Coupling of Ca2+ transport to ATP hydrolysis by the Ca2+-ATPase of sarcoplasmic reticulum: potential role of the 53-kilodalton glycoprotein.

An essential feature of the function of the Ca2+-ATPase of sarcoplasmic reticulum (SR) is the close coupling between the hydrolysis of ATP and the active transport of Ca2+. The purpose of this study is to investigate the role of other components of the SR membrane in regulating the coupling of Ca2+-ATPase in SR isolated from rabbit skeletal muscle, reconstituted SR, and purified Ca2+-ATPase/phospholipid complexes. Our results suggest that (1) it is possible to systematically alter the degree of coupling obtained in reconstituted SR preparations by varying the [KC1] present during cholate solubilization, (2) the variation in coupling is not due to differences in the permeability of the reconstituted SR vesicles to Ca2+, and (3) vesicles reconstituted with purified Ca2+-ATPase are extensively uncoupled under our experimental conditions regardless of the lipid/protein ratio or phospholipid composition. In reconstituted SR preparations prepared by varying the [KC1] present during cholate treatment, we find a direct correlation between the relative degree of coupling between ATP hydrolysis and Ca2+ transport and the level of the 53-kilodalton (53-kDa) glycoprotein of the SR membrane. These results suggest that the 53-kDa glycoprotein may be involved in regulating the coupling between ATP hydrolysis and Ca2+ transport in the SR.