Biochemical and ultrastructural characterization of the 1,4-dihydropyridine receptor from rabbit skeletal muscle. Evidence for a 52,000 Da subunit

Localization of the alpha 1 and alpha 2 subunits of the dihydropyridine receptor and ankyrin in skeletal muscle triads

We have studied the subcellular distribution of the alpha 1 and alpha 2 subunits of the dihydropyridine (DHP) receptor and ankyrin in rat skeletal muscle with immunofluorescence and immunogold labeling techniques. All three proteins were concentrated in the triad junction formed between the T-tubules and sarcoplasmic reticulum. The alpha 1 and alpha 2 subunits of the DHP receptor were colocalized in the junctional T-tubule membrane, supporting their proposed association in a functional complex and the possible participation of the alpha 2 subunit in excitation-contraction coupling. Ankyrin label in the triad showed a distribution different from that of the DHP receptor subunits. In addition, ankyrin was found in longitudinally oriented structures outside the triad. Thus, ankyrin might be involved in organizing the triad and in immobilizing integral membrane proteins in T-tubules and the sarcoplasmic reticulum.

Subunit structure and localization of dihydropyridine-sensitive calcium channels in mammalian brain, spinal cord, and retina

Monoclonal antibodies that recognize the alpha 2 delta subunits of calcium channels from skeletal muscle immunoprecipitate a complex of alpha 1, alpha 2 delta, beta, and gamma subunits. They also immunoprecipitate 64% of rabbit brain dihydropyridine-sensitive calcium channels. Iodination of partially purified brain calcium channels followed by immunoprecipitation reveals alpha 1-, alpha 2 delta-, and beta-like subunits that have apparent molecular masses of 175, 142, and 57 kd, respectively. A polypeptide of 100 kd is also specifically immunoprecipitated. Immunocytochemical studies identify dihydropyridine-sensitive calcium channels in neuronal somata and proximal dendrites in rat brain, spinal cord, and retina. Staining of many neuronal somata is uneven, revealing relatively high densities of dihydropyridine-sensitive calcium channels at the base of major dendrites. L-type calcium channels in this location may serve to mediate long-lasting increases in intracellular calcium in the cell body in response to excitatory inputs to the dendrites.

Toward an understanding of the dihydropyridine-sensitive calcium channel

The dihydropyridine-sensitive calcium channel continues to fascinate scientific investigators. Each new discovery leads to more complexity. Further work elucidating the molecular structures and functions of the various isoforms of the channel will lead us to a better understanding of its nature. We are well on our way towards understanding the molecular structure and mechanisms involved in calcium permeability, and the coming decade promises to reveal numerous breakthroughs in our understanding of this channel.

Two subtypes of dihydropyridine-sensitive calcium channels in rat ventricular muscle

Two opposite inotropic effects of the dihydropyridine activators, CGP 28392 and Bay K 8644, given at the same concentration (1-2 microM) were found in rat papillary muscles: a positive effect in polarized tissue (4 mM KCl) and a negative one during partial depolarization. The depressive effect found at a low rate or after a short rest was associated with marked prolongation of the Ca2(+)-mediated action potential, indicating that the drugs behave as Ca channel stimulators. The depressive effect of the activation on the resting state contraction was antagonized by nifedipine (2 microM) and high Mg2+ (5 mM). It was suggested that at least two subtypes of the L-type, dihydropyridine-sensitive channels underlie the opposite inotropic responses of the activators. The positive effect is apparently caused by conventional stimulation of Ca2+ entry through the cell membrane, whereas the negative effect is probably due to the stimulation of Ca2+ efflux from the sarcoplasmic reticulum, leading to depletion of intracellular stores. The effect was proposed to be mediated by activation of junctional channels linked to sarcoplasmic reticulum Ca2+ release. An important role for these channels in triggering the sarcoplasmic reticulum Ca2+ release and regulation of force-frequency relation is proposed.

Identification of two types of specific endothelin receptors in rat mesangial cell

Two types of receptors specific for endothelin were identified using cross-linking technique in cultured rat mesangial cells. The molecular weights of these receptors were approximately 58,000 and 34,000 by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The binding of radioiodinated-endothelin to its receptors was inhibited by excess of unlabeled endothelin, but not by nifedipine, nicardipine, verapamil, diltiazem, angiotensin II or [Arg8]-vasopressin. The endothelin binding proteins were solubilized with 1% digitonin and fractionated under non-denaturing conditions by gel filtration. Two endothelin binding peaks eluted at the positions corresponding to the molecular weights of 65,000 and 43,000. These observations indicate that there are two types of specific endothelin receptors in rat mesangial cells which are distinct from voltage dependent L-type calcium channel.

32,000-Dalton subunit of the 1,4-dihydropyridine receptor

Polyclonal antibodies to the 32,000-Da polypeptide of the 1,4-dihydropyridine receptor of the voltage-dependent Ca2+ channel have been produced and used to characterize the association of the 32,000-Da polypeptide (gamma subunit) with other subunits of the dihydropyridine receptor. The 32,000-Da polypeptide was found to copurify with alpha 1 and alpha 2 subunits at each step of the purification of the dihydropyridine receptor. Monoclonal antibodies against the alpha 1 and beta subunits immunoprecipitate the digitonin-solubilized dihydropyridine receptor as a multisubunit complex that includes the 32,000-Da polypeptide. Polyclonal antibodies generated against both the nonreduced and reduced forms of the alpha 2 subunit and the gamma subunit have been used to show that the 32,000-Da polypeptide is not a proteolytic fragment of a larger component of the dihydropyridine receptor and not disulfide linked to the alpha 2 subunit. Our results demonstrate that the 32,000-Da polypeptide (gamma subunit) is an integral and distinct component of the dihydropyridine receptor.

