Involvement of 60-kilodalton phosphoprotein in the regulation of calcium release from skeletal muscle sarcoplasmic reticulum

Aldolase potentiates DIDS activation of the ryanodine receptor in rabbit skeletal sarcoplasmic reticulum

DIDS (4,4'-di-isothiocyanostilbene-2,2'-disulfonate), an anion channel blocker, triggers Ca2+ release from skeletal muscle SR (sarcoplasmic reticulum). The present study characterized the effects of DIDS on rabbit skeletal single Ca2+-release channel/RyR1 (ryanodine receptor type 1) incorporated into a planar lipid bilayer. When junctional SR vesicles were used for channel incorporation (native RyR1), DIDS increased the mean P(o) (open probability) of RyR1 without affecting unitary conductance when Cs+ was used as the charge carrier. Lifetime analysis of single RyR1 activities showed that 10 microM DIDS induced reversible long-lived open events (P(o)=0.451+/-0.038) in the presence of 10 microM Ca2+, due mainly to a new third component for both open and closed time constants. However, when purified RyR1 was examined in the same condition, 10 microM DIDS became considerably less potent (P(o)=0.206+/-0.025), although the caffeine response was similar between native and purified RyR1. Hence we postulated that a DIDS-binding protein, essential for the DIDS sensitivity of RyR1, was lost during RyR1 purification. DIDS-affinity column chromatography of solubilized junctional SR, and MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight) MS analysis of the affinity-column-associated proteins, identified four major DIDS-binding proteins in the SR fraction. Among them, aldolase was the only protein that greatly potentiated DIDS sensitivity. The association between RyR1 and aldolase was further confirmed by co-immunoprecipitation and aldolase-affinity batch-column chromatography. Taken together, we conclude that aldolase is physically associated with RyR1 and could confer a considerable potentiation of the DIDS effect on RyR1.

Enhanced phosphorylation and enzymatic activity of phosphoglucomutase by the Btk29A tyrosine kinase in Drosophila

The Drosophila Btk29A tyrosine kinase is suggested to be involved in diverse processes, although its target proteins are unknown. In the present study, we investigated substrates of Btk29A tyrosine kinase by expressing a catalytically activated form of Btk29A-P1 (Btk-EG) in Drosophila compound eyes. Expression in eye disks led to the development of the rough-eye phenotype and increased tyrosine phosphorylation of a 65-kDa protein. Partial amino acid sequence analysis of this protein showed that it was phosphoglucomutase. Phosphoglucomutase activity in heads from Btk-EG-expressing flies was higher than that in controls, suggesting that the levels of tyrosine phosphorylation and activity of the enzyme are associated with Btk29A tyrosine kinase activity.

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.

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.

Parafusin is a membrane and vesicle associated protein that cycles at exocytosis

In the unicellular eukaryote Paramecium tetraurelia, stimulation of exocytosis leads to Ca2+ activation of an alpha Glc-1-phosphodiesterase that dephosphoglucosylates the phosphoglycoprotein parafusin (PFUS). This process fails in exo mutant nd9 and also in the absence of Ca2+ influx upon stimulation suggesting that PFUS dephosphoglucosylation may be causally related to exocytosis. To further corroborate the hypothesis that PFUS is involved in the molecular events in exocytosis, we used laser confocal scanning microscopy and a PFUS specific peptide antibody to perform localization studies of PFUS in wild type (wt) and mutant Paramecium. In unstimulated wt cells, PFUS was associated both with the exocytic site of the cell membrane and with the membrane of the dense core secretory vesicles. Localization at these two sites was shown to be independent of each other since the exocytosis mutant (exo-) tam8, in which docking of its vesicles is blocked, still showed cell membrane staining. Immunofluorescence and immunoblotting of isolated intact secretory vesicles also revealed PFUS association. Upon stimulation of exocytosis, PFUS dissociated from both the dense core secretory vesicles and the cell membrane in a Ca(2+)-dependent manner. During recovery of exocytic capacity, PFUS reassociated with the newly formed secretory vesicles in the cytoplasm prior to their docking at the exocytic sites. Immunoblot analysis of PFUS during this time showed no changes in levels of the protein. Stimulation of exocytosis in wt in Mg2+ buffer or in the exo- temperature sensitive mutant (nd9) at the non-permissive temperature did not lead to dissociation of the PFUS. We conclude that PFUS is a novel critical component whose cycling probably participates in the molecular exocytic fusion machinery in these cells.

