Voltage-gated Ca(2+) channels mediate Ca(2+) entry into cells in response to membrane depolarization. Electrophysiological studies reveal different Ca(2+) currents designated L-, N-, P-, Q-, R-, and T-type. The high-voltage-activated Ca(2+) channels that have been characterized biochemically are complexes of a pore-forming alpha1 subunit of approximately 190-250 kDa; a transmembrane, disulfide-linked complex of alpha2 and delta subunits; an intracellular beta subunit; and in some cases a transmembrane gamma subunit. Ten alpha1 subunits, four alpha2delta complexes, four beta subunits, and two gamma subunits are known. The Cav1 family of alpha1 subunits conduct L-type Ca(2+) currents, which initiate muscle contraction, endocrine secretion, and gene transcription, and are regulated primarily by second messenger-activated protein phosphorylation pathways. The Cav2 family of alpha1 subunits conduct N-type, P/Q-type, and R-type Ca(2+) currents, which initiate rapid synaptic transmission and are regulated primarily by direct interaction with G proteins and SNARE proteins and secondarily by protein phosphorylation. The Cav3 family of alpha1 subunits conduct T-type Ca(2+) currents, which are activated and inactivated more rapidly and at more negative membrane potentials than other Ca(2+) current types. The distinct structures and patterns of regulation of these three families of Ca(2+) channels provide a flexible array of Ca(2+) entry pathways in response to changes in membrane potential and a range of possibilities for regulation of Ca(2+) entry by second messenger pathways and interacting proteins.

Structure and regulation of voltage-gated Ca2+ channels.

The family of voltage-gated sodium channels initiates action potentials in all types of excitable cells. Nine members of the voltage-gated sodium channel family have been characterized in mammals, and a 10th member has been recognized as a related protein. These distinct sodium channels have similar structural and functional properties, but they initiate action potentials in different cell types and have distinct regulatory and pharmacological properties. This article presents the molecular relationships and physiological roles of these sodium channel proteins and provides comprehensive information on their molecular, genetic, physiological, and pharmacological properties.

https://pharmrev.aspetjournals.org/content/pharmrev/57/4/411.full.pdf

International Union of Pharmacology. XLVIII. Nomenclature and Structure-Function Relationships of Voltage-Gated Calcium Channels

The human ATP-binding cassette (ABC) transporter superfamily

The ATP-binding cassette (ABC) transporter superfamily contains membrane proteins that translocate a variety of substrates across extra- and intra-cellular membranes. Genetic variation in these genes is the cause of or contributor to a wide variety of human disorders with Mendelian and complex inheritance, including cystic fibrosis, neurological disease, retinal degeneration, cholesterol and bile transport defects, anemia, and drug response. Conservation of the ATP-binding domains of these genes has allowed the identification of new members of the superfamily based on nucleotide and protein sequence homology. Phylogenetic analysis is used to divide all 48 known ABC transporters into seven distinct subfamilies of proteins. For each gene, the precise map location on human chromosomes, expression data, and localization within the superfamily has been determined. These data allow predictions to be made as to potential functions or disease phenotypes associated with each protein. In this paper, we review the current state of knowledge on all human ABC genes in inherited disease and drug resistance. In addition, the availability of the complete Drosophila genome sequence allows the comparison of the known human ABC genes with those in the fly genome. The combined data enable an evolutionary analysis of the superfamily. Complete characterization of all ABC from the human genome and from model organisms will lead to important insights into the physiology and the molecular basis of many human disorders.

The Human ATP-Binding Cassette (ABC) Transporter Superfamily

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through L...

Molecular Physiology of Low-Voltage-Activated T-type Calcium Channels

A member of a new subclass of the voltage-activated sodium channel genes has been cloned from the human medullary thyroid carcinoma (hMTC) cell line. The cDNA of hNE-Na (human neuroendocrine sodium channel) encodes a 1977 amino acid protein which phylogenetically represents a link between sodium channels isolated from skeletal muscle and brain. The hNE-Na alpha subunit was transiently expressed in human embryonic kidney cells either alone or in combination with the human sodium channel beta 1 subunit. The channel exhibited rapid activation and inactivation kinetics, and was blocked by tetrodotoxin and cadmium with IC50 values of 24.5 nM and 1.1 mM, respectively. Action potentials were generated in cells expressing high levels of hNE-Na. Northern blot and reverse transcription-polymerase chain reaction (RT-PCR) analyses demonstrated its expression in hMTC cells, in a C-cell carcinoma, and in thyroid and adrenal gland. Transcripts were not identified in pituitary gland, brain, heart, liver or kidney, indicating that the hNE-Na is a sodium channel solely expressed in neuroendocrine cells.

