Identification and purification of a transverse tubule coupling protein which binds to the ryanodine receptor of terminal cisternae at the triad junction in skeletal muscle

Skeletal muscle calcium channel ryanodine binding activity in genetically unimproved and commercial turkey populations

The biochemical basis for the incidence of pale, soft, exudative (PSE) turkey meat was investigated by conducting ryanodine binding experiments on sarcoplasmic reticulum (SR) vesicles prepared from genetically unimproved and commercial turkeys. Ryanodine binding to the Ca2+ channel protein in SR vesicles from both populations of turkeys was activated at a threshold concentration of approximately 0.2 microM Ca2+, reached a plateau over the range of 3 to 30 microM free Ca2+, and was only slightly inhibited at 1 mM Ca2+. The SR fractions, enriched in the Ca(2+)-channel protein, from commercial turkeys exhibited a higher (P < 0.05) mean affinity for ryanodine when compared to that from unimproved turkeys (Kd = 12.2 vs 20.5 nM, respectively). A fourfold difference (P < 0.05) in mean Ca(2+)-channel protein content or Bmax (1.10 pmol/mg vs 4.01 pmol/mg) was observed between commercial and unimproved turkey SR fractions. The apparent difference in channel protein content between the two populations may be partially accounted for by the high abundance of a 75-kDa protein, as yet unidentified, observed in most commercial turkey samples on SDS polyacrylamide gels. The differences in ryanodine binding activity between these two populations of turkeys suggest that altered SR calcium channel protein activity, or altered channel regulation, may be associated with the increased incidence of PSE meat from turkeys selected for growth characteristics.

Purification, amino-terminal sequence and functional properties of a 64 kDa cytosolic protein from heart muscle capable of modulating calcium transport across the sarcoplasmic reticulum in vitro

In previous studies we have described the inhibitory action of a cytosolic protein fraction from heart muscle on ATP-dependent Ca2+ uptake by the sarcoplasmic reticulum (SR); further this inhibition was shown to be blocked by an inhibitor antagonist, also derived from the cytosol (Narayanan et al., Biochim. Biophys. Acta. 735: 53-66, 1983; Can. J. Physiol. Pharmacol. 67: 999-1006, 1989). Here we report the complete purification of the antagonist protein (AP) and characterization of its functional properties. AP was purified to homogeneity from rabbit heart cytosol using two procedures, one utilizing sequential DE52-cellulose and hydroxylapatite chromatography, and the other utilizing anion exchange chromatography on Mono Q HR 5/5 column in a Pharmacia FPLC system. The purified AP has an apparent molecular weight of 64 kDa; it is made up of about 43% hydrophobic and 57% hydrophilic residues with the following amino-terminal sequence: E-A-H-K-S-E-I-A-H-R-F-N-D-V-G-E-E-H-F-I-G-L-V-L-I-T-F-S-Q-Y-L-Q-K-X-P-Y- E-E-H-A . This partial amino acid sequence data indicate strong sequence homology to serum albumin (sequence homology: 85% to rat serum albumin and 74% to sheep and bovine serum albumin). The purified AP caused concentration-dependent-blockade of the inhibition of Ca2+ uptake by SR observed in the presence of the cytosolic Ca2+ uptake inhibitor protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of ryanodine receptors

Recent findings on the ryanodine receptor of vertebrates, a Ca-release channel protein for the caffeine- and ryanodine-sensitive Ca pools, are reviewed in this article. Three distinct genes, i.e., ryr1, ryr2, and ryr3, express different isoforms in specific locations: Ryr1 in skeletal muscle and Purkinje cells of cerebellum; Ryr2 in cardiac muscle and brain, especially cerebellum; Ryr3 in skeletal muscle of nonmammalian vertebrates, the corpus striatum, and limbic cortex of brain, smooth muscles, and the other cells in vertebrates. While only one isoform (Ryr1) is expressed in mammalian skeletal muscles, two isoforms (alpha- and beta-isoforms expressed by ryr1 and ryr3, respectively) are found in nonmammalian vertebrate skeletal muscles. Although the coexistence of two isoforms may merely be related to differentiation and specialization, the biological significance remains to be clarified. Ryanodine receptors in vertebrate skeletal muscles are believed to mediate two different modes of Ca release: Ca(2+)-induced Ca release and action potential-induced Ca release. All results obtained so far with any isoform of ryanodine receptor are related to Ca(2+)-induced Ca release and show very similar characteristics. Ca(2+)-induced Ca release, however, cannot be the underlying mechanism of Ca release on skeletal muscle activation. Susceptibility of the ryanodine receptor's ryanodine-binding activity to modification by physical factors, such as osmolality of the medium, might be related to action potential-induced Ca release. A hypothesis of molecular interaction in view of the plunger model of action potential-induced Ca release is discussed, suggesting that the model could be compatible with Ryr1 and Ryr3, but incompatible with Ryr2. The functional relevance of ryanodine receptor isoforms, especially Ryr3, in brain also remains to be clarified. Among ryr1 gene-related diseases, malignant hyperthermia was the first to be identified; however, there is still the possibility of involvement of the other genes. Central core disease has been added to the list recently. A molecular approach for the diagnosis and treatment of diseases is now in progress.