Phosphorylation of the porcine skeletal and cardiac muscle sarcoplasmic reticulum ryanodine receptor

Porcine skeletal and cardiac muscle sarcoplasmic reticulum (SR) vesicle fractions enriched in the ryanodine receptor were phosphorylated in the presence of [gamma-32P]MgATP and either exogenous cAMP-dependent protein kinase (cAMP-PK), or Ca2+ plus calmodulin. Phosphorylation of the cardiac muscle ryanodine receptor in the presence of either cAMP-PK or calmodulin (6.4 and 10.6 pmol Pi/mg SR respectively) was approximately equal to or twice the [3H]ryanodine binding activity of this preparation (5.2 pmol/mg). Furthermore, cardiac muscle ryanodine receptor Pi incorporation catalyzed by cAMP-PK and calmodulin was approximately additive. In skeletal muscle SR, however, the level of cAMP-PK or calmodulin catalyzed phosphorylation of the intact ryanodine receptor (0.2 or 2.9 pmol Pi/mg SR, respectively) was much less than the [3H]ryanodine binding activity of this fraction (11.6 pmol/mg). Furthermore, Pi incorporation into the intact skeletal muscle ryanodine receptor was 3-8-fold less than that incorporated into a component of slightly lower M(r). Although this latter component comigrated with an immunoreactive fragment of the ryanodine receptor on polyacrylamide gels, it did not appear to be derived from the ryanodine receptor. We conclude that the significant phosphorylation of the cardiac muscle SR ryanodine receptor indicates a likely physiological role for protein kinase-mediated regulation of this Ca(2+)-channel. In contrast, the minimal phosphorylation of the skeletal muscle SR ryanodine receptor indicates that such a role of protein kinases is unlikely in this tissue.

Inactivation of the sarcoplasmic reticulum calcium channel by protein kinase

The ryanodine receptor protein of skeletal muscle sarcoplasmic reticulum (SR) membranes is a calcium ion channel which allows movement of calcium from the SR lumen into the cytoplasm during muscle activation. Gating of this channel is modulated by a number of physiologically important substances including calcium. Interestingly, calcium has both activating and inactivating effects which are concentration- and tissue-specific. In skeletal muscle, calcium-dependent inactivation of calcium release occurs at concentrations reached physiologically, suggesting that calcium may modulate the release process by a negative feedback mechanism. To determine the cellular mechanism responsible for calcium-dependent inactivation, we have investigated the ability of protein phosphorylation to affect single channel gating behaviour using the patch clamp technique. Here we demonstrate that the ryanodine receptor protein/calcium release channel of skeletal muscle SR is inactivated under conditions permissive for protein phosphorylation. This inactivation is reversed by the application of phosphatase and prevented by a peptide inhibitor specific for calcium/calmodulin-dependent protein kinase II. The results provide evidence for an endogenous protein kinase which is closely associated with the ryanodine receptor protein and regulates channel gating.