Structure and functional expression of a new member of the tetrodotoxin-sensitive voltage-activated sodium channel family from human neuroendocrine cells.

Sequence database searches with the α 2 δ subunit as probe led to the identification of two new genes encoding proteins with the essential properties of this calcium channel subunit. Primary structure comparisons revealed that the novel α 2 δ-2 and α 2 δ-3 subunits share 55.6 and 30.3% identity with the α 2 δ-1 subunit, respectively. The number of putative glycosylation sites and cysteine residues, hydropathicity profiles, and electrophysiological character of the α 2 δ-3 subunit indicates that these proteins are functional calcium channel subunits. Coexpression of α 2 δ-3 with α 1C and cardiac β2a or α 1E and β3 subunits shifted the voltage dependence of channel activation and inactivation in a hyperpolarizing direction and accelerated the kinetics of current inactivation. The kinetics of current activation were altered only when α 2 δ-1 or α 2 δ-3 was expressed with α 1C . The effects of α 2 δ-3 on α 1C but not α 1E are indistinguishable from the effects of α 2 δ-1. Using Northern blot analysis, it was shown that α 2 δ-3 is expressed exclusively in brain, whereas α 2 δ-2 is found in several tissues. In situ hybridization of mouse brain sections showed mRNA expression of α 2 δ-1 and α 2 δ-3 in the hippocampus, cerebellum, and cortex, with α 2 δ-1 strongly detected in the olfactory bulb and α 2 δ-3 in the caudate putamen.

https://www.jneurosci.org/content/jneuro/19/2/684.full.pdf

Molecular Diversity of the Calcium Channel α2δ Subunit

High-voltage activated calcium channels are modulated by a series of auxiliary proteins, including those of the alpha(2)delta family. Until recently, only a single alpha(2)delta subunit was known, but two further members, alpha(2)delta-2 and -3, have since been identified. In this study, the structure of these two novel subunits has been characterized and binding of the antiepileptic drug gabapentin investigated. Using antibodies directed against the amino terminal portion of the proteins, the gross structure of the subunits could be analyzed by Western blotting. Similar to alpha(2)delta-1, both alpha(2)delta-2 and -3 subunits consist of two proteins-a larger alpha(2) and a smaller delta that can be separated by reduction. The subunits are also highly N-glycosylated with approximately 30 kDa of their mass consisting of oligosaccharides. alpha(2)delta-1 was detected in all mouse tissues studied, whereas alpha(2)delta-2 was found at high levels in brain and heart. The alpha(2)delta-3 subunit was observed only in brain. alpha(2)delta-1 and alpha(2)delta-2, but not alpha(2)delta-3, were found to bind gabapentin. The K(d) value of gabapentin binding to alpha(2)delta-2 was 153 nM compared with the higher affinity binding to alpha(2)delta-1 (K(d) = 59 nM).

Calcium channel alpha(2)delta subunits-structure and Gabapentin binding.

Dihydropyridines (DHPs) block the vascular smooth muscle L-type Ca2+ channel at lower concentrations than the cardiac Ca2+ channel, although their α1 subunit, which binds the DHPs, is derived from the same gene. This α1C gene gives rise to several splice variants, among which the α1C-b variant is affected by lower concentrations of nisoldipine than the α1C-a variant. Functional expression of chimeras of α1C-a and α1C-b subunits demonstrated that the transmembrane segment IS6 is responsible for the different dihydropyridine sensitivity. Northern blot analysis showed that transcripts coding for the IS6 segment of the α1C-a subunit were expressed in heart but not in aorta, whereas the IS6 segment of the α1C-b subunit was expressed predominantly in vascular smooth muscle. In situ hybridization of rat heart sections confirmed this expression pattern of IS6 α1C-a and IS6 α1C-b in ventricular and smooth muscle myocytes, respectively. These results suggest that the different dihydropyridine sensitivities...

Alternatively Spliced IS6 Segments of the α1C Gene Determine the Tissue-Specific Dihydropyridine Sensitivity of Cardiac and Vascular Smooth Muscle L-Type Ca2+ Channels

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/S0014-5793%2800%2901306-5

Norbert Klugbauer

Papers

Structure and functional expression of a new member of the tetrodotoxin-sensitive voltage-activated sodium channel family from human neuroendocrine cells.

Molecular Diversity of the Calcium Channel α2δ Subunit

Calcium channel alpha(2)delta subunits-structure and Gabapentin binding.

Alternatively Spliced IS6 Segments of the α1C Gene Determine the Tissue-Specific Dihydropyridine Sensitivity of Cardiac and Vascular Smooth Muscle L-Type Ca2+ Channels

A family of γ‐like calcium channel subunits