Identification of a region of the H,K-ATPase alpha subunit associated with the beta subunit.

The gastric H,K-ATPase consists of an alpha, beta heterodimer. To determine which part of the alpha subunit is associated with the beta subunit, hog gastric vesicles were trypsinized, