Skeletal muscle excitation–contraction coupling is independent of a conserved heptad repeat motif in the C-terminus of the DHPRβ1a subunit
In skeletal muscle excitation–contraction (EC) coupling the sarcolemmal L-type Ca2+ channel or 1,4-dihydropyridine receptor (DHPR) transduces the membrane depolarization signal to the sarcoplasmic Ca2+ release channel RyR1 via protein–protein interaction. While it is evident that the pore-forming and voltage-sensing DHPRα1S subunit is essential for this process, the intracellular DHPRβ1a subunit was also shown to be indispensable. We previously found that the β1a subunit is essential to target the DHPR into groups of four (tetrads) opposite the RyR1 homotetramers, a prerequisite for skeletal muscle EC coupling. Earlier, a unique hydrophobic heptad repeat motif (L⋯V⋯V) in the C-terminus of β1a was postulated by others to be essential for skeletal muscle EC coupling, as substitution of these residues with alanines resulted in 80% reduction of RyR1 Ca2+ release. Therefore, we wanted to address the question if the proposed β1a heptad repeat motif could be an active element of the DHPR–RyR1 signal transduction mechanism or already contributes at the ultrastructural level i.e. DHPR tetrad arrangement. Surprisingly, our experiments revealed full tetrad formation and an almost complete restoration of EC coupling in β1-null zebrafish relaxed larvae and isolated myotubes upon expression of a β1a-specific heptad repeat mutant (LVV to AAA) and thus contradict the earlier results.