensen A., Kelley J. J Biol Chem 1995; 270: 30787-30796] having, in common, the property to bind calsequestrin (CS) in overlays in the presence of EGTA. The substrate specificity of endogenous CaM I PK is also the same as that of parent TC vesicles. Phosphorylation of mainly triadin and of a high M(r) polypeptide, and not of the RyR, is the most remarkable common property. Retention of peripheral proteins, like CS and histidine-rich Ca(2+)-binding protein, although not that endogenous CaM, and of a unique set of CaM-binding proteins, unlike that of junctional SR-specific integral proteins, is shown to be influenced by the concentration of Ca2+ during incubation of TC vesicles with Chaps. Characterization of RyR functional behaviour with [3H]-ryanodine has indicated extensive similarities between the enriched JFM and parent TC vessicles, as far as the characteristic bell shaped Ca(2+)-dependence of [3H]-ryanodine binding and the dose-dependent sensitization to Ca2+ by caffeine, reflecting the inherent properties of SR Ca(2+)-release channel, as well as concerning the stimulation of [3H]-ryanodine binding by increasing concentrations of KCl. Stabilizing the RyR in a maximally active state by optimizing concentrations of KCl (1 M), at also optimal concentrations of Ca2+ (pCa 4), rendered the receptor less sensitive to inhibition by 1 microM CaM, to a greater extent in the case of enriched JFM. That was not accounted for by any significant difference in the IC50 concentrations of CaM varying between 40 nM to approximately 80 nM, at low-intermediate and at high KCl concentrations, respectively. Additional results with enriched JFM using doxorubicin, a pharmacological Ca2+ channel allosteric modifier, strengthen the hypothesis that the conformational state at which RyR is stabilized, according to the experimental assay conditions for [3H]-ryanodine binding, directly influences CaM-sensitivity.

Age-related abnormalities in regulation of the ryanodine receptor in rat fast-twitch muscle

The tibialis anterior (TA) muscles of 6-month-old and 24-month-old male Wistar rats, after being characterized, at the fast motor unit level, for twitch properties, were dissected and processed by a procedure [Margreth A., Damiani E., Tobaldin G. Biochem Biophys Res Commun 1993; 197: 1303-1311] aimed at obtaining a representative total membrane fraction comprising 70-80% of the total muscle content of sarcoplasmic reticulum (SR) and transverse tubule (TT) membranes (about 20 mg protein/g). Skeletal muscle membranes were analyzed for protein composition, and the content and functional properties of specific components of the free and junctional subcompartments of the SR and of junctional TT. Our results, while confirming a twitch prolongation in TA of old rats, do not demonstrate any associated age-related change concerning: (a) the overall number and functional properties of Ca2+ pumps, as characterized by kinetic parameters, Ca(2+)-dependency, and the protein isoform specificity of SR Ca(2+)-ATPase; (b) the number of functional junctional SR Ca(2+)-release channels, on the basis of Bmax values for high-affinity binding of [3H]-ryanodine to skeletal muscle membranes at optimal Ca2+; (c) the overall muscle dihydropyridine receptor/ryanodine receptor (RyR) ratio. We conclude from these findings, and the additional negative evidence for changes in membrane density of specific components of junctional SR, including 60 kDa Ca(2+)-calmodulin protein kinase, that this membrane domain, like the Ca(2+)-pump domain of the SR, are in no way basically altered at early stages of the aging process, as investigated here. Because of that, we allege particular significance to the occurrence of age-related, specific abnormalities in regulation of RyR in rat TA. The main supportive evidence is as follows: (a) an increased sensitivity to Ca2+ of the RyR of old muscle, and, more importantly; (b) an increased sensitivity to caffeine of [3H]ryanodine binding to the RyR at optimal Ca2+ and also optimal for the activity of the Ca(2+)-release channel. The results reported here also demonstrate that there are two classes of caffeine sites in rat TA muscle, as defined by differences in EC50 values at resting (pCa 7) and at high Ca2+ (pCa 4-5), that sites involved in stimulation of [3H]-ryanodine binding to the RyR are distinguished by a higher affinity (caffeine below mM), and that only these sites undergo age-related changes. Thus, although the underlying age-related abnormality of the RyR remains to be elucidated, it appears to satisfy the requirement for being regarded as a specific change, which in itself might argue for its being fundamentally related to the twitch prolongation of the muscle.

Skeletal muscle sarcoplasmic reticulum phenotype in myotonic dystrophy

In this study we investigated the sarcoplasmic reticulum (SR), alongside myofibrillar phenotype, in muscle samples from five Myotonic Dystrophy (DM) patients and five control individuals. DM muscles exhibited as a common feature, a decrease in the slow isoform of myosin heavy chain (MHC) and of troponin C in myofibrils. We observed a match between myofibrillar changes and changes in SR membrane markers specific to fiber type, i.e. the fast (SERCA1) Ca(2+)-ATPase isoform increased concomitantly with a decrease of protein phospholamban (PLB), which in native SR membranes colocalizes with the slow (SERCA2a) SR Ca(2+)-ATPase, and regulates its activity depending on phosphorylation by protein kinases. Our results outline a cellular process selectively affecting slow-twitch fibers, and non-degenerative in nature, since neither the total number of Ca(2+)-pumps or of ryanodine receptor/Ca(2+)-release channels, or their ratio to the dihydropyridine receptor/voltage sensor in junctional transverse tubules, were found to be significantly changed in DM muscle. The only documented, apparently specific molecular changes associated with this process in the SR of DM muscle, are the defective expression of the slow/cardiac isoform of Ca(2+)-binding protein calsequestrin, together with an increased phosphorylation activity of membrane-bound 60 kDa Ca(2+)-calmodulin (CaM) dependent protein kinase. Enhanced phosphorylation of PLB by membrane-bound Ca(2+)-CaM protein kinase also appeared to be most pronounced in biopsy from a patient with a very high CTG expansion, as was the overall 'slow-to-fast' transformation of the same muscle biopsy. Animal studies showed that endogenous Ca(2+)-CaM protein kinase exerts a dual activatory role on SERCA2a SR Ca(2+)-ATPase, i.e. either by direct phosphorylation of the Ca(2+)-ATPase protein, or mediated by phosphorylation of PLB. Our results seem to be consistent with a maturational-related abnormality and/or with altered modulatory mechanisms of SR Ca(2+)-transport in DM slow-twitch muscle fibers.

