Abstract
Decavanadate produces extensive ordered arrays of Ca 2+-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 Ca 2+-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 Ca 2+-translocation from Ca 2+-dependent ATP hydrolysis [(1986) Arch. Biochem. Biophys. 246, 90–97]. The effect of myotoxin a on decavanadate-induced two-dimensional Ca 2+-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 Ca 2+ 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 Ca 2+-ATPase enzyme-myotoxin a complex. The present results suggest that a Ca 2+-ATPase dimer is required for coupling Ca 2+ translocation to Ca 2+-dependent ATP hydrolysis.
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