The alpha 1 and alpha 2 polypeptides of the dihydropyridine-sensitive calcium channel differ in developmental expression and tissue distribution

The dihydropyridine (DHP) binding complex isolated from skeletal muscle contains two large proteins, alpha 1 and alpha 2, and at least two smaller polypeptides. The alpha 1 subunit has a primary structure expected for ion channels and is a functional component of a DHP-sensitive, voltage-activated calcium channel. The functions of the alpha 2 protein and the smaller polypeptides are unknown. We prepared monoclonal antibodies to the alpha 1 and alpha 2 polypeptides and studied the developmental appearance and tissue distribution of these two proteins. In rat skeletal muscle, the levels of alpha 1 are quite low during the first 10 days after birth, then rise dramatically, and, by day 20, approach those found in adult muscle. In direct contrast, alpha 2 is present in substantial amounts in rat muscle at birth and increases only slightly during this same period of development. Furthermore, alpha 1 is detected only in skeletal muscle, whereas alpha 2 is present in a variety of tissues. These results provide evidence for the segregation of these two polypeptides and suggest that the function of alpha 2 is not limited to that associated with the DHP-sensitive calcium channel.

Association of dystrophin and an integral membrane glycoprotein

Duchenne muscular dystrophy (DMD) is caused by a defective gene found on the X-chromosome. Dystrophin is encoded by the DMD gene and represents about 0.002% of total muscle protein. Immunochemical studies have shown that dystrophin is localized to the sarcolemma in normal muscle but is absent in muscle from DMD patients. Many features of the predicted primary structure of dystrophin are shared with membrane cytoskeletal proteins, but the precise function of dystrophin in muscle is unknown. Here we report the first isolation of dystrophin from digitonin-solubilized skeletal muscle membranes using wheat germ agglutinin (WGA)-Sepharose. We find that dystrophin is not a glycoprotein but binds to WGA-Sepharose because of its tight association with a WGA-binding glycoprotein. The association of dystrophin with this glycoprotein is disrupted by agents that dissociate cytoskeletal proteins from membranes. We conclude that dystrophin is linked to an integral membrane glycoprotein in the sarcolemma. Our results indicate that the function of dystrophin could be to link this glycoprotein to the underlying cytoskeleton and thus help either to preserve membrane stability or to keep the glycoprotein non-uniformly distributed in the sarcolemma.

Structure and function of voltage-sensitive ion channels

Voltage-sensitive ion channels mediate action potentials in electrically excitable cells and play important roles in signal transduction in other cell types. In the past several years, their protein components have been identified, isolated, and restored to functional form in the purified state. Na+ and Ca2+ channels consist of a principal transmembrane subunit, which forms the ion-conducting pore and is expressed with a variable number of associated subunits in different cell types. The principal subunits of voltage-sensitive Na+, Ca2+, and K+ channels are homologous members of a gene family. Models relating the primary structures of these principal subunits to their functional properties have been proposed, and experimental results have begun to define a functional map of these proteins. Coordinated application of biochemical, biophysical, and molecular genetic methods should lead to a clear understanding of the molecular basis of electrical excitability.

Receptors for calcium antagonists

Calcium antagonists have been divided into 3 different subclasses represented by nifedipine, verapamil and diltiazem. These drugs have different pharmacologic effects and are not interchangeable. Previous studies suggested that all calcium antagonists bind to a 170 kd polypeptide (now called the alpha 2 subunit of the voltage-dependent calcium channel). The apparent molecular weight of this polypeptide characteristically decreased from 170 to 140 kd upon disulfide reduction as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Recent studies demonstrated that calcium antagonists bind to a previously unrecognized 165 kd polypeptide (alpha 1 subunit) that does not change its electrophoretic mobility on disulfide reduction. Because of their similar molecular weights, the 2 polypeptides may overlap each other on polyacrylamide gels. The primary structure of both polypeptides clearly shows, however, that they are different from each other and only the alpha 1 subunit has the features expected of an ion channel.

Sequence and expression of mRNAs encoding the alpha 1 and alpha 2 subunits of a DHP-sensitive calcium channel

Complementary DNAs were isolated and used to deduce the primary structures of the alpha 1 and alpha 2 subunits of the dihydropyridine-sensitive, voltage-dependent calcium channel from rabbit skeletal muscle. The alpha 1 subunit, which contains putative binding sites for calcium antagonists, is a hydrophobic protein with a sequence that is consistent with multiple transmembrane domains and shows structural and sequence homology with other voltage-dependent ion channels. In contrast, the alpha 2 subunit is a hydrophilic protein without homology to other known protein sequences. Nucleic acid hybridization studies suggest that the alpha 1 and alpha 2 subunit mRNAs are expressed differentially in a tissue-specific manner and that there is a family of genes encoding additional calcium channel subtypes.

Heterologous expression of excitability proteins: route to more specific drugs?

Many clinically important drugs act on the intrinsic membrane proteins (ion channels, receptors, and ion pumps) that control cell excitability. A major goal of pharmacology has been to develop drugs that are more specific for a particular subtype of excitability molecule. DNA cloning has revealed that many excitability proteins are encoded by multigene families and that the diversity of previously recognized pharmacological subtypes is matched, and probably surpassed, by the diversity of messenger RNAs that encode excitability molecules. In general, the diverse subtypes retain their properties when the excitability proteins are expressed in foreign cells such as oocytes and mammalian cell lines. Such heterologous expression may therefore become a tool for testing drugs against specific subtypes. In a systematic research program to exploit this possibility, major considerations include alternative processing of messenger RNA for excitability proteins, coupling to second-messenger systems, and expression of enough protein to provide material for structural studies.