The distal C terminus of the dihydropyridine receptor β(1a) subunit is essential for tetrad formation in skeletal muscle

The skeletal muscle dihydropyridine receptor (DHPR) β(1a) subunit is indispensable for full trafficking of DHPRs into triadic junctions (i.e., the close apposition of transverse tubules and sarcoplasmic reticulum [SR]), facilitation of DHPRα(1S) voltage sensing, and arrangement of DHPRs into tetrads as a consequence of their interaction with ryanodine receptor (RyR1) homotetramers. These three features are obligatory for skeletal muscle excitation–contraction (EC) coupling. Previously, we showed that all four vertebrate β isoforms (β(1)–β(4)) facilitate α(1S) triad targeting and, except for β(3), fully enable DHPRα(1S) voltage sensing [Dayal et al., Proc. Natl. Acad. Sci. U.S.A. 110, 7488–7493 (2013)]. Consequently, β(3) failed to restore EC coupling despite the fact that both β(3) and β(1a) restore tetrads. Thus, all β-subunits are able to restore triad targeting, but only β(1a) restores both tetrads and proper DHPR–RyR1 coupling [Dayal et al., Proc. Natl. Acad. Sci. U.S.A. 110, 7488–7493 (2013)]. To investigate the molecular region(s) of β(1a) responsible for the tetradic arrangement of DHPRs and thus DHPR–RyR1 coupling, we expressed loss- and gain-of-function chimeras between β(1a) and β(4), with systematically swapped domains in zebrafish strain relaxed (β(1)-null) for patch clamp, cytoplasmic Ca(2+) transients, motility, and freeze-fracture electron microscopy. β(1a)/β(4) chimeras with either N terminus, SH3, HOOK, or GK domain derived from β(4) showed complete restoration of SR Ca(2+) release. However, chimera β(1a)/β(4)(C) with β(4) C terminus produced significantly reduced cytoplasmic Ca(2+) transients. Conversely, gain-of-function chimera β(4)/β(1a)(C) with β(1a) C terminus completely restored cytoplasmic Ca(2+) transients, DHPR tetrads, and motility. Furthermore, we found that the nonconserved, distal C terminus of β(1a) plays a pivotal role in reconstitution of DHPR tetrads and thus allosteric DHPR–RyR1 interaction, essential for skeletal muscle EC coupling.