Characterization of human cardiac calsequestrin and its deleterious mutants

Mutations of conserved residues of human cardiac calsequestrin (hCSQ2), a high-capacity, low-affinity Ca2+-binding protein in the sarcoplasmic reticulum, have been associated with catecholamine-induced polymorphic ventricular tachycardia (CPVT). In order to understand the molecular mechanism and pathophysiological link between these CPVT-related missense mutations of hCSQ2 and the resulting arrhythmias, we generated three CPVT-causing mutants of hCSQ2 (R33Q, L167H, and D307H) and two non-pathological mutants (T66A and V76M) and investigated the effect of these mutations. In addition, we determined the crystal structure of the corresponding wild-type hCSQ2 to gain insight into the structural effects of those mutations. Our data show clearly that all three CPVT-related mutations lead to significant reduction in Ca2+-binding capacity in spite of the similarity of their secondary structures to that of the wild-type hCSQ2. Light-scattering experiments indicate that the Ca2+-dependent monomer-polymer transitions of the mutants are quite different, confirming that the linear polymerization behavior of CSQ is linked directly to its high-capacity Ca2+ binding. R33Q and D307H mutations result in a monomer that appears to be unable to form a properly oriented dimer. On the other hand, the L167H mutant has a disrupted hydrophobic core in domain II, resulting in high molecular aggregates, which cannot respond to Ca2+. Although one of the non-pathological mutants, T66A, shares characteristics with the wild-type, the other null mutant, V76M, shows significantly altered Ca2+-binding and polymerization behaviors, calling for careful reconsideration of its status.

Surface plasmon resonance studies prove the interaction of skeletal muscle sarcoplasmic reticular Ca(2+) release channel/ryanodine receptor with calsequestrin

A high affinity molecular interaction is demonstrated between calsequestrin and the sarcoplasmic reticular Ca(2+) release channel/ryanodine receptor (RyR) by surface plasmon resonance. K(D) values of 92 nM and 102 nM for the phosphorylated and dephosphorylated calsequestrin have been determined, respectively. Phosphorylation of calsequestrin seems not to influence this high affinity interaction, i.e. calsequestrin might always be bound to RyR. However, the phosphorylation state of calsequestrin determines the amount of Ca(2+) released from the lumen. Dephosphorylation of approximately 1% of the phosphorylated calsequestrin could be enough to activate the RyR channel half-maximally, as we have shown previously [Szegedi et al., Biochem. J. 337 (1999) 19].

Cardiac sarcalumenin: phosphorylation, comparison with the skeletal muscle sarcalumenin and modulation of ryanodine receptor

Cardiac sarcoplasmic reticulum (SR) contains an endogenous phosphorylation system that under specific conditions phosphorylates two proteins with apparent molecular masses of 150 and 130 kDa. The conditions for their phosphorylation are as for the skeletal muscle sarcalumenin and the histidine-rich Ca2+ binding protein (HCP) with respect to: (i) Ca2+ and high concentrations of NaF are required; (ii) phosphorylation is obtained with no added Mg2+ and shows a similar time course and ATP concentration dependence; (iii) inhibition by similar concentrations of La3+; (iv) phosphorylation is obtained with [gamma-32P]GTP; (v) ryanodine binding is inhibited parallel to the phosphorylation of the two proteins. The endogenous kinase is identified as casein kinase II (CK II) based on its ability to use GTP as effectively as ATP, and its inhibition by La3+. The association of CK II with the cardiac SR, even after EGTA extraction at alkaline pH, is demonstrated using antibodies against CK II. The cardiac 130 kDa protein is identified as sarcalumenin based on its partial amino acid sequence and its blue staining with Stains-All. Cardiac sarcalumenin is different from the skeletal muscle protein based on electrophoretic mobilities, immunological analysis, peptide and phosphopeptide maps, as well as amino acid sequencing. Preincubation of SR with NaF and ATP, but not with NaF and AMP-PNP caused strong inhibition of ryanodine binding. This is due to decrease in Ca2+- and ryanodine-binding affinities of the ryanodine receptor (RyR) by about 6.6 and 18-fold, respectively. These results suggest that cardiac sarcalumenin is an isoform of the skeletal muscle protein. An endogenous CK II can phosphorylate sarcalumenin, and in parallel to its phosphorylation the properties of the ryanodine receptor are modified.

