Mass spectrometric identification of dystrophin isoform Dp427 by on-membrane digestion of sarcolemma from skeletal muscle

Although the membrane cytoskeletal protein dystrophin of 427 kDa and its tightly associated glycoprotein complex are drastically affected in muscular dystrophy, recent large-scale proteomic investigations did not identify full-length dystrophin in muscle preparations and were unable to determine its molecular fate in dystrophinopathy. Because conventional two-dimensional gel electrophoresis underrepresents many low-abundance and membrane-associated protein species and in-gel trypsination is often hampered by an inefficient digestion of certain target proteins, here we have applied direct on-membrane digestion of one-dimensional blots of the sarcolemma-enriched fraction and the isolated dystrophin-glycoprotein complex. This method succeeded in the mass spectrometric identification of dystrophin isoform Dp427 and associated glycoproteins as well as sarcolemmal dysferlin. In addition, protein bands representing established signature molecules of cross-contaminating membrane systems, such as the voltage-sensing dihydropyridine receptor of transverse tubules, the ryanodine receptor Ca²⁺-release channel of triad junctions, and the Ca²⁺-ATPase of the sarcoplasmic reticulum, were identified by mass spectrometry. Thus, proteomic approaches using on-membrane digestion might be suitable for future studies of low-abundance proteins, integral proteins, peripheral membrane proteins, and high-molecular-mass proteins. On-membrane digestion has the potential to develop into the method of choice for studying these classes of proteins, whose presence is otherwise missed by conventional gel electrophoresis-based proteomics.     

Evidence that platelet and skeletal sarcoplasmic reticulum Ca2+-ATPase are structurally distinct

Proteolytic digestion and indirect immunostaining were used to compare the platelet and sarcoplasmic reticulum Ca2+-ATPase proteins. When the platelet and sarcoplasmic reticulum Ca2+-ATPase proteins were digested in the native state with trypsin, the platelet Ca2+-ATPase, which had an apparent undigested molecular mass of 103 kDa, yielded 78-kDa and 25-kDa fragments. Calcium transport activity depended on the integrity of the 103-kDa protein, while the digested protein had residual ATPase activity. Tryptic digestion of the sarcoplasmic reticulum pump protein, which also had an undigested molecular mass of 103 kDa, yielded products with apparent molecular masses of 55 kDa, 36 kDa, and 26 kDa. Distinct patterns were also observed when the platelet and sarcoplasmic reticulum calcium pump proteins were digested with chymotrypsin and Staphylococcus aureus protease in the presence of sodium dodecyl sulfate. Chymotrypsin digestion of the platelet protein resulted in the appearance of products with apparent molecular masses of 70 kDa, 39 kDa, and 31 kDa, while a similar digestion of the sarcoplasmic reticulum calcium pump protein yielded 54-kDa, 52.5-kDa, 46-kDa, 41-kDa, and 36-kDa fragments. Exposure of the sarcoplasmic reticulum and platelet Ca2+-ATPase proteins to S. aureus protease also yielded dissimilar fragmentation patterns. These results indicate that the Ca2+-ATPases from platelets and sarcoplasmic reticulum are distinct proteins.     

Distinct immunopeptide maps of the sarcoplasmic reticulum Ca2+ release channel in malignant hyperthermia

Sarcoplasmic reticulum isolated from malignant hyperthermia-susceptible (MHS) muscle exhibits abnormalities in the regulation of calcium release. To identify the molecular basis of this abnormality, the Ca2+ release channel from both normal and MHS sarcoplasmic reticulum was examined using proteolytic digestion followed by immunoblot staining with a polyclonal antibody against the rabbit Ca2+ release channel protein. Under appropriate conditions, trypsin digestion of isolated sarcoplasmic reticulum vesicles from the two types of pigs revealed a distinct difference in the immunostaining pattern of the Ca2+ release channel-derived peptides. An approximate 86-kDa peptide was the predominant fragment in normal sarcoplasmic reticulum while an approximate 99-kDa peptide fragment was the major peptide detected in MHS sarcoplasmic reticulum. Digestion of sarcoplasmic reticulum vesicles isolated from four normal and four MHS pigs showed that the differences were highly reproducible. Trypsin digestion of sarcoplasmic reticulum isolated from heterozygous pigs, which contain one normal and one MHS allele, showed an antibody staining pattern that was intermediate between MHS and normal sarcoplasmic reticulum. These results can be explained by a primary amino acid sequence difference between the normal and MHS Ca2+ release channels and support the hypothesis that a mutation in the gene coding for the sarcoplasmic reticulum Ca2+ release channel is responsible for malignant hyperthermia.     

