Reconstitution experiments provide no evidence for a role for the 53-kDa glycoprotein in coupling Ca2+ transport to ATP hydrolysis by the (Ca(2+)-Mg2+)-ATPase in sarcoplasmic reticulum.

The sarcoplasmic reticulum (SR) of skeletal muscle contains a 53 kDa glycoprotein of unknown function, as well as the (Ca(2+)-Mg2+)-ATPase. It has been suggested that the glycoprotein couples the hydrolysis of ATP by the ATPase to the transport of calcium. It has been shown that if SR vesicles are solubilized in cholate in media containing low K+ concentrations followed by reconstitution, then vesicles are formed containing the glycoprotein and with ATP hydrolysis coupled to Ca2+ accumulation, as shown by a large stimulation of ATPase activity by addition of A23187. In contrast, if SR vesicles are solubilized in media containing a high concentration of K+, then the vesicles that are produced following reconstitution lack the glycoprotein and show low stimulation by A23187 (Leonards, K.S. and Kutchai, H. (1985) Biochemistry 24, 4876-4884). We show that the effect of K+ on reconstitution does not follow from any changes in the amount of glycoprotein but rather from an effect of K+ on the detergent properties of cholate. In low K+ media, the cmc of cholate is high, cholate is a relatively poor detergent and incomplete solubilization results in 'reconstitution' of vesicles with the correct orientation of ATPase molecules. In high K+ media, the cmc of cholate is reduced and more complete solubilization of the SR leads to a true reconstitution with the formation of vesicles with a random orientation of ATPase molecules. The experiments provide no evidence for an effect of the glycoprotein on the (Ca(2+)-Mg2+)-ATPase.