Identification, characterization and partial purification of a thiol-protease which cleaves specifically the skeletal muscle ryanodine receptor/Ca2+ release channel

A 94 kDa large subunit thiol-protease, as identified by anti-calpain antibodies, has been isolated from skeletal muscle junctional sarcoplasmic reticulum (SR). This protease cleaves specifically the skeletal muscle ryanodine receptor (RyR)/Ca2+ release channel at one site resulting in the 375 kDa and 150 kDa fragments. The 94 kDa thiol-protease degrades neither other SR proteins nor the ryanodine receptor of cardiac nor brain membranes. The partially purified 94 kDa protease, like the SR associated protease, had an optimal pH of about 7.0, was absolutely dependent on the presence of thiol reducing reagents, and was completely inhibited by HgCl2, leupeptin and the specific calpain I inhibitor. However, while the SR membrane-associated protease requires Ca2+ at a submicromolar concentration, the isolated thiol-protease has lost the Ca2+ requirement. The 94 kDa thiol-protease had no effect on ryanodine binding but modified the channel activity of RyR reconstituted into planar lipid bilayer: in a time-dependent manner, the channel activity decreases and within several minutes the channel is converted into a subconducting state. The protease-modified channel activity is still Ca2+-dependent and ryanodine sensitive. This 94 kDa thiol-protease cross react with anti-calpain antibodies thus, may represent the novel large subunit of the skeletal muscle specific calpain p94.

The identification of the phosphorylated 150/160-kDa proteins of sarcoplasmic reticulum, their kinase and their association with the ryanodine receptor

In the present work we studied the relationship between the phosphorylated 150- and 160-kDa proteins and other SR proteins in the 150,000-170,000 range of molecular masses. on SDS-PAGE, the identification of their kinase, as well as the purification and structural interactions between these proteins and the rynodine receptor (RyR). The phosphorylated 150-kDa protein was identified as sarcalumenin based on: (a) its cross-reactivity with three different monoclonal antibodies specific for sarcalumenin. (b) its mobility in SDS-PAGE which was modified upon digestion with endoglycosidase H, (c) its elution from lentil-lectin column by alpha-methyl mannoside, (d) its resistance to trypsin, (e) its ability to bind Ca2+ and to stain blue with Stains-All. The phosphorylated 160-kDa protein was identified as the histidine-rich Ca2+ binding protein (HCP) based on: (a) its Ca(2+)-binding property and staining blue with Stains-All, (b) phosphorylation with the catalytic subunit of cAMP-dependent kinase. (c) its increased mobility in SDS-PAGE in the presence of Ca2+ (d) its heat stability and (e) its partial amino acid sequence. The endogenous kinase was identified as casein kinase II (CK II) based on the inhibition of the endogenous phosphorylation 160/150-kDa proteins by heparin, 5.6-dichlorobenzimidazole riboside, polyaspartyl peptide and hemin, and its ability to use [gamma-32P]GTP as the phosphate donor. The association of CK II with SR membranes, was demonstrated using specific polyclonal anti-CK II antibodies. The luminal location of CK II is suggested because CK II was extracted from the SR by l M NaCl only after their treatment with hypotonic medium, and CK II activity was inhibited with the charged inhibitors heparin and polyaspartyl peptide only after their incubation with the SR in the presence of NP-40. The 160- and 150-kDa proteins were purified on spermine-agarose column, and were phosphorylated by CK II. Like the endogenous phosphorylation of the 150/160-kDa proteins in SR. the phosphorylation of the purified proteins by CK II was inhibited by La3+ (Cl50 = 4 microM) and hemin. The results suggest the phosphorylation of the luminally located sarcalumenin and HCP with CK II.