Regulation of Ca2+ release from sarcoplasmic reticulum in skeletal muscles

Ca2+ release from skeletal sarcoplasmic reticulum (SR) could be regulated by at least three mechanisms: 1) Ca2+, 2) calmodulin, and 3) Ca2+/calmodulin-dependent phosphorylation. Bell-shaped Ca(2+)-dependence of Ca2+ release from both actively- and passively-loaded SR vesicles suggest that opening and closing of the Ca2+ release channel could be regulated by [Ca2+o]. The time- and concentration-dependent inhibition of Ca2+ release from skeletal SR by calmodulin was also studied using passively-Ca2+ loaded SR vesicles. Up to 50% of Ca2+ release was inhibited by calmodulin (0.01-0.5 microM); this inhibition required 5-15 min preincubation time. The hypothesis that Ca2+/calmodulin-dependent phosphorylation of a 60 kDa protein regulates Ca2+ release from skeletal SR was tested by stopped-flow fluorometry using passively-Ca2+-loaded SR vesicles. Approximately 80% of the initial rates of Ca(2+)-induced Ca2+ release was inhibited by the phosphorylation within 2 min of incubation of the SR with Mg-ATP and calmodulin. We identified two types of 60 kDa phosphoproteins in the rabbit skeletal SR, which was distinguished by solubility of the protein in CHAPS. The CHAPS-soluble 60 kDa phosphoprotein was purified by column chromatography on DEAE-Sephacel, heparin-agarose, and hydroxylapatite. Analyses of the purified protein indicate that the CHAPS-soluble 60 kDa protein is an isoform of phosphoglucomutase (PGM). cDNAs encoding isoforms of PGM were cloned and sequenced using synthetic oligonucleotides. Two types of PGM isoforms (Type I and Type II) were identified. The translated amino acid sequences show that Type II isoform is SR-form.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-induced inactivation of calcium release from the sarcoplasmic reticulum of skeletal muscle

The ability of myofilament space Ca2+ to modulate Ca2+ release from the sarcoplasmic reticulum (SR) of skeletal muscle was investigated. Single fibers of the frog Rana pipiens belindieri were manually skinned (sarcolemma removed). Following a standard load and pre-incubation in varying myoplasmic Ca2+ concentrations, SR Ca2+ release was initiated by caffeine. Ca2+ release rates were calculated from the changes in absorbance of a Ca2+ sensitive dye, antipyrylazo III. An apparent dissociation constant (Kd) for dye-Ca2+ binding of 8000 microM 2 was determined by comparing the buffering action of the dye with that of ethylenebis(oxonitrilo)tetraacetate (EGTA) using the contractile proteins of the skinned fiber as a measure of free Ca2+. This value for Kd was used in the calculation of Ca2+ release rates. As the myoplasmic space Ca2+ was increased from pCa 7.4, Ca2+ release rates declined sharply such that at pCa 6.9 the calculated release rate was 72 +/- 3% (mean +/- SEM) of control (pCa 8.4). Further increases in myoplasmic Ca2+ from pCa 6.9 to pCa 6.1 did not result in a further decline in release rate. The effect of a decreased driving force on Ca2+ ions was investigated to determine whether it could account for the change in release rates observed. At pCa 6.9, where the greatest degree of inactivation occurred, the measured effects of a change in driving force could account for at most 40% of the observed inactivation. Varying concentrations of Ba2+ and Sr2+ in the myofilament space had no inactivating effect on the SR Ca2+ release rates. The ability of myofilament Ca2+ to inhibit SR Ca2+ release at concentrations normally encountered during muscle activation suggests a role for released Ca2+ as a modulator of the SR Ca2+ channel.

Effects of azumolene on doxorubicin-induced Ca2+ release from skeletal and cardiac muscle sarcoplasmic reticulum

