Localization in the II-III loop of the dihydropyridine receptor of a sequence critical for excitation-contraction coupling

Skeletal and cardiac muscles express distinct isoforms of the dihydropyridine receptor (DHPR), a type of voltage-gated Ca²⁺ channel that is important for excitation-contraction (EC) coupling. However, entry of Ca²⁺ through the channel is not required for skeletal muscle-type EC coupling. Previous work (Tanabe, T., Beam, K. G., Adams, B. A., Niidome, T., and Numa, S. (1990) Nature 346, 567-569) revealed that the loop between repeats II and III (II-III loop) is an important determinant of skeletal- type EC coupling. In the present study we have further dissected the regions of the II-III loop critical for skeletal-type EC coupling by expression of cDNA constructs in dysgenic myotubes. Because Ser⁶⁸⁷ of the skeletal II- III loop has been reported to be rapidly phosphorylated in vitro, we substituted this serine with alanine, the corresponding cardiac residue. This alanine-substituted skeletal DHPR retained the ability to mediate skeletal- type EC coupling. Weak skeletal-type EC coupling was produced by a chimeric DHPR, which was entirely cardiac except for a small amount of skeletal sequence (residues 725-742) in the II-III loop. Skeletal-type coupling was stronger when both residues 725-742 and adjacent residues were skeletal (e.g. a chimera containing skeletal residues 711-765). However, residues 725-742 appeared to be critical because skeletal-type coupling was not produced either by a chimera with skeletal residues 711-732 or by one with skeletal residues 734-765.