Adhesion of Golgi cisternae by proteinaceous interactions: intercisternal bridges as putative adhesive structures

We have investigated the nature of the component(s) responsible for holding the cisternal membranes of the Golgi complex into a stacked unit. Isolated Golgi complexes were treated with a variety of agents to induce the separation of intact Golgi stacks into single cisternal elements, i.e. “unstacking”, and the effects were analyzed and quantitated by electron microscopy. In control experiments, isolated, intact Golgi stacks were stable at 4 degrees C and 20 degrees C for &gt; or = 1 h; however, some unstacking occurred at 32 degrees C. Treatment of intact Golgi stacks with a variety of proteolytic enzymes resulted in a time- and dose-dependent unstacking of the cisternae, although stacks were resistant to various other proteases. Following liberation from the stack, single cisternae remained flattened with dilated rims. The integrity of intact Golgi stacks was unaffected by treatment with various concentrations and combinations of monovalent and divalent cations, or chelators of divalent cations. Electron microscopic observations of tannic acid- or negatively stained Golgi complexes, revealed the presence of highly structured, intercisternal “bridges”. When seen within intact Golgi complexes, these bridges were only consistently found between closely apposed cisternae and were not observed on dilated rims or secretory vesicles. These bridges, on both intact stacks and physically disrupted cisternae, were rectangular, being approximately 8.5 nm in width, approximately 11 nm in height. Treatment with proteases under conditions that resulted in the with proteases under conditions that resulted in the unstacking of intact complexes also removed these bridge structures. These data show that proteinaceous components are responsible for holding Golgi cisternae together into a cohesive, stacked unit, and identify a candidate bridge structure that could serve this purpose.

Isolation of two saxitoxin-sensitive sodium channel subtypes from rat brain with distinct biochemical and functional properties

Two different 3H-saxitoxin-binding proteins, with distinct biochemical and functional properties, were isolated from rat brain using a combination of anion exchange and lectin affinity chromatography as well as high resolution size exclusion and anion exchange HPLC. The alpha subunits of the binding proteins had different apparent molecular weights on SDS-PAGE (Type A: 235,000; Type B: 260,000). When reconstituted into planar lipid bilayers, the two saxitoxin-binding proteins formed sodium channels with different apparent single-channel conductances in the presence of batrachotoxin (Type A: 22 pS; Type B: 12 pS) and veratridine (Type A: 9 pS; Type B: 5 pS). The subtypes were further distinguished by scorpion (Leiurus quinquestriatus) venom which had different effects on single-channel conductance and gating of veratridine-activated Type A and Type B channels. Scorpion venom caused a 19% increase in single-channel conductance of Type A channels and a 35-mV hyperpolarizing shift in activation. Scorpion venom doubled the single-channel conductance of Type B channels and shifted activation by at least 85 mV.

Molecular interactions of the junctional foot protein and dihydropyridine receptor in skeletal muscle triads

Isolated triadic proteins were employed to investigate the molecular architecture of the triad junction in skeletal muscle. Immunoaffinity-purified junctional foot protein (JFP), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), aldolase and partially purified dihydropyridine (DHP) receptor were employed to probe protein-protein interactions using affinity chromatography, protein overlay and crosslinking techniques. The JFP, an integral protein of the sarcoplasmic reticulum (SR) preferentially binds to GAPDH and aldolase, peripheral proteins of the transverse (T)-tubule. No direct binding of JFP to the DHP receptor was detected. The interactions of JFP with GAPDH and aldolase appear to be specific since other glycolytic enzymes associated with membranes do not bind to the JFP. The DHP receptor, an integral protein of the T-tubule, also binds GAPDH and aldolase. A ternary complex between the JFP and the DHP receptor can be formed in the presence of GAPDH. In addition, the DHP receptor binds to a previously undetected Mr 95 K protein which is distinct from the SR Ca2+ pump and phosphorylase b. The Mr 95 K protein is an integral protein of the junctional domain of the SR terminal cisternae. It is also present in the newly identified "strong triads" (accompanying paper). From these findings, we propose a new model for the triad junction.

Does muscle activation occur by direct mechanical coupling of transverse tubules to sarcoplasmic reticulum?

Our knowledge of the physiological and biochemical constituents of skeletal muscle excitation has increased greatly during the last few years but this has not led to a consensus of the physiological mode of muscle activation. Three hypotheses of transmission, involving either transmitter-receptor interaction or direct mechanical coupling, are still under active consideration. The hypothesis of direct mechanical coupling currently being evaluated proposes that the dihydropyridine receptor in the transverse tubules serves as a voltage sensor that communicates directly with the junctional foot protein/Ca2+ channel of sarcoplasmic reticulum to initiate opening of the channel.

Biochemical properties of isolated transverse tubular membranes

This review addresses the major biochemical and structural characteristics of isolated transverse tubule (T-tubule) membranes, including methods of isolation and morphology of purified membranes, evaluation of attendant membrane activities, including ion pumps and channels, and structural and compositional analyses of functionally relevant components. Particular emphasis is placed on the Mg2+-ATPase, its localization in the T-system, its unusual kinetic properties, its possible functions, and its potential regulation by diacylglycerol and other biologically-relevant lipids. Conclusions are drawn with respect to the biochemical markers characteristic of T-tubule membranes and the criteria to be applied in the assessment of isolated T-tubule membrane purity.

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.

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.