The mechanism of doxorubicin-induced Ca2+ release from skeletal and cardiac muscle sarcoplasmic reticulum (SR) was studied by examining the effects of azumolene (a water soluble dantrolene analog) on doxorubicin-mediated Ca2+ release and ryanodine binding. Doxorubicin induced a rapid Ca2+ release from both skeletal and cardiac SR in a similar concentration range (EC50 = 5-10 microM). Maximal doxorubicin-induced Ca2+ release was seen at 2 and 0.2 microM Ca2+ for skeletal and cardiac SR, respectively. Addition of 400 microM azumolene caused approx. 30% inhibition of doxorubicin-induced Ca2+ release from both skeletal and cardiac SR; skeletal SR had significantly higher sensitivity to azumolene than cardiac SR. In the presence of Ca2+, doxorubicin increased [3H]ryanodine binding to both skeletal and cardiac SR; whereas in the absence of Ca2+, doxorubicin led to significant ryanodine binding to skeletal SR, but not to cardiac SR. In both types of SR, doxorubicin-activated, but not Ca2+ activated ryanodine binding was inhibited by azumolene. Azumolene sensitivity for inhibition of doxorubicin-activated ryanodine binding was much higher in skeletal SR than cardiac SR, consistent with the results for effects of azumolene on Ca2+ release. Our results are consistent with the possibility that azumolene inhibits doxorubicin binding by direct competition for the drug receptor(s).

Kinase activity and protein phosphorylation in control and malignant hyperthermic skeletal muscle

1. Native 6% Laemmli gels were used to resolve 7 protein kinase activity bands in control and malignant hyperthermia (MH)-susceptible porcine and human skeletal muscle extracts. 2. MH-susceptible samples were consistently more active than the controls. 3. Following halothane treatment, a 43 kDa component displayed increased phosphorylation by a calcium-calmodulin dependent kinase in MH-susceptible vs control human samples. 4. Increased phosphorylation of additional endogenous protein components of molecular mass 116 and 60 kDa was observed.

Biphasic effects of doxorubicin on the calcium release channel from sarcoplasmic reticulum of cardiac muscle

To define the mechanism of doxorubicin cardiotoxicity, the effects of doxorubicin and caffeine were examined on calcium release channels from cardiac sarcoplasmic reticulum. We found that calcium release from cardiac sarcoplasmic reticulum vesicles was induced by both compounds. When sarcoplasmic reticulum vesicles were incorporated into planar lipid bilayers, calcium-permeable channels were observed. Addition of caffeine (2.5-10 mM) increased channel open probability from less than 0.1% to 40%, and this effect persisted for a mean of 44 minutes. In contrast, doxorubicin (2.5-10 microM) had a biphasic effect; initially, doxorubicin activated the channel, whereas after a mean of 8 minutes, the channel became irreversibly inhibited. Although the degree of channel activation by doxorubicin was concentration dependent, the time needed to inactivate the channel was concentration independent. Pretreatment with dithiothreitol (0.2 mM) prevented doxorubicin-induced channel inactivation, and channel activity persisted for an average of 58 minutes. Dithiothreitol alone did not alter channel open probability. Our results support the hypotheses that 1) the integrity of sulfhydryl groups is important for some aspects of calcium release channel function and 2) activation and inactivation of the channel are separable processes. The biphasic effect of doxorubicin on channel function may also correspond to the clinically observed adverse effects of doxorubicin, a widely used chemotherapeutic agent that, after prolonged usage, causes a dilated cardiomyopathy.

Effects of compound 48/80 on the Ca2+ release by reversal of the Ca2+ pump and by the Ca2+ channel of sarcoplasmic reticulum membranes

The effect of the calmodulin antagonist, compound 48/80, on the Ca2+ release from skeletal muscle sarcoplasmic reticulum was investigated. Both the Ca2+ release by reversal of the Ca2+ pump and the Ca2+ release by the Mg2(+)-controlled Ca2+ channel were studied. It was observed that, when reversal of the pump is inoperative and Mg2+ is not present in the reaction medium, 48/80 stimulates Ca2+ release from the vesicles. In contrast, in the presence of Mg2+, which blocks the Ca2+ channel, 48/80 inhibits Ca2+ release induced by ADP and Pi. This effect is strong at low concentrations of Pi (approximately 1 mM), whereas high concentrations (approximately 15 mM) protect the system against the drug. Furthermore, it was observed that 48/80 has a maximum effect on the channel-mediated Ca2+ release at concentrations of about 20 micrograms/ml, whereas maximal inhibition of the pump-mediated Ca2+ release occurs at concentrations of about 60-80 micrograms/ml. The results indicate that both the Ca2+ channel complex and the Ca2(+)-ATPase may be target systems for the effects of 48/80 on the Ca2+ transport activity of sarcoplasmic reticulum. However, the Ca2+ channel is more sensitive to the drug, suggesting an involvement of calmodulin on this mechanism of Ca2+ release.