VDAC/porin is present in sarcoplasmic reticulum from skeletal muscle

In this study we demonstrate the existence of a protein with properties of the voltage-dependent anion channel (VDAC) in the sarcoplasmic reticulum (SR) using multiple approaches as summarized in the following: (a) 35 and 30 kDa proteins in different SR preparations, purified from other membranal systems by Ca2+/oxalate loading and sedimentation through 55% sucrose, cross-react with four different VDAC monoclonal antibodies. (b) Amino acid sequences of three peptides derived from the SR 35 kDa protein are identical to the sequences present in VDAC1 isoform. (c) Similar to the mitochondrial VDAC, the SR protein is specifically labeled by [14C]DCCD. (d) Using a new method, a 35 kDa protein has been purified from SR and mitochondria with a higher yield for the SR. (e) Upon reconstitution into a planar lipid bilayer, the purified SR protein shows voltage-dependent channel activity with properties similar to those of the purified mitochondrial VDAC or VDAC1/porin 31HL from human B lymphocytes, and its channel activity is completely inhibited by the anion transport inhibitor DIDS and about 80% by DCCD. We also demonstrate the translocation of ATP into the SR lumen and the phosphorylation of the luminal protein sarcalumenin by this ATP. Both ATP translocation and sarcalumenin phosphorylation are inhibited by DIDS, but not by atractyloside, a blocker of the ATP/ADP exchanger. These results indicate the existence of VDAC, thought to be located exclusively in mitochondria, in the SR of skeletal muscle, and its possible involvement in ATP transport. Together with recent studies on VDAC multicompartment location and its dynamic association with enzymes and channels, our findings suggest that VDAC deserves attention and consideration as a protein contributing to various cellular functions.

Endogenous, Ca(2+)-dependent cysteine-protease cleaves specifically the ryanodine receptor/Ca2+ release channel in skeletal muscle

The association of an endogenous, Ca(2+)-dependent cysteine-protease with the junctional sarcoplasmic reticulum (SR) is demonstrated. The activity of this protease is strongly stimulated by dithiothreitol (DTT), cysteine and beta-mercaptoethanol, and is inhibited by iodoacetamide, mercuric chloride and leupeptin, but not by PMSF. The activity of this thiol-protease is dependent on Ca2+ with half-maximal activity obtained at 0.1 microM and maximal activity at 10 microM. Mg2+ is also an activator of this enzyme (CI50 = 22 microM). These observations, together with the neutral pH optima and inhibition by the calpain I inhibitor, suggest that this enzyme is of calpain I type. This protease specifically cleaves the ryanodine receptor monomer (510 kD) at one site to produce two fragments with apparent molecular masses of 375 and 150 kD. The proteolytic fragments remain associated as shown by purification of the cleaved ryanodine receptor. The calpain binding site is identified as a PEST (proline, glutamic acid, serine, threonine-rich) region in the amino acid sequence GTPGGTPQPGVE, at positions 1356-1367 of the RyR and the cleavage site, the calmodulin binding site, at residues 1383-1400. The RyR cleavage by the Ca(2+)-dependent thiol-protease is prevented in the presence of ATP (1-5 mM) and by high NaCl concentrations. This cleavage of the RyR has no effect on ryanodine binding activity but stimulates Ca2+ efflux. A possible involvement of this specific cleavage of the RyR/Ca2+ release channel in the control of calpain activity is discussed.

Relation of phosphatidylinositol metabolism to glycolytic pathway in skeletal muscle membranes

