2 REGIONS OF THE RYANODINE RECEPTOR INVOLVED IN COUPLING WITH L-TYPE CA2+ CHANNELS

Ryanodine receptors (RyRs) are present in the endoplasmic reticulum of virtually every cell type and serve critical roles, including excitat ion-contraction (EC) coupling in muscle cells. In skeletal muscle the primary control of RyR-1 (the predominant skeletal RyR isoform) occurs via an interaction with plasmalemmal dihydropyridine receptors (DHPRs ), which function as both voltage sensors for EC coupling and as L-typ e Ca2+ channels (Rios, E., and Brum, G. (1987) Nature 325, 717-720). I n addition to ''receiving'' the EC coupling signal from the DHPR, RyR- 1 also ''transmits'' a retrograde signal that enhances the Ca2+ channe l activity of the DHPR (Nakai, J., Dirksen, R. T., Nguyen, H. T., Pess ah, I. N., Beam, K. G., and Alien, P. D. (1996) Nature 380, 72-76). A similar kind of retrograde signaling (from RyRs to L-type Ca2+ channel s) has also been reported in neurons (Chavis, P., Fagni, L., Lansman, J. B., and Bockaert, J. (1996) Nature 382, 719-722). To investigate th e molecular mechanism of reciprocal signaling, we constructed cDNAs en coding chimeras of RyR-1 and RyR-2 (the predominant cardiac RyR isofor m) and expressed them in dyspedic myotubes, which lack an endogenous R yR-1. We found that a chimera that contained residues 1,635-2,636 of R yR-1 both mediated skeletal-type EC coupling and enhanced Ca2+ channel function, whereas a chimera containing adjacent RyR-1 residues (2,659 -3,720) was only able to enhance Ca2+ channel function. These results demonstrate that two distinct regions are involved in the reciprocal i nteractions of RyR-1 with the skeletal DHPR.