The muscle ryanodine receptor and its intrinsic Ca2+ channel activity

In skeletal and cardiac muscle, contraction is initiated by the rapid release of Ca2+ ions from the intracellular membrane system, sarcoplasmic reticulum. Rapid-mixing vesicle ion flux and planar lipid bilayer-single-channel measurements have shown that Ca2+ release is mediated by a high-conductance, ligand-gated Ca2+ channel. Using the Ca2+ release-specific probe ryanodine, a 30 S protein complex composed of four polypeptides of Mr approximately 400,000 has been isolated. Reconstitution of the purified skeletal and cardiac muscle 30 S complexes into planar lipid bilayers induced single Ca2+ channel currents with conductance and gating kinetics similar to those of native Ca2+ release channels. Electron microscopy revealed structural similarity with the protein bridges ("feet") that span the transverse-tubule-sarcoplasmic reticulum junction. These results suggest that striated muscle contains an intracellular Ca2+ release channel that is identical with the ryanodine receptor and the transverse-tubule-sarcoplasmic reticulum spanning feet structures.

The unraveling architecture of the junctional sarcoplasmic reticulum

The sarcoplasmic reticulum (SR) of skeletal muscle controls the contraction-relaxation cycle by raising and lowering the myoplasmic free-Ca2+ concentration. The coupling between excitation, i.e., depolarization of sarcolemma and transverse tubule (TT) and Ca2+ release from the terminal cisternae (TC) of SR takes place at the triad. The triad junction is formed by a specialized region of the TC, the junctional SR, and the TT. The molecular architecture and protein composition of the junctional SR are under active investigation. Since the junctional SR plays a central role in excitation-contraction coupling and Ca2+ release, some of its protein constituents are directly involved in these processes. The biochemical evidence supporting this contention is reviewed in this article.

Kinetic analysis of excitation-contraction coupling

Recent studies of isolated muscle membrane have enabled induction and monitoring of rapid Ca2+ release from sarcoplasmic reticulum (SR)5 in vitro by a variety of methods. On the other hand, various proteins that may be directly or indirectly involved in the Ca2+ release mechanism have begun to be unveiled. In this mini-review, we attempt to deduce the molecular mechanism by which Ca2+ release is induced, regulated, and performed, by combining the updated information of the Ca2+ release kinetics with the accumulated knowledge about the key molecular components.

Calmodulin modulation of single sarcoplasmic reticulum Ca2+-release channels from cardiac and skeletal muscle

Sarcoplasmic reticulum (SR) contains a Ca2+-conducting channel that is believed to play a central role in excitation-contraction coupling by releasing the Ca2+ necessary for muscle contraction. The effects of calmodulin on single cardiac and skeletal muscle SR Ca2+-release channels were studied using the planar lipid bilayer-vesicle fusion technique. Calmodulin inhibited Ca2+-release channel opening by reducing the mean duration of single-channel open events without having an effect on single-channel conductance. Inhibition by calmodulin was dependent on Ca2+ concentration and occurred in the absence of ATP. The effects of calmodulin were reversed by mastoparan, a calmodulin-binding peptide. Two other calmodulin antagonists [calmidazolium and N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide] modified the gating behavior of the channel in the absence of exogenous calmodulin in a concentration- and Ca2+-dependent manner. Our results suggest that calmodulin can modulate excitation-contraction coupling by directly interacting with the SR Ca2+-release channel of cardiac and skeletal muscle.

Involvement of the 60 kDa phosphoprotein in the regulation of Ca2+ release from sarcoplasmic reticulum of normal and malignant hyperthermia susceptible pig muscles

Junctional sarcoplasmic reticulum (SR) vesicles isolated from back muscles of normal and malignant hyperthermia susceptible (MHS) pigs were phosphorylated by addition of MgATP in the presence of 5 mM Ca2+ and 1 microM calmodulin (CaM). The major site of phosphorylation was a 60 kDa protein both in normal and MHS SR. The maximal amount of phosphorylation in MHS SR (5 pmol P/mg SR) was significantly lower than that in the normal SR (12 pmol P/mg SR). The phosphorylated 60 kDa protein was spontaneously dephosphorylated both in normal and MHS SR. Ca2+ release from the passively loaded SR was induced by a Ca2+-jump, and monitored by stopped-flow fluorometry using chlorotetracycline. In the absence of preincubation with MgATP, no significant difference was found in any of the kinetic parameters of Ca2+ release between normal and MHS SR. Upon addition of 20 microM MgATP to the passively loaded SR to phosphorylate the 60 kDa protein, the initial rate of Ca2+ release in normal SR significantly decreased from 659 +/- 102 to 361 +/- 105 nmol Ca2+/mg SR per s, whereas in MHS SR the rate decreased from 749 +/- 124 to 652 +/- 179 nmol Ca2+/mg SR per s. Addition of 20 microM adenosine 5'-[beta, gamma-imido]triphosphate (p[NH]ppA) did not significantly alter the initial rate of Ca2+ release both in normal and MHS SR. These results suggest that the previously reported higher Ca2+ release rate in MHS SR (Kim et al. (1984) Biochim. Biophys. Acta 775, 320-327) is at least partly due to the reduced extent of the Ca2+/CaM-dependent phosphorylation of the 60 kDa protein. Two-dimensional gel electrophoresis study showed that amount of a protein with Mr = 55,000 was significantly lower in MHS SR than in normal SR suggesting that the abnormally lower amount of 55 kDa protein would cause the lower amount of phosphorylation of the 60 kDa protein in MHS SR.

Skeletal muscle sarcolemma proteins as targets for adenosine 3':5'-monophosphate-dependent and calcium-dependent protein kinases

The present study documents the existence in rat skeletal muscle plasma membrane (sarcolemma) of a distinct set of proteins, most of which represent unknown protein species, which can be phosphorylated in vitro by addition of cAMP-dependent or calcium-dependent protein kinases. Under the experimental conditions used, cAMP-regulated protein phosphorylation appeared to be the most important phosphorylation system in these membranes, followed by the calcium/phospholipid-regulated, and, with only a few substrates detected, the calcium/calmodulin-regulated systems. No specific substrate for cGMP-dependent protein kinase was found. In contrast, calcium/calmodulin-regulated protein phosphorylation was the most important in the sarcoplasmic reticulum fraction. Most of the cAMP-regulated and calcium/phospholipid-regulated sarcolemma phosphoproteins appeared to be intrinsic membrane proteins, at least three of which appeared to be phosphorylated by both these protein kinases. These phosphoproteins may represent membrane targets for multiple hormone or transmitter actions in skeletal muscle cells. Our results, therefore, suggest that protein phosphorylation systems, particularly those regulated by cAMP or calcium/phospholipid, may be more important in the regulation of sarcolemma function than hitherto believed.