Altered stored calcium release in skeletal myotubes deficient of triadin and junctin

Triadin and junctin are integral sarcoplasmic reticulum membrane proteins that form a macromolecular complex with the skeletal muscle ryanodine receptor (RyR1) but their roles in skeletal muscle calcium homeostasis remain incompletely understood. Here we report that delivery of siRNAs specific for triadin or junctin into C2C12 skeletal myoblasts reduced the expression of triadin and junctin in 8-day-old myotubes by 80 and 100%, respectively. Knocking down either triadin or junctin in these cells reduced Ca2+ release induced by depolarization (10mM KCl) by 20-25%. Unlike triadin knockdown myotubes, junctin knockdown and junctin/triadin double knockdown myotubes also had reduced Ca2+ release induced by 400 microM 4-chloro-m-cresol, 10mM caffeine, 400 microM UTP, or 1 microM thapsigargin. Thus, knocking down junctin compromised the Ca2+ stores in the sarcoplasmic reticulum of these cells. Our subsequent studies showed that in junctin knockdown myotubes at least two sarcoplasmic reticulum proteins (RyR1 and skeletal muscle calsequestrin) were down-regulated while these proteins' mRNA expression was not affected. The results suggest that triadin has a role in facilitating KCl depolarization-induced Ca2+ release in contrast to junctin which has a role in maintaining sarcoplasmic reticulum Ca2+ store size in C2C12 myotubes.     

Fluorimetric coupled enzyme assay for lysoplasmalogenase activity in liver.

Two new proteins with apparent molecular masses of 53 kDa and 190 kDa have been identified in both sarcoplasmic reticulum and human blood platelets using a monoclonal antibody, FII1b5. The sarcoplasmic reticulum FII1b5 antigens were present in the terminal cisternae fraction, but were absent from light sarcoplasmic reticulum. The platelet and skeletal muscle proteins were not sensitive to digestion with endoglycosidase H under conditions that removed carbohydrate from the 53 kDa glycoprotein in sarcoplasmic reticulum or GPIIIa in platelet microsomes and did not bind 45Ca in a nitrocellulose overlay calcium-binding assay. These results distinguished the FII1b5 antigens from the 53 kDa glycoprotein and calsequestrin of sarcoplasmic reticulum. The 190 kDa platelet and sarcoplasmic reticulum proteins were extracted from membranes with high concentrations of NaCl, indicating that the high molecular mass FII1b5 antigens are peripherally associated with the bilayers. In contrast, the platelet and muscle 53 kDa proteins remained membrane-bound in the presence of high salt concentrations, suggesting that they are integral proteins.     

Comparison of the (Ca2+ + Mg2+)-ATPase proteins from normal and dystrophic chicken sarcoplasmic reticulum.

The involvement of membrane protein in dystrophic chicken fragmented sarcoplasmic reticulum alterations has been examined. A purified preparation of the (Ca2+ + Mg2+)-ATPase protein from dystrophic fragmented sarcoplasmic reticulum was found to have a reduced calcium-sensitive ATPase activity and phosphoenzyme level, in agreement with alterations found in dystrophic chicken fragmented sarcoplasmic reticulum. An amino acid analysis of the ATPase preparations showed no difference in the normal and dystrophic (Ca2+ + Mg2+)-ATPase. The (Ca2+ + Mg2+)-ATPase was investigated further by isoelectric focusing and proteolytic digestion of the fragmented sarcoplasmic reticulum. Neither of these methods indicated any alteration in the composition of the dystrophic (Ca2+ + Mg2+)-ATPase. We have concluded that the alterations observed in dystrophic fragmented sarcoplasmic reticulum are not due to increased amounts of non-(Ca2+ + Mg2+)-ATPase protein, and that the normal and dystrophic (Ca2+ + Mg2+)-ATPase protein are not detectably different.     

