Biochemical properties of purified transverse tubules isolated from skeletal muscle triads

Myotubularin and PtdIns3P remodel the sarcoplasmic reticulum in muscle in vivo

The sarcoplasmic reticulum (SR) is a specialized form of endoplasmic reticulum (ER) in skeletal muscle and is essential for calcium homeostasis. The mechanisms involved in SR remodeling and maintenance of SR subdomains are elusive. In this study, we identified myotubularin (MTM1), a phosphoinositide phosphatase mutated in X-linked centronuclear myopathy (XLCNM, or myotubular myopathy), as a key regulator of phosphatidylinositol 3-monophosphate (PtdIns3P) levels at the SR. MTM1 is predominantly located at the SR cisternae of the muscle triads, and Mtm1-deficient mouse muscles and myoblasts from XLCNM patients exhibit abnormal SR/ER networks. In vivo modulation of MTM1 enzymatic activity in skeletal muscle using ectopic expression of wild-type or a dead-phosphatase MTM1 protein leads to differential SR remodeling. Active MTM1 is associated with flat membrane stacks, whereas dead-phosphatase MTM1 mutant promotes highly curved cubic membranes originating from the SR and enriched in PtdIns3P. Overexpression of a tandem FYVE domain with high affinity for PtdIns3P alters the shape of the SR cisternae at the triad. Our findings, supported by the parallel analysis of the Mtm1-null mouse and an in vivo study, reveal a direct function of MTM1 enzymatic activity in SR remodeling and a key role for PtdIns3P in promoting SR membrane curvature in skeletal muscle. We propose that alteration in SR remodeling is a primary cause of X-linked centronuclear myopathy. The tight regulation of PtdIns3P on specific membrane subdomains may be a general mechanism to control membrane curvature.

Interaction of triadin with histidine-rich Ca(2+)-binding protein at the triadic junction in skeletal muscle fibers

The present study documents the binding interaction of skeletal muscle sarcoplasmic reticulum (SR) transmembrane protein triadin with peripheral histidine-rich, Ca(2+)-binding protein (HCP). In addition to providing further evidence that HCP coenriches with RyR1, FKBP-12, triadin and calsequestrin (CS) in sucrose-density-purified TC vesicles, using specific polyclonal antibody, we show it to be expressed as a single protein species, both in fast-twitch and slow-twitch fibers, and to identically localize to the I-band. Colocalization of HCP and triadin at junctional triads is supported by the overlapping staining pattern using monoclonal antibodies to triadin. We show a specific binding interaction between digoxigenin-HCP and triadin, using ligand blot techniques. The importance of this finding is strengthened by the similarities in binding affinity and in Ca2+ dependence, (0.1-1 mM Ca2+) of the interaction of digoxigenin-HCP with immobilized TC vesicles. Suggesting that triadin dually interacts with HCP and with CS, at distinct sites, we have found that triadin-CS interaction in overlays does not require the presence of Ca2+. Consistent with the binding of CS to triadin luminal domain (Guo and Campbell, 1995), we show that binding sites for digoxigenin-CS, although not binding sites for digoxigenin-HCP, can be recovered in the 92 kDa triadin fragment, after chymotryptic cleavage of the NH2-terminal end of the folded molecule in intact TC vesicles. These differential effects form the basis for the hypothesis that HCP anchors to the junctional membrane domain of the SR, through binding to triadin short cytoplasmic domain at the NH2 terminus. Although the function of this interaction, as such, is not well understood, it seems of potential biological interest within the more general context of the structural-functional role of triadin at the triadic junction in skeletal muscle.

