A combined non-denaturing and denaturing gel electrophoretic analysis of the subunit composition of a membrane protein: the skeletal muscle L-type calcium channel

The Three-dimensional Structure of the Cardiac L-type Voltage-gated Calcium Channel

We describe here the first three-dimensional structure of the cardiac L-type voltage-gated calcium channel (VGCC) purified from bovine heart. The structure was determined by electron microscopy and single particle analysis of negatively stained complexes, using the angular reconstitution method. The cardiac VGCC can be isolated as a stable dimer, as reported previously for the skeletal muscle VGCC, with a central aqueous chamber formed by the two halves of the complex. Moreover, we demonstrate that the dimeric cardiac VGCC binds the dihydropyridine [3H]azidopine with a Kd approximately 310 pM. We have compared the cardiac VGCC structure with the skeletal muscle form, determined using the same reconstructive methodology, allowing us to identify common and distinct features of the complexes. By using antibody and lectin-gold labeling, we have localized the intracellular beta polypeptides and the extracellular glycosylation sites of the skeletal muscle VGCC, which can be correlated to the cardiac three-dimensional structure. From the data presented here the assignment of the orientation of the VGCC complexes with respect to the lipid bilayer is now possible. A difference between the cardiac and skeletal muscle ion channels is apparent in the putative transmembrane region, which would be consistent with the absence of the gamma subunit in the cardiac VGCC assembly.

Effect of halothane on the oligomerization of the sarcoplasmic reticulum Ca(2+)-ATPase

The exact molecular mechanism of inhalational anesthetics remains obscure. Since the enzyme activity of the sarcoplasmic reticulum Ca(2+)-ATPase from skeletal muscle fibres is modified by halothane and because protein-protein interactions play an important role in the regulation of Ca(2+)-regulatory proteins, we investigated the effect of this volatile drug on the oligomerization of the fast-twitch Ca(2+)-ATPase. Using electrophoretic separation following incubation with halothane, increases in relative molecular mass were determined by immunoblotting with a monoclonal antibody to the SERCA1 isoform of the Ca(2+)-ATPase. Distinct drug-induced decreases in electrophoretic mobility indicated oligomerization of the native Ca(2+)-pump by halothane, comparable to crosslinking-mediated formation of homo-tetramers. Determination of the effect of halothane on enzyme activity suggested that halothane-mediated protein aggregation triggers a partial inhibition of Ca(2+)-pump units. Thus, halothane appears to exert its action via specific peptide binding sites and not indirectly by lipid perturbation. These findings support the protein theory of anesthetic action.

Complex formation of skeletal muscle Ca2+-regulatory membrane proteins by halothane

In skeletal muscle, halothane affects the functions of several Ca2+-regulatory membrane proteins involved in the excitation-contraction-relaxation cycle. To investigate the mechanism by which this volatile anesthetic interferes with Ca2+-homeostasis, we studied potential changes in protein-protein interactions by halothane. Using comparative immunoblotting of microsomal muscle proteins separated on native and denaturing gels, we show here that halothane induces oligomerization of the terminal cisternae Ca2+-binding protein calsequestrin, the junctional ryanodine receptor Ca2+-release channel and the transverse-tubular alpha1-dihydropyridine receptor. This agrees with previous reports on the modulation of Ca2+-release activity by halothane since interactions between the voltage-sensing alpha1-dihydropyridine receptor, the ryanodine receptor and the luminal Ca2+-reservoir might result in a rapid release of Ca2+-ions. Furthermore, this study supports the idea that specific protein sites are involved in the action of inhalational anesthetics and that halothane might trigger abnormal Ca2+-homeostasis in malignant hyperthermia via oligomerization of the mutated ryanodine receptor.

A large scale preparative gel electrophoresis separation of alpha 1 and alpha 2 subunits of the voltage-gated Ca2+ channel from rabbit skeletal muscle

A large-scale preparative gel electrophoresis method effectively separates individual voltage-gated calcium channel (VGCC) subunits with high resolution, starting with up to 4 mg of rabbit skeletal muscle L-type VGCC complex. Using this method, we separated alpha 1 and alpha 2 subunits of rabbit skeletal muscle VGCC with a high efficiency and with protein recoveries of 83%. The separated alpha 1 and alpha 2 subunits eluted from the gel in a 1:1 molar ratio. The method should be applicable for separating the other VGCC subunits or subunits of other protein complexes.

Relationship between functional properties and structure of ovalbumin

The effects of ovalbumin (OVA) denaturation using urea, guanidinium chloride (GdnHCl), sodium dodecyl sulphate (SDS), cetylpyridinium chloride (CPC), 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), and 5 different cationic detergents with various side chains, HCl, and CH3COOH were observed. Progressive unfolding in ovalbumin was measured as a function of fluorescent light intensity, peak response and shift in the maximum of emission. Kinetic measurements demonstrated that the rate of denaturation usually followed a double exponential decay pattern, but at small concentrations of urea and acids first-order reaction was indicated. The reversibility of the unfolding-folding transitions was confirmed from tryptophan fluorescence and circular dichroism (CD) measurements. Differences in secondary structure were observed and changes of alpha-helical content were calculated. Polyacrylamide gel electrophoresis (PAGE) with and without sodium dodecyl sulphate (SDS-PAGE) showed differences in the structure of native and denatured ovalbumin. Native protein samples in PAGE demonstrated smaller number and larger mobilities of subunits than denatured ones with different reductants, such as SDS and 2-mercaptoethanol (2 ME). Scanning of SDS protein patterns showed the appearance of aggregated forms in region of 45 kD.