Isolation of highly active preparations of sarcoplasmic reticulum and Ca2-dependent ATPase from cardiac muscle]

A microsomal preparation with a high ability for Ca2+ uptake has been isolated from pigeon heart. A method of further purification of Ca2+-accumulating system of heart, based on the ability of sarcoplasmic reticulum for the energy-dependent Ca2+ accumulation in the presence of oxalate, has been developed. Upon centrifugation in the gradient of sucrose and KCl concentration the fragments of sarcoplasmic reticulum, rendered "heavy" by calcium oxalate, can be separated from foreign cell membranes. The main component of heart "calcium pump" is Ca2+-dependent ATPase (making up to about 50% of all proteins of the purified reticulum), having a molecular weight of 100.000--105.000. Specific activity of heart Ca2+-ATPase as well as the ability of purified heart sarcoplasmic reticulum for Ca2+ uptake are only slightly less than those of the skeletal muscle reticulum. The data obtained suggest that heart sarcoplasmic reticulum may be efficient for providing heart muscle relaxation.     

Studies on the heterogeneity of sarcoplasmic reticulum vesicles.

Isolated sarcoplasmic reticulum vesicles from rabbit white muscle were separated into a light (15--20% of total microsomes) and a heavy (80--85%) fraction by density gradient centifugation. The ultrastructure, chemical composition, enzymic activities and localization of membrane components in the vesicles of both fractions were investigated. From the following results it was concluded that both fractions are derived from the membranes of the sarcoplasmic reticulum system of the muscle: (i) The protein pattern of both fractions is essentially the same, except for different ratios of acidic, Ca2+-binding proteins. (ii) The 105000 dalton protein of the light fraction cross-reacts immunologically with the Ca2+-dependent ATPase of the heavy fraction. (iii) Ca2+-dependent ATPase, although of different specific activity, is found in both fractions. After rendering the vesicles leaky, specific activities in both fractions reach the same value. The light fraction was found to consist of "inside-out" vesicles by the following criteria: (i) No Ca2+ accumulation can be measured and the Ca2+-dependent ATPase activity is low and variable. (ii) The rate of trypsin digestion is lower and, compared to the heavy microsomes, a different ratio of degradation products is obtained. (iii) The sarcoplasmic reticulum membrane has a highly asymmetrical lipid distribution. This distribution of aminophospholipids is opposite to that in vesicles of heavy fraction. The light sarcoplasmic reticulum fraction has a higher phospholipid to protein ratio than the heavy one. This is consistent with the possibility that the two fractions derive from different parts of the sarcoplasmic reticulum system.     

Molecular cloning and expression of cDNA encoding the 53,000-dalton glycoprotein of rabbit skeletal muscle sarcoplasmic reticulum

The 53-kDa glycoprotein of rabbit skeletal muscle sarcoplasmic reticulum was purified by lentil lectin affinity chromatography and preparative polyacrylamide gel electrophoresis and partially sequenced. Polyclonal and monoclonal antibodies were raised against the 53-kDa glycoprotein and found to cross-react with the 160-kDa glycoprotein. A combination of antibody and synthetic oligonucleotide screening was used to isolate a cDNA encoding the 53-kDa glycoprotein of rabbit fast-twitch skeletal muscle sarcoplasmic reticulum. The cDNA encodes a protein of 453 amino acids with Mr of 52,421 and a 19-residue amino-terminal signal sequence. The deduced sequence contains two potential glycosylation sites and is largely hydrophilic. The presence of a glycine-rich sequence in the glycoprotein with homology to mononucleotide binding domains supports earlier observations that the glycoprotein binds ATP with high affinity. Although two sequences appear to be hydrophobic on a hydropathy plot, they are not sufficiently long nor sufficiently hydrophobic to qualify unambiguously as transmembrane sequences. The glycoprotein, like calsequestrin, was shown to be inaccessible to trypsin in intact sarcoplasmic reticulum. It can be eluted from the sarcoplasmic reticulum by extraction with [ethylenebis(oxyethylenenitrilo)]tetraacetic acid under hypotonic conditions. Thus, the glycoprotein appears to be localized entirely in the lumen of the sarcoplasmic reticulum and to be associated with the inner membrane surface through Ca2+-dependent mechanisms. Cotransfection of COS-1 cells with cDNAs encoding the glycoprotein and the Ca2+-ATPase led to expression of both proteins with a common localization in the microsomal fraction. The Ca2+ pumping activity of the microsomes isolated from transfected cells was unaltered by the presence of the glycoprotein. Thus the glycoprotein does not appear to modulate Ca2+-ATPase function.     

