A proteolipid associated with Na,K-ATPase is not essential for ATPase activity

1. 11,000 dalton proteins were detected in membrane-bound preparations of Na,K-ATPase from shark rectal glands and avian nasal salt glands. 2. 2. In shark rectal gland microsomal Na,K-ATPase the stoichiometry of the 11,000 dalton protein to the catalytic and glycoprotein subunits of the ATPase was 2.0 moles per mole α 2 β 4 . 3. 3. Non-ionic detergent treatment under non-denaturing conditions of the membrane-bound Na,K-ATPase from avian nasal salt glands and shark rectal glands simultaneously separated the small proteins from the α-β complex and the ATPase activity, indicating that the small proteins are not essential for the hydrolysis of ATP. 4. 4. A fundamental role for the small proteins is suggested by the great similarity of the amino acid compositions of the 11,000 dalton protein from shark rectal gland Na,K-ATPase and the γ2 proteolipid from lamb kidney outer medulla reported by Reeves, A.S., Collins, J.H. and Schwartz, A. (1980) Biochem. Biophys. Res. Comm. 95 , 1591–1598, which implies that there is a strong sequence homology between the two small proteins, i.e. that the primary structure is highly conserved.