The "gamma subunit" of Na,K-ATPase: a small, amphiphilic protein with a unique amino acid sequence

The "gamma subunit", or "proteolipid", of Na,K-ATPase is a small, membrane-bound protein that copurifies with the alpha and beta subunits of this enzyme. The importance of gamma in the function of Na,K-ATPase remains to be established, but some evidence indicates that it may be involved in forming a receptor site for cardiac glycosides. We have previously communicated [Reeves, A. S., Collins, J. H., & Schwartz, A. (1980) Biochem. Biophys. Res. Commun. 95, 1591-1598] the purification and amino acid composition of sheep kidney gamma, and in this paper we present the first available sequence information on this protein. Although the amino terminus of gamma seems to be blocked and it is resistant to proteolytic cleavage, we have determined approximately half of its amino acid sequence. Our results indicate that gamma contains a total of 68 amino acid residues, with a calculated Mr of 7675. The sequenced portion appears to be at the carboxyl terminus of the polypeptide chain. The gamma sequence is unique, providing strong evidence for its homogeneity and establishing for the first time that it is not a breakdown product of the alpha or beta subunits. gamma is not a true proteolipid, but rather it is an amphiphilic protein with two distinct structural domains. The amino-terminal domain (residues 1-49) is very hydrophilic, with many charged amino acid side chains, and must be extracellular. This domain includes a concentrated segment of four aromatic residues which may be involved in glycoside binding. The carboxyl-terminal domain (residues 50-68) is hydrophobic and probably spans the cell membrane.