Domain cooperativity in the β_1asubunit is essential for dihydropyridine receptor voltage sensing in skeletal muscle

Abstract

The dihydropyridine receptor (DHPR) β 1a subunit is crucial for enhancement of DHPR triad expression, assembly of DHPRs in tetrads, and elicitation of DHPRα 1S charge movement-the three prerequisites of skeletal muscle excitation-contraction coupling. Despite the ability to fully target α 1S into triadic junctions and tetradic arrays, the neuronal isoform β 3 was unable to restore considerable charge movement (measure of α 1S voltage sensing) upon expression in β 1 -null zebrafish relaxed myotubes, unlike the other three vertebrate β-isoforms (β 1a , β 2a , and β 4 ). Thus, we used β 3 for chimerization with β 1a to investigate whether any of the five distinct molecular regions of β 1a is dominantly involved in inducing the voltage-sensing function of α 1S . Surprisingly, systematic domain swapping between β 1a and β 3 revealed a pivotal role of the src homology 3 (SH3) domain and C terminus of β 1a in charge movement restoration. More interestingly, β 1a SH3 domain and C terminus, when simultaneously engineered into β 3 sequence background, were able to fully restore charge movement together with proper intracellular Ca 2+ release, suggesting cooperativity of these two domains in induction of the α 1S voltage-sensing function in skeletal muscle excitation-contraction coupling. Furthermore, substitution of a proline by alanine in the putative SH3-binding polyproline motif in the proximal C terminus of β 1a (also of β 2a and β 4 ) fully obstructed α 1S charge movement. Consequently, we postulate a model according to which β subunits, probably via the SH3-C-terminal polyproline interaction, adapt a discrete conformation required to modify the α 1S conformation apt for voltage sensing in skeletal muscle.

Congenital myopathy results from misregulation of a muscle Ca2+channel by mutant Stac3

“…In addition, our freezefracture results support previous work indicating that complete tetrads represent the critical DHPR structural unit required for functional EC coupling (17,(23)(24)(25). Indeed, the number of complete tetrads and amount of DHPR charge movement are tightly correlated in each genotype studied (Table 1).…”

“…DHPRβ 1a -null zebrafish also display reduced DHPR triadic levels, charge movements, tetrad formation, and voltage-gated Ca 2+ release. However, unlike DHPRβ 1a , a chaperone that is cotransported with DHPRα 1S to the triadic junction, Stac3 is required instead for stability of DHPRα 1S within the triadic junction (17,23,24). Interestingly, whereas both Stac3-null and DHPRβ 1a -null fibers show a 90% reduction in Q max , Stac3-null shows a rightward shift in voltage dependence whereas DHPRβ 1a -null exhibits a leftward shift.…”

Fluorescence Resonance Energy Transfer-based Structural Analysis of the Dihydropyridine Receptor α1S Subunit Reveals Conformational Differences Induced by Binding of the β1a Subunit

“…Thus, deletion of the ␤ 1a C terminus, which prevents bidirectional signaling (22,23), also may affect the conformation/orientation of both the ␣ 1S II-III loop and the ␤ 1a subunit. These results are consistent with the hypothesis that the C terminus of the ␤ 1a subunit is required for inducing conformational changes in the ␣ 1S subunit necessary to transmit the EC coupling signal (20,40).…”

“…6B), thus suggesting that deletion of the ␤ 1a C terminus may affect the relative ␣ 1S /␤ 1a orientation. This finding is consistent with the suggested role of the ␤ 1a C-terminal tail in supporting domain cooperativity within the subunit (40). Binding specificity was confirmed using the Y366S-␣ 1S mutation, which prevented specific energy transfer between ␣ 1S and ␤ 1a (Fig.…”

Bridging the myoplasmic gap II: more recent advances in skeletal muscle excitation–contraction coupling

“…Their finding that β 1a is a critical element for the alignment of Ca V 1.1 with RyR1 is arguably the most significant advance in our knowledge of the function of β 1a in EC coupling since the generation of the β 1 null mouse (Schredelseker et al, 2005(Schredelseker et al, , 2009). Continuing on with this approach, they determined that Ca V 1.1 voltage sensing is facilitated by a cooperative intramolecular relationship between the carboxyl terminus and a conserved SH3-like domain of β 1a (Dayal et al, 2013).…”