Repeat I of the dihydropyridine receptor is critical in determining calcium channel activation kinetics

Abstract

Membrane depolarization causes many kinds of ion channels to open, a process termed activation. For both Na+ channels and Ca2+ channels, kinetic analysis of current has suggested that during activation the channel undergoes several conformational changes before reaching the open state. Structurally, these channels share a common motif: the central element is a large polypeptide with four repeating units of homology (repeats I-IV), each containing a voltage-sensing region, the S4 segment. This suggests that the distinct conformational transitions inferred from kinetic analysis may be equated with conformational changes of the individual structural repeats. To investigate the molecular basis of channel activation, we constructed complementary DNAs encoding chimaeric Ca2+ channels in which one or more of the four repeats of the skeletal muscle dihydropyridine receptor are replaced by the corresponding repeats derived from the cardiac dihydropyridine receptor. We report here that repeat I determines whether the chimaeric Ca2+ channel shows slow (skeletal muscle-like) or rapid (cardiac-like) activation.

Tissue-specific expression of calcium channels

“…The primary sequences of these cDNAs are homologous to one another and encode proteins of predicted molecular masses of 212-273 kD and homologies of 4 l%-70%. Hydropathicity analysis of all a, subunits predicts a transmembrane topology similar to that of other voltagedependent ion channels with four homologous repeats, each containing five hydrophobic putative transmembrane a helices and one amphipathic segment (S4) (Figure 2 and the cardiac muscle aI subunit, specific properties of the calcium channel were assigned to distinct parts of the ion-conducting pore: repeat I determines the activation time of the chimeric channel, that is, slow activation upon membrane depolarization with the repeat from skeletal muscle and rapid activation with that from cardiac muscle (Tanabe et al 1991); the putative cytoplasmic loop between repeats II and III determines the type of excitation-contraction coupling. The loop from the skeletal muscle calcium channel aI subunit induces contraction in the absence of calcium influx, whereas the loop fi-om the cardiac calcium channel a, subunit induces contraction only in the presence of calcium influx (Tanabe et al 1990).…”

Structural Determinants of the High Affinity Extracellular Zinc Binding Site on Cav3.2 T-type Calcium Channels

“…The ability of zinc to decrease gating charge 25% is consistent with immobilization of one of the four voltage-sensor paddles. It is interesting to note that repeat I plays a dominant role in the opening of both HVA and LVA channels (37,38). These studies also provide evidence for considerable structural similarity between voltage-gated K ϩ and Ca 2ϩ channels, and combined with the established role of Ca v 3.2 in pain and epilepsy, provide a structural model for the development of novel therapeutics (17,24,39,40).…”

Molecular Basis for CA2+ Channel Diversity

“…By functional expression of chimeras of the skeletal (CaChl) and the cardiac (CaCh2a) muscle subunit, specific properties of the calcium channel were assigned to distinct parts of the ion conducting pore: Repeat I determines the activation time of the chimeric channel, i.e. slow activation upon membrane depolarization with the repeat from skeletal muscle and rapid activation with that from cardiac muscle (Tanabe et al 1991); the putative cytoplasmic loop between repeats II and III determines the type of excitationcontraction coupling. The loop from the skeletal muscle calcium channel subunit induces contraction in the absence of calcium influx, whereas the loop from the cardiac calcium channel a} subunit induces contraction only in the presence of calcium influx (Tanabe et al 1990).…”