Interaction of triadin with histidine-rich Ca(2+)-binding protein at the triadic junction in skeletal muscle fibers

The present study documents the binding interaction of skeletal muscle sarcoplasmic reticulum (SR) transmembrane protein triadin with peripheral histidine-rich, Ca(2+)-binding protein (HCP). In addition to providing further evidence that HCP coenriches with RyR1, FKBP-12, triadin and calsequestrin (CS) in sucrose-density-purified TC vesicles, using specific polyclonal antibody, we show it to be expressed as a single protein species, both in fast-twitch and slow-twitch fibers, and to identically localize to the I-band. Colocalization of HCP and triadin at junctional triads is supported by the overlapping staining pattern using monoclonal antibodies to triadin. We show a specific binding interaction between digoxigenin-HCP and triadin, using ligand blot techniques. The importance of this finding is strengthened by the similarities in binding affinity and in Ca2+ dependence, (0.1-1 mM Ca2+) of the interaction of digoxigenin-HCP with immobilized TC vesicles. Suggesting that triadin dually interacts with HCP and with CS, at distinct sites, we have found that triadin-CS interaction in overlays does not require the presence of Ca2+. Consistent with the binding of CS to triadin luminal domain (Guo and Campbell, 1995), we show that binding sites for digoxigenin-CS, although not binding sites for digoxigenin-HCP, can be recovered in the 92 kDa triadin fragment, after chymotryptic cleavage of the NH2-terminal end of the folded molecule in intact TC vesicles. These differential effects form the basis for the hypothesis that HCP anchors to the junctional membrane domain of the SR, through binding to triadin short cytoplasmic domain at the NH2 terminus. Although the function of this interaction, as such, is not well understood, it seems of potential biological interest within the more general context of the structural-functional role of triadin at the triadic junction in skeletal muscle.

Use of thermal analysis to distinguish magnesium and calcium stimulated ATPase activity in isolated transverse tubules from skeletal muscle

The presence of calcium stimulated adenosine triphosphatase (Ca2+,Mg(2+)-ATPase) activity in isolated transverse tubule (t-tubule) membranes is distinguished from magnesium adenosine triphosphatase (Mg(2+)-ATPase) activity on the basis of differing thermal stabilities. The Mg(2+)-ATPase is the major protein component of the t-tubule membrane, and it can be difficult to discriminate between the low levels of Ca2+ stimulated ATPase activity found in isolates of t-tubules compared to the much higher Mg(2+)-ATPase activity. Thermal analysis reveals different inactivation temperatures (Ti) for the proteins responsible for ATP dependent calcium transport (Ti = 49 degrees C) and Mg(2+)-ATPase activity (Ti = 57 degrees C) in isolated t-tubule membranes. The differential scanning calorimetry profile of t-tubule membranes consists of three major components with transition temperatures (Tm) of 51 degrees C, 57 degrees C and 63 degrees C. Denaturation of the component with Tm = 57 degrees C correlates with inactivation of Mg(2+)-ATPase activity, and denaturation of the Tm = 51 degrees C component correlates with the inactivation of Ca2+,Mg(2+)-ATPase activity and calcium transport. The functions of the t-tubule membrane component or components that denature with Tm = 63 degrees C have yet to be identified. The lack of stimulation of calcium transport in isolated t-tubules by oxalate, the impermeability of isolated t-tubules to oxalate, and experiments performed on t-tubules with defined amounts of sarcoplasmic reticulum (SR) added suggest that contamination of the isolated t-tubules by SR is unlikely to account for the level of Ca2+,Mg(2+)-ATPase activity detected. The presence of a Ca2+,Mg(2+)-ATPase in the t-tubule membrane would provide a mechanism that may be involved in the partial removal of calcium that is accumulated in the junctional space during muscle relaxation or calcium that is released from the terminal cisternae of sarcoplasmic reticulum during excitation-contraction coupling.

Ecto-ATPases: identities and functions

Ecto-ATPases are ubiquitous in eukaryotic cells. They hydrolyze extracellular nucleoside tri- and/or diphosphates, and, when isolated, they exhibit E-type ATPase activity, (that is, the activity is dependent on Ca2+ or Mg2+, and it is insensitive to specific inhibitors of P-type, F-type, and V-type ATPases; in addition, several nucleotide tri- and/or diphosphates are hydrolysed, but nucleoside monophosphates and nonnucleoside phosphates are not substrates). Ecto-ATPases are glycoproteins; they do not form a phosphorylated intermediate during the catalytic cycle; they seem to have an extremely high turnover number; and they present specific experimental problems during solubilization and purification. The T-tubule Mg2+-ATPase belongs to this group of enzymes, which may serve at least two major roles: they terminate ATP/ADP-induced signal transduction and participate in adenosine recycling. Several other functions have been discussed and identity to certain cell adhesion molecules and the bile acid transport protein was suggested on the basis of cDNA clone isolation and immunological work.