Skeletal muscle triads are possessing the whole set of enzymes of the phosphatidylinositol (PI)-linked signal generating pathway, PI-kinase, PI(4)P-kinase, and PI(4,5)P2-phospholipase C (PLC). The activities of these enzymes are comparable to those found in other cell types for which a functional role of the PI-pathway in intracellular signal transduction has been established. For skeletal muscle an unequivocal function and an initiating signal for Ins(1,4,5)P3-liberation is still unknown. However, the observed Ca-dependency of PLC activity suggests that here Ins(1,4,5)P3 production is a consequence rather than a cause of increasing cytosolic Ca2+. Recently, the glycolytic enzyme aldolase, whose activity can be modulated by inositol polyphosphates, has been localized in the triadic structure. The enzyme which has a high affinity to Ins(1,4)P2, Ins(1,4,5)P3 and Ins(1,3,4,5)P4, seems to be compartmentalized to the junctional foot structure from which it is released upon binding of these molecules. This phenomenon could reflect a capability for regulation of the glycolytic flux even for aldolase, especially if a non steady-state situation in the junctional gap is considered. Meanwhile we have accumulated evidence for the operation of a partial glycolytic sequence in the junctional region established by the enzymes aldolase, glyceraldehyde-3-P (GAP) dehydrogenase and phosphoglycerate kinase. This system is able to produce ATP upon oxidation of GAP and could be, because of the inositol polyphosphate-sensing abilities of aldolase, a target for the membrane associated PI-pathway. The ATP production is however transient which indicates the coupling to an ATP hydrolyzing reaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of phosphorylase kinase subunits at the sarcoplasmic reticulum of rabbit skeletal muscle by monoclonal and polyclonal antibodies

Molecular structures related to phosphorylase kinase have been localized by light and electron microscopy in tissue sections of rabbit skeletal muscle employing polyclonal antibodies directed against the holoenzyme as well as monoclonal antibodies specific for its alpha-, beta- or gamma-subunits. In frozen sections of prefixed muscle fibres both known major regions of glycogen deposition, the intermyofibrillar space and the perinuclear area, are stained predominantly. In sections of unfixed muscle in which cytosolic phosphorylase kinase was removed by extensive washes prior to immunostaining the immunolabel is mainly associated with the sarcoplasmic reticulum (SR). This membrane location is further confirmed by immunoblot analysis of proteins solubilized from isolated SR with Triton X-114. Employing monoclonal antibodies two membrane proteins are identified as the alpha- and beta-subunits of phosphorylase kinase by Western blots. Immunoprecipitates reveal also the gamma-subunit; the delta-subunit, i.e., calmodulin, is enriched with the solubilized enzyme. It proves that a SR membrane associated form of holophosphorylase kinase exists in muscle. Functionally, this kinase might be involved in phosphorylation of phosphatidylinositol present on the SR Ca2+ transport ATPase and thereby might play a role in regulation of Ca2+ transport.

Activation of sarcoplasmic reticular Ca2+ transport ATPase by phosphorylation of an associated phosphatidylinositol

Approximately 1 mol phosphatidylinositol phosphate is formed per mol isolated Ca2+ transport ATPase when the enzyme is incubated with ATP/Mg2+. The phosphorylation of this enzyme-associated phosphatidylinositol represents the alkylphosphate formation described earlier. The phosphatidylinositol phosphate has been found in the hydrophobic core of the enzyme. A complex of phosphatidylinositol phosphate with protein can be extracted with acidic chloroform/methanol. The protein behaves like proteolipid during chromatography on Sephadex LH 60 and binds the radioactively labelled phosphatidylinositol phosphate. The phosphorylation of approximately 1 mol phosphatidylinositol per 100,000 g protein correlates with an enhancement of the Ca2+ transport ATPase activity which is due to an approximately 7-fold enhanced affinity for Ca2+ and an approximately 2-fold enhanced maximal turnover.

Regulation of the sarcoplasmic reticular Ca2+ transport ATPase by phosphorylation and dephosphorylation

At 0.1 mg/ml protein and 0.45 micrometers free Ca2+ l mol trichloroacetic acid precipitable phosphate is incorporated into 100,000 g SR protein as hydroxylamine sensitive acylphosphate. At nearly physiological protein concentration (ca. 7 mg/ml) a total of ca. 0.8 mol phosphate/100,000 g protein is incorporated, from which a fraction of 0.3 mol/100,000 g protein is insensitive to the hydroxylamine treatment, i.e. it is alkylphosphate. Phosphorylase kinase accelerates the alkylphosphate incorporation ca. 3-fold and enhances its final level to 0.7 mol/100,000 g protein. At 1.6 nm free Ca2+ alkylphosphate incorporation occurs at high SR concentration to a maximal extent of 0.5 mol/ 100,000 g protein. The incorporated alkylphosphate is present in comparable amounts in the 100,000 Mr Ca2+-transport ATP-ase and a polypeptide of Mr 9,000.