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ABSTRACT The functional state of the skeletal muscle Ca2+ release channel is modulated by a number of endogenous molecules during excitation-contraction. Using electron cryomicroscopy and angular reconstitution techniques, we determined the three-dimensional (3D) structure of the skeletal muscle Ca^sup 2+^ release channel activated by a nonhydrolyzable analog of ATP in the presence of Ca^sup 2+^. These ligands together produce almost maximum activation of the channel and drive the channel population toward a predominately open state. The resulting 30-Angstrom 3D reconstruction reveals long-range conformational changes in the cytoplasmic region that might affect the interaction of the Ca^sup 2+^ release channel with the t-tubule voltage sensor. In addition, a central opening and mass movements, detected in the transmembrane domain of both the Ca^sup 2+^- and the Ca^sup 2+^/nucleotide-activated channels, suggest a mechanism for channel opening similar to opening-closing of the iris in a camera diaphragm.
INTRODUCTION
Excitation-contraction (E-C) coupling is the process in muscle that links depolarization of the plasmalemma membrane to Ca^sup 2+^ release from the sarcoplasmic reticulum (SR), the main source of Ca^sup 2+^ in muscle. The release of Ca^sup 2+^ occurs via the cation-selective, ligand-regulated Ca^sup 2+^ release channel located in the junctional membrane of the SR in response to signals arising from the voltage-dependent Ca^sup 2+^ channels (dihydropyridine receptors) in the t-tubule. The increase in the intracellular Ca^sup 2+^ concentration initiates muscle contraction. Thus the Ca^sup 2+^ release channel plays a critical role in the regulation of muscle contraction. The native form of the skeletal muscle Ca^sup 2+^ release channel is a tetramer (Lai et al., 1989) with a subunit molecular mass of 565 kDa (Takeshima et al., 1989; Zorzato et al., 1990). Because the 12-kDa FK506-binding protein, FKBP12, is considered an integral part of the functional Ca^sup 2+^ release channel (Jayaraman et al., 1992; Timerman et al., 1993, 1995), the entire channel assembly represents a heterooligomer with a molecular mass of over 2.3 MDa.
The skeletal muscle Ca^sup 2+^ release channel probably exists in several distinct functional states during the excitation-- contraction coupling process. The functional channel transitions are regulated by a wide variety of endogenous molecules and pharmacological modifiers (see reviews: Coronado et al., 1994; Fleischer and Inui, 1989; Meissner, 1994; Ogawa, 1994). It has been suggested that the Ca^sup...