Purification and characterization of a calcium-calmodulin-dependent phospholamban kinase from canine myocardium

A Ca2+-calmodulin-dependent protein kinase was purified to apparent homogeneity from the cytosolic fraction of canine myocardium, with phospholamban as substrate. Purification involved sequential chromatography on DEAE-cellulose, calmodulin-agarose, DEAE-Bio-Gel A, and phosphocellulose. This procedure resulted in a 987-fold purification with a 5.4% yield. The purified enzyme migrated as a single band on native polyacrylamide gels, and it exhibited an apparent molecular weight of 550,000 upon gel filtration. Gel electrophoresis under denaturing conditions revealed a single protein band with Mr 55,000. The purified kinase could be autophosphorylated in a Ca2+-calmodulin-dependent manner, and under optimal conditions, 6 mol of Pi was incorporated per mole of 55,000-dalton subunit. The activity of the enzyme was dependent on Ca2+, calmodulin, and ATP.Mg2+. Other ions which could partially substitute for Ca2+ in the presence of Mg2+ and saturating calmodulin concentrations were Sr2+ greater than Mn2+ greater than Zn2+ greater than Fe2+. The substrate specificity of the purified Ca2+-calmodulin-dependent protein kinase for cardiac proteins was determined by using phospholamban, troponin I, sarcoplasmic reticulum membranes, myofibrils, highly enriched sarcolemma, and mitochondria. The protein kinase could only phosphorylate phospholamban and troponin I either in their purified forms or in sarcoplasmic reticulum membranes and myofibrils, respectively. Exogenous proteins which could also be phosphorylated by the purified protein kinase were skeletal muscle glycogen synthase greater than gizzard myosin light chain greater than brain myelin basic protein greater than casein. However, phospholamban appeared to be phosphorylated with a higher rate as well as affinity than glycogen synthase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Calmodulin-mediated regulation of calcium transport and (Ca2+ + Mg2+)-activated ATPase activity in isolated cardiac sarcoplasmic reticulum

A severalfold activation of calcium transport and (Ca2+ + Mg2+)-activated ATPase activity by micromolar concentrations of calmodulin was observed in sarcoplasmic reticulum vesicles obtained from canine ventricles. This activation was seen in the presence of 120 mM KCl. The ratio of moles of calcium transported per mol of ATP hydrolyzed remained at about 0.75 when calcium transport and (Ca2+ + Mg2+)-activated ATPase activity were measured in the presence and absence of calmodulin. Thus, the efficiency of the calcium transport process did not change. Stimulation of calcium transport by calmodulin involves the phosphorylation of one or more proteins. The major 32P-labeled protein, as determined by sodium dodecyl sulfate slab gel electrophoresis, was the 22,000-dalton protein called phospholamban. The Ca2+ concentration dependency of calmodulin-stimulated microsomal phosphorylation corresponded to that of calmodulin-stimulated (Ca2+ + Mg2+)-activated ATPase activity. Proteins of 11,000 and 6,000 daltons and other proteins were labeled to a lesser extent. A similar phosphorylation pattern was obtained when microsomes were incubated with cAMP-dependent protein kinase and ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. Phosphorylation produced by added cAMP-dependent protein kinase and calmodulin was additive. These studies provided further evidence for Ca2+-dependent regulation of calcium transport by calmodulin in sarcoplasmic reticulum that could play a role in the beat-to-beat regulation of cardiac relaxation in the intact heart.     

Synthesis of the calcium transport ATPase of sarcoplasmic reticulum and other muscle proteins during development of muscles cells in vivo and in vitro.