Inhibition of calcium release from skeletal muscle sarcoplasmic reticulum by calmodulin

The effect of calmodulin on calcium release from heavy sarcoplasmic reticulum isolated from rabbit skeletal muscle was investigated with actively and passively calcium loaded sarcoplasmic reticulum vesicles and measured either spectrophotometrically with arsenazo III or by Millipore filtration technique. The transient calcium-, caffeine- and AMP-induced calcium release from actively loaded sarcoplasmic reticulum vesicles was reduced to 29%, 51% and 59% of the respective control value by 1 microM exogenous calmodulin. Stopped-flow measurements demonstrate that calmodulin reduces the apparent rate of caffeine-induced calcium release from actively loaded sarcoplasmic reticulum. The rate of calcium uptake measured in the presence of ruthenium red, which blocks the calcium release channel, was not affected by calmodulin or calmodulin-dependent phosphorylation of sarcoplasmic reticulum vesicles with ATP[S]. The rate of the calcium-, caffeine- and AMP-induced calcium release from passively loaded sarcoplasmic reticulum vesicles was reduced 1.4-2.0-fold by 1 microM exogenous calmodulin, i.e. the half-time of release was maximally increased by a factor of two, whilst calmodulin-dependent phosphorylation of a 57 kDa protein with ATP[S] had no effect. The data indicate that calmodulin itself regulates the calcium release channel of sarcoplasmic reticulum.

Identification of the multifunctional calmodulin-dependent protein kinase in the cytosol, sarcoplasmic reticulum, and sarcolemma of rabbit skeletal muscle

A multifunctional calmodulin-dependent protein kinase (calmodulin kinase) was purified from the cytosol of rabbit skeletal muscle as a subunit of 58 kDa. A 58-kDa protein in sarcoplasmic reticulum (SR) and sarcolemma (SL) of rabbit skeletal muscle was endogenously phosphorylated in a calmodulin-dependent manner. The 58-kDa protein in SR and SL was considered to be identical to the subunit of cytosol calmodulin kinase on the basis of immunoreactivity, calmodulin binding, and autophosphorylation studies and on the patterns of protease-treated phosphopeptides. Calmodulin kinase showed broad substrate specificity and phosphorylated troponins I and T.

Lack of effects of calcium X calmodulin-dependent phosphorylation on Ca2+ release from cardiac sarcoplasmic reticulum

Canine cardiac sarcoplasmic reticulum is phosphorylated by an endogenous calcium X calmodulin-dependent protein kinase and phosphorylation occurs mainly on a 27 kDa proteolipid, called phospholamban. To determine whether this phosphorylation has any effect on Ca2+ release, sarcoplasmic reticulum vesicles were phosphorylated by the calcium X calmodulin-dependent protein kinase, while non-phosphorylated vesicles were preincubated under identical conditions but in the absence of ATP to avoid phosphorylation. Both non-phosphorylated and phosphorylated vesicles were centrifuged to remove calmodulin, and subsequently used for Ca2+ release studies. Calcium loading was carried out either by the active calcium pump or by incubation with high (5 mM) calcium for longer periods. Phosphorylation of sarcoplasmic reticulum by calcium X calmodulin-dependent protein kinase had no appreciable effect on the initial rates of Ca2+ released from cardiac sarcoplasmic reticulum vesicles loaded under passive conditions and on the apparent 45Ca2+-40Ca2+ exchange from cardiac sarcoplasmic reticulum vesicles loaded under active conditions. Thus, it appears that calcium X calmodulin-dependent protein kinase mediated phosphorylation of cardiac sarcoplasmic reticulum is not involved in the regulation of Ca2+ release and 45Ca2+-40Ca2+ exchange.

Mono(ADP-ribosyl)ation of Ca2+-dependent ATPase in rabbit skeletal muscle sarcoplasmic reticulum and the effect of poly L-lysine

We investigated the endogenous mono(ADP-ribosyl)ation of the sarcoplasmic reticulum from rabbit skeletal muscle. The autoradiogram obtained after sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the [adenylate-32P]NAD-treated sarcoplasmic reticulum vesicles revealed a major band corresponding to the MW 105 K Ca2+-dependent ATPase and other bands corresponding to proteins of MW 153, 60 and 38 K and those of 125 to 135 K range. The addition of poly L-lysine during the incubation led to an enhancement of the modification. Poly L-lysine is proving to be a pertinent tool for identifying acceptor proteins.