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 dihydrop yridine 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 c oupling. 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 o f skeletal-type EC coupling. In the present study we have further diss ected the regions of the II-III loop critical for skeletal-type EC cou pling by expression of cDNA constructs in dysgenic myotubes. Because S er(687) of the skeletal II-III loop has been reported to be rapidly ph osphorylated in vitro, we substituted this serine with alanine, the co rresponding cardiac residue. This alanine-substituted skeletal DHPR re tained 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 725-7 42) in the II-III loop. Skeletal-type coupling was stronger when both residues 725-742 and adjacent residues were skeletal (e.g. a chimera c ontaining skeletal residues 711-765). However, residues 725-742 appear ed to be critical because skeletal-type coupling was not produced eith er by a chimera with skeletal residues 711-732 or by one with skeletal residues 734-765.