The effect of medium Ca2+ concentration upon the concentration and the rate of synthesis of muscle proteins was investigated in chicken pectoralis muscle cultures. There is an easily identifiable class of muscle protein which includes the Ca2+-ATPase of sarcoplasmic reticulum, myosin, troponin C, ATP : creatine phosphotransferase, muscle specific actin, tropomysin 1 and 2, and muscle hemagglutinin, which show a large increase in concentration during normal development. The increased synthesis of these proteins was inhibited, without inhibition of cell proliferation, in culture media of relatively low Ca2+ concentration, 0.05--0.3 mM, where fusion was prevented. Similar medium Ca2+ concentration was required for the expression of all these proteins, suggesting their coordinate regulation. The proteins are denoted as 'calcium-modulated proteins'. The increased Ca2+ transport activity of sarcoplasmic reticulum in cultured chicken pectoralis muscle cells during development at 1.8 mM medium calcium concentration represents de novo synthesis of the Ca2+ transport ATPase, as shown by immunoprecipitation, active site labeling and direct identification of the Ca2+ transport ATPase on two-dimensional gel electropherograms of whole muscle homogenates. The concentration and the turnover rate of the majority of the muscle proteins is not affected significantly by medium Ca2+ concentration between 0.06 and 1.8 mM. It is proposed that increase in cytoplasmic free Ca2+ concentration during fusion plays a central role in the regulation of the synthesis of calcium-modulated proteins.     

Hydrophobic photolabelling of sarcoplasmic reticulum with [125I]TID

[125I]TID, a small photoreactive lipophylic reagent, was used to label intrinsic proteins of rabbit and rat sarcoplasmic reticulum membranes. A 160,000 glycoprotein, the Ca2+-ATPase and polypeptides of mol. wt 53-55,000, 30,000, 20,000 and 6000 dalton were labelled suggesting that these proteins are integral membrane components.     

Ca2+ signalling and muscle disease.

Transient elevations of intracellular Ca2+ play a signalling role in such complex cellular functions as contraction, secretion, fertilization, proliferation, metabolism, heartbeat and memory. However, prolonged elevation of Ca2+ above about 10 microM is deleterious to a cell and can activate apoptosis. In muscle, there is a narrow window of Ca2+ dysregulation in which abnormalities in Ca2+ regulatory proteins can lead to disease, rather than apoptosis. Key proteins in the regulation of muscle Ca2+ are the voltage-dependent, dihydropyridine-sensitive, L-type Ca2+ channels located in the transverse tubule and Ca2+ release channels in the junctional terminal cisternae of the sarcoplasmic reticulum. Abnormalities in these proteins play a key role in malignant hyperthermia (MH), a toxic response to anesthetics, and in central core disease (CCD), a muscle myopathy. Sarco(endo)plasmic reticulum Ca2+ ATPases (SERCAs) return sarcoplasmic Ca2+ to the lumen of the sarcoplasmic reticulum. Loss of SERCA1a Ca2+ pump function is one cause of exercise-induced impairment of the relaxation of skeletal muscle, in Brody disease. Phospholamban expressed in cardiac muscle and sarcolipin expressed in skeletal muscle regulate SERCA activity. Studies with knockout and transgenic mice show that gain of inhibitory function of phospholamban alters cardiac contractility and could be a causal feature in some cardiomyopathies. Calsequestrin, calreticulin, and a series of other acidic, lumenal, Ca2+ binding proteins provide a buffer for Ca2+ stored in the sarcoplasmic reticulum. Overexpression of cardiac calsequestrin leads to cardiomyopathy and ablation of calreticulin alters cardiac development.     

Rapid purification of calsequestrin from cardiac and skeletal muscle sarcoplasmic reticulum vesicles by Ca2+-dependent elution from phenyl-sepharose

Treatment of cardiac or skeletal muscle sarcoplasmic reticulum vesicles with 0.1 M sodium carbonate selectively extracts both the Ca2+-binding protein calsequestrin and the two "intrinsic glycoproteins," while leaving the Ca2+-dependent ATPase membrane bound. Phenyl-Sepharose chromatography in the presence of ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) and high salt (0.5 M NaCl) readily fractionates these solubilized proteins into a Ca2+-elutable fraction, which contains purified calsequestrin, and a low ionic strength elutable fraction, which contains one of the two intrinsic glycoproteins. Elution of calsequestrin from phenyl-Sepharose occurs near 1 mM Ca2+. Copurifying with calsequestrin are an homologous set of high molecular weight proteins, which like calsequestrin stain blue with Stains-All. These proteins are present in trace amounts and do not correspond to any sarcoplasmic reticulum proteins previously identified. Elution of calsequestrin from phenyl-Sepharose is consistent with the Ca2+-binding protein losing its hydrophobic character in the presence of millimolar Ca2+. This behavior is converse to that observed for several calmodulin-like proteins, which are eluted from hydrophobic gels in the presence of EGTA. The high yield and purity of calsequestrin prepared by this method makes possible a unique system for studying what may be a distinct class of Ca2+-binding proteins.     

