High-affinity and low-affinity vanadate binding to sarcoplasmic reticulum Ca2+-ATPase labeled with fluorescein isothiocyanate.
Conditions were found that allowed both the fluorescence detection of vanadate binding to the Ca2+-ATPase of skeletal muscle sarcoplasmic reticulum and the vanadate-induced formation of two-dimensional arrays of the enzyme. The fluorescence intensity of fluorescein isothiocyanate-labeled Ca2+-ATPase increased with high-affinity vanadate binding (Ka = 10(6) M-1) as reported by Pick and Karlish (Pick, U. and Karlish, S.D. (1982) J. Biol. Chem. 257, 6120-6126). The Ca2+ and Mg2+ dependencies for high-affinity vanadate binding were similar but not identical to those for orthophosphate. In addition, it was found that there is low-affinity (Ka = 380 M-1) vanadate binding, which causes a 25% decrease in fluorescence. The Ca2+ and Mg2+ dependencies of the low-affinity vanadate binding were different from those of orthophosphate or high-affinity vanadate binding. The covalent attachment of fluorescein isothiocyanate (FITC) in the ATP site of the Ca2+-ATPase did not affect the formation of two-dimensional arrays, as detected by negatively stained electron micrographs. Vanadate concentrations high enough to saturate the low-affinity binding caused two-dimensional arrays as reported by Dux and Martonosi (Dux, L. and Martonosi, A. (1983) J. Biol. Chem. 258, 2599-2603). In addition, freeze-fracture replicas of quick-frozen specimens showed rows of indentations in the inner leaflet of the bilayer that corresponds to the arrays seen on the outer leaflet. This appearance of indentations suggests that low-affinity vanadate binding causes a transmembrane movement of the Ca2+-ATPase. By contrast, high-affinity vanadate binding was shown to cause neither array formation nor the appearance of indentations.
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D000818:Animals; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D016650:Fluorescein-5-isothiocyanate; D005452:Fluoresceins; D005456:Fluorescent Dyes; D007700:Kinetics; D008854:Microscopy, Electron; D009132:Muscles / Q000201:enzymology; D011485:Protein Binding; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology* / Q000648:ultrastructure; D013861:Thiocyanates; D014638:Vanadates; D014639:Vanadium / Q000378:metabolism*
D005452:Fluoresceins; D005456:Fluorescent Dyes; D013861:Thiocyanates; D014639:Vanadium; D014638:Vanadates; D000252:Calcium-Transporting ATPases; D016650:Fluorescein-5-isothiocyanate