A 63 kDa phosphoprotein undergoing rapid dephosphorylation during exocytosis in Paramecium cells shares biochemical characteristics with phosphoglucomutase

We have enriched phosphoglucomutase (PGM; EC 5.4.2.2) approximately 20-fold from Paramecium tetraurelia cells by combined fractional precipitation with (NH4)2SO4, gel filtration and anion-exchange chromatography yielding two PGM peaks. Several parameters affecting PGM enzymic activity, molecular mass and pI were determined. Phosphorylation studies were done with isolated endogenous protein kinases. Like the 63 kDa phosphoprotein PP63, which is dephosphorylated within 80 ms during synchronous trichocyst exocytosis [Höhne-Zell, Knoll, Riedel-Gras, Hofer and Plattner (1992) Biochem. J. 286, 843-849], PGM has a molecular mass of 63 kDa and forms of identical pI. Since mammalian PGM activity depends on the presence of glucose 1,6-bisphosphate (Glc-1,6-P2) (which is lost during anion-exchange chromatography), we analysed this aspect with Paramecium PGM. In this case PGM activity was shown not to be lost, due to p-nitrophenyl phosphate-detectable phosphatase(s) (which we have separated from PGM), but also due to loss of Glc-1,6-P2. Like PGM from various vertebrate species, PGM activity from Paramecium can be fully re-established by addition of Glc-1,6-P2 at 10 nM, and it is also stimulated by bivalent cations and insensitive to chelating or thiol reagents. The PGM which we have isolated can be phosphorylated by endogenous cyclic-GMP-dependent protein kinase or by endogenous casein kinase. This results in three phosphorylated bands of identical molecular mass and pI values, as we have shown to occur with PP63 after phosphorylation in vivo (forms with pI 6.05, 5.95, 5.85). In ELISA, antibodies raised against PGM from rabbit skeletal muscle were reactive not only with original PGM but also with PGM fractions from Paramecium. Therefore, PGM and PP63 seem to be identical with regard to widely different parameters, i.e. co-elution by chromatography, molecular mass, phosphorylation by the two protein kinases tested, pI values of isoforms, and immuno-binding. Recent claims that PP63 ('parafusin') would not be identical with PGM specifically in Paramecium are critically evaluated. Since some glycolytic enzymes are discussed as being associated with the Ca(2+)-release channel in muscle sarcoplasmic reticulum, and since sub-plasmalemmal Ca2+ stores in Paramecium closely resemble sarcoplasmic reticulum, a possible function of PP63/PGM in exocytosis regulation is discussed, particularly since dephosphorylation strictly parallels exocytosis.

Phosphorylation modulates the function of the calcium release channel of sarcoplasmic reticulum from skeletal muscle

The modulation of the calcium release channel (CRC) by protein kinases and phosphatases was studied. For this purpose, we have developed a microsyringe applicator to achieve sequential and multiple treatments with highly purified kinases and phosphatases applied directly at the bilayer surface. Terminal cisternae vesicles of sarcoplasmic reticulum from rabbit fast twitch skeletal muscle were fused to planar lipid bilayers, and single-channel currents were measured at zero holding potential, at 0.15 microM free Ca2+, +/- 0.5 mM ATP and +/- 2.6 mM free Mg2+. Sequential dephosphorylation and rephosphorylation rendered the CRC sensitive and insensitive to block by Mg2+, respectively. Channel recovery from Mg2+ block was obtained by exogenous protein kinase A (PKA) or by Ca2+/calmodulin-dependent protein kinase II (CalPK II). Somewhat different characteristics were observed with the two kinases, suggesting two different states of phosphorylation. Channel block by Mg2+ was restored by dephosphorylation using protein phosphatase 1 (PPT1). Before application of protein kinases or phosphatases, channels were found to be "dephosphorylated" (inactive) in 60% and "phosphorylated" (active) in 40% of 51 single-channel experiments based on the criterion of sensitivity to block by Mg2+. Thus, these two states were interconvertable by treatment with exogenously added protein kinases and phosphatases. Endogenous Ca2+/calmodulin-dependent protein kinase (end CalPK) had an opposite action to exogenous CalPK II. Previously, dephosphorylated channels using PPT (Mg2+ absent) were blocked in the closed state by action of endogenous CalPK. This block was removed to normal activity by the action of either PPT or by exogenous CalPK II. Our findings are consistent with a physiological role for phosphorylation/dephosphorylation in the modulation of the calcium release channel of sarcoplasmic reticulum from skeletal muscle. A corollary of our studies is that only the phosphorylated channel is active under physiological conditions (mM Mg2+). Our studies suggest that phosphorylation can be at more than one site and, depending on the site, can have different functional consequences on the CRC.