Evidence of endoplasmic reticulum-related Ca2+ ATPase in human microvascular endothelial cells

We have demonstrated by immunological and molecular methods the presence of a reticulum endoplasmic-related Ca2+-ATPase in human omental microvascular endothelial cells (HOME cells). HOME cells reacted positively with a previously characterized sarcoplasmic reticulum Ca2+-ATPase antibody as demonstrated by indirect immunofluorescence. Western blotting revealed that the antibody recognized a 95-100 kDa protein. 35S-Metabolic labeling led to the detection of a similar protein with which the purified sarcoplasmic reticulum Ca2+-ATPase competed. Dot-blotting experiments indicated that a substantial amount of Ca2+-ATPase was present in HOME cell membranes. In addition, Northern blot analysis using a cDNA probe from cardiac sarcoplasmic reticulum showed the presence of mRNA species of 4-kb. As these experiments were conducted in comparison with cell types with well-defined Ca2+-ATPase in HOME cells.     

Trypsin destruction of the high affinity ryanodine binding sites of the junctional sarcoplasmic reticulum

Tryptic digestion of the junctional sarcoplasmic reticulum membranes in sucrose but not NaCl buffer leads to complete loss of ryanodine binding capacity. The presence of MgCl2 in the sucrose buffer prevents the loss of ryanodine binding by the trypsin treatment. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the treated membranes reveal that the 400-kDa protein band disappeared under all the different digestion conditions. However, the presence of 135-kDa tryptic fragment is observed only when ryanodine binding is retained. Quantitative analysis of the gels shows that the loss of ryanodine binding is well correlated with the cleavage of the 135-kDa tryptic fragment. This correlation is obtained when the cleavage was controlled either by the digestion time or by NaCl or MgCl2 concentrations. The same concentrations of MgCl2 and NaCl affect the ryanodine binding activity, the cleavage of the 135-kDa tryptic fragment, and the solubility and stability of the [3H]ryanodine-receptor complex in a detergent-containing medium. Tryptic digestion of the ryanodine receptor/junctional Ca2+ release channel, which leads to complete loss of ryanodine binding capacity, has no effect or slightly stimulates the Ca2+ accumulation activity of these membranes.     

Separation of vesicles of cardiac sarcolemma from vesicles of cardiac sarcoplasmic reticulum. Comparative biochemical analysis of component activities.

Sarcolemmal and sarcoplasmic reticulum membrane vesicle fractions were isolated from cardiac microsomes. Separation of sarcolemmal and sarcoplasmic reticulum membrane markers was documented by a combination of correlative assay and centrifugation techniques. To facilitate the separation, the crude microsomes were incubated in the presence of ATP, Ca2+, and oxalate to increase the density of the sarcoplasmic reticulum vesicles. After sucrose gradient centrifugation, the densest subfraction (sarcoplasmic reticulum) contained the highest (K+,Ca2+)-ATPase activity and virtually no (Na2+,K+)-ATPase activity, even when latent (Na+,K+)-ATPase activity was unmasked. In addition, the sarcoplasmic reticulum fraction contained no significant sialic acid, beta receptor binding activity, or adenylate cyclase activity. Sarcolemmal membrane fractions were of low buoyant density. Preparations most enriched in sarcolemmal vesicles contained the highest level of all the other parameters and only about 10% of the (K+,Ca2+)-ATPase activity of the sarcoplasmic reticulum fraction. The results suggest that (Na+,K+)-ATPase, sialic acid, beta-adrenergic receptors, and adenylate cyclase can be entirely accounted for by the sarcolemmal content of cardiac microsomes. Gel electrophoresis of the sarcolemmal and sarcoplasmic reticulum membrane fractions showed distinct bands. Membrane proteins exclusive to each of the fractions were also demonstrated by phosphorylation. Cyclic AMP stimulated phosphorylation by [gamma-32P]ATP of two proteins of apparent Mr = 20,000 and 7,000 that were concentrated in sarcoplasmic reticulum, but the stimulation was markedly dependent on the presence of added soluble cyclic AMP-dependent protein kinase. Cyclic AMP also stimulated phosphorylation of membrane proteins in sarcolemma, but this phosphorylation was mediated by an endogenous protein kinase activity. The apparent molecular weights of these phosphorylated proteins were 165,000, 90,000, 56,000, 24,000, and 11,000. The results suggest that sarcolemma may contain an integral enzyme complex, not present in sarcoplasmic reticulum, that contains beta-adrenergic receptors, adenylate cyclase, cyclic AMP-dependent protein kinase, and several substrates of the protein kinase.     

