The α1S III-IV Loop Influences 1,4-Dihydropyridine Receptor Gating but Is Not Directly Involved in Excitation-Contraction Coupling Interactions with the Type 1 Ryanodine Receptor

• Published in: The Journal of biological chemistry Vol. 283; no. 34; p. 23217
• Main Authors: Roger A. Bannister, Manfred Grabner, Kurt G. Beam
• Format: Journal Article
• Language: English
• Published: American Society for Biochemistry and Molecular Biology 22.08.2008
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M804312200

In skeletal muscle, coupling between the 1,4-dihydropyridine receptor (DHPR) and the type 1 ryanodine receptor (RyR1) underlies excitation-contraction (EC) coupling. The III-IV loop of the DHPR α 1S subunit binds to a segment of RyR1 in vitro , and mutations in the III-IV loop alter the voltage dependence of EC coupling, raising the possibility that this loop is directly involved in signal transmission from the DHPR to RyR1. To clarify the role of the α 1S III-IV loop in EC coupling, we examined the functional properties of a chimera (GFP-α 1S [III-IVa]) in which the III-IV loop of the divergent α 1A isoform replaced that of α 1S . Dysgenic myotubes expressing GFP-α 1S [III-IVa] yielded myoplasmic Ca 2+ transients that activated at ∼10 mV more hyperpolarized potentials and that were ∼65% smaller than those of GFP-α 1S . A similar reduction was observed in voltage-dependent charge movements for GFP-α 1S [III-IVa], indicating that the chimeric channels trafficked less well to the membrane but that those that were in the membrane functioned as efficiently in EC coupling as GFP-α 1S . Relative to GFP-α 1S , L-type currents mediated by GFP-α 1S [III-IVa] were ∼40% smaller and activated at ∼5 mV more hyperpolarized potentials. The altered gating of GFP-α 1S [III-IVa] was accentuated by exposure to ±Bay K 8644, which caused a much larger hyperpolarizing shift in activation compared with its effect on GFP-α 1S . Taken together, our observations indicate that the α 1S III-IV loop is not directly involved in EC coupling but does influence DHPR gating transitions important both for EC coupling and activation of L-type conductance.