Co-localization of the dihydropyridine receptor and the cyclic AMP-binding subunit of an intrinsic protein kinase to the junctional membrane of the transverse tubules of skeletal muscle

Junctional transverse tubules (TT) isolated from triads of rabbit skeletal muscle by centrifugation in an ion-free sucrose gradient were compared with membrane subfractions, predominantly derived from the free portion of TT, that had been purified from sarcoplasmic reticulum membrane contaminants by three different methods. The markers used were diagnostic membrane markers and the dihydropyridine (DHP) receptor, which is a specific marker of the junctional membrane of TT. Junctional TT have a high membrane density (Bmax. 60 pmol/mg of protein) of high-affinity (Kd 0.25 nM) DHP-binding sites using [3H]PN200-110 as the specific ligand. When analysed by SDS/PAGE under reducing conditions and by Western blot techniques, the TT were found to contain a concanavalin A-binding 150 kDa glycoprotein which probably corresponds to the alpha 2-subunit of the DHP receptor. This conclusion was supported by correlative immunoblot experiments with a specific antibody. Junctional TT are further distinguished from free TT by the presence of a high number (Bmax. 20 pmol/mg of protein) of [3H]cyclic AMP receptor sites, as determined by the Millipore filtration technique of Gill & Walton [(1974) Methods Enzymol. 38, 376-381]. Use of this method means that the number of receptors may have been underestimated. The TT-bound cyclic AMP receptor was identified as a 55 kDa protein by specific photoaffinity labelling with 8-N3-[3H]cyclic AMP, and had similar phosphorylation properties and apparent molecular mass to the RII form of the regulatory subunit of cyclic AMP-dependent protein kinase. Co-localization of the intrinsic cyclic AMP-dependent protein kinase and of the DHP receptor complex to the junctional membrane of TT supports the hypothesis that the 170 kDa alpha 1-subunit of the receptor is a substrate for the kinase.

Abnormal properties of Mg2(+)-ATPase in transverse tubule membranes from dystrophic chicken

Purified transverse tubule membranes from normal and dystrophic chicken skeletal muscle were isolated by a calcium-loading procedure. Normal and dystrophic T-tubules were similar in cholesterol content and (Na+,K+)-ATPase and 5'-nucleotidase activities but a significant decrease of Mg2(+)-ATPase activity was observed in dystrophic membranes. A comparative analysis of the enzyme properties revealed that the kinetic parameters were altered in dystrophic T-tubules and the ATP-hydrolyzing activity was differently affected by the ionic strength. However, the influence of temperature and the regulatory effect of concanavalin A were the same as in normal T-tubules. Membrane fluidity was similar in both preparations as estimated by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene and trimethylammonium diphenylhexatriene. These results point to an impairment in the function of Mg2(+)-ATPase due to structural alterations of the enzyme.

Morphological, immunological and biochemical characterization of purified transverse tubule membranes isolated from rabbit skeletal muscle

A microsomal fraction consisting of membranes of transverse tubule origin has been purified by a modification of the calcium-loading procedure initially described by Rosemblatt et al. (J Biol Chem 256:8140-8, 1981). Enzymatic analysis of this fraction shows an enrichment of the vesicles in the Mg++ ATPase (basal) activity characteristic of the T-tubules and an absent or very low Ca++-dependent-ATPase activity. Stereological analysis of freeze fracture replica of the membranes in the purified fraction indicates that they have a very low density of particles in their P faces and lack the structural manifestation of the caveolae typical of the sarcolemma. Immunological analysis performed with monoclonal antibodies prepared against purified T-tubule and sarcoplasmic reticulum membranes define some T-tubule specific antigens and confirm the morphological and biochemical data regarding the origin and purity of the T-tubule preparation.

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.