Influence of monovalent cations on the Ca2+-ATPase of sarcoplasmic reticulum isolated from rabbit skeletal and dog cardiac muscles. An interpretation of transient-state kinetic data

Transient-state kinetics of phosphorylation and dephosphorylation of the Ca2+-ATPase of sarcoplasmic reticulum vesicles from rabbit skeletal and dog cardiac muscles were studied in the presence of varying concentrations of monovalent and divalent cations. Monovalent cations affect the two types of sarcoplasmic reticulum differently. When the rabbit skeletal sarcoplasmic reticulum was Ca2+ deficient, preincubation with K+ (as compared with preincubation with choline chloride) did not affect initial phosphorylation at various concentrations of Ca2+, added with ATP to phosphorylate the enzyme. This is in contrast to preincubation with K+ of the Ca2+-deficient dog cardiac sarcoplasmic reticulum, which resulted in an increase in the phosphoenzyme level. When Ca2+ was bound to the rabbit skeletal sarcoplasmic reticulum, K+ inhibited E - P formation; but under the same conditions, E - P formation of dog cardiac sarcoplasmic reticulum was activated by K+ at 12 microM Ca2+ and inhibited at 0.33 and 1.3 microM Ca2+. Li+, Na+ and K+ also have different effects on E - P decomposition of skeletal and cardiac sarcoplasmic reticulum. The latter responded less to these cations than the former. Studies with ADP revealed differences between the two types of sarcoplasmic reticulum. For rabbit skeletal sarcoplasmic reticulum, 40% of the phosphoenzyme formed was 'ADP sensitive', and the decay of the remaining E - P was enhanced by K+ and ADP. Dog cardiac sarcoplasmic reticulum yielded about 40--48% ADP-sensitive E - P, but the decomposition rate of the remaining E - P was close to the rate measured in the absence of ADP. Thus, these studies showed certain qualitative differences in the transformation and decomposition of phosphoenzymes between skeletal and cardiac muscle which may have bearing on physiological differences between the two muscle types.     

Optical probe responses on sarcoplasmic reticulum: oxacarbocyanines as probes of membrane potential.

The relationship between Ca2+ fluxes and the ion diffusion potential was analyzed on sarcoplasmic reticulum membranes using oxacarbocyanine dyes as optical probes for membrane potential. 3.3'-Diethyloxodicarbocyanine responds to ATP-induced Ca2+ uptake by isolated sarcoplasmic reticulum vesicles with a decrease in absorbance at 600 nm. The optical change is reversed during Ca2+ release from sarcoplasmic reticulum induced by KCl or by ADP and inorganic phosphate. The absorbance changes are largely attributable to the binding of accumulated Ca2+ to the membrane. There is no indication that sustained changes in membrane diffusion potential would accompany pump-mediated Ca2+ fluxes. A large change in the absorbance of 3,3'-diethyloxodicarbocyanine was observed on sarcoplasmic reticulum vesicles under the influence of membrane potential generated by valinomycin in the presence of a K+ gradient or by ionophore A23187 in the presence of a Ca2+ gradient. The maximum of the potential-dependent absorbance change is at 575--580 nm. The potentials generated by valinomycin or ionophore A23187 are short-lived due to the high permeability of sarcoplasmic reticulum membranes for cations and anions. There is no correlation between the direction and magnitude of the artifically imposed membrane potential and the rate of Ca2+ uptake or release by isolated sarcoplasmic reticulum vesicles.