Crystallization of the Ca2+‐ATPase of skeletal muscle sarcoplasmic reticulum Inhibition by myotoxin a

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Abstract
Decavanadate produces extensive ordered arrays of Ca2+‐ATPase molecules on sarcoplasmic reticulum (SR) vesicle surfaces [(1984) J. Bioenerg. Biomembranes 16, 491–505] and the basic unit of these crystalline structures seems to be a dimer of Ca2+‐ATPase [(]983) J. Ultrastruct. Res. 24, 454–464; (1984) J. Mol. Biol. 174, 193–204]. Myotoxin a, isolated from the venom of the prairie rattlesnake Crotalus viridis viridis, is a muscle‐degenerating polypeptide and its primary site of interaction is the SR membrane, where it uncouples Ca2+‐translocation from Ca2+‐dependent ATP hydrolysis [(1986) Arch. Biochem. Biophys. 246, 90–97]. The effect of myotoxin a on decavanadate‐induced two‐dimensional Ca2+‐ATPase crystals of SR membranes has been investigated. The toxin inhibits the formation of two‐dimensional SR‐membrane crystals and disrupts previously formed crystals in a time‐ and concentration‐dependent manner, which parallels the uncoupling of ATP hydrolysis from Ca2+ translocation. Two‐dimensional crystalline arrays of the SR membrane have a typical diffraction pattern which, after myotoxin a treatment, displays a progressive loss of order. Decavanadate is an uncompetitive inhibitor of the Ca2+‐ATPase enzyme‐myotoxin a complex. The present results suggest that a Ca2+‐ATPase dimer is required for coupling Ca2+ translocation to Ca2+‐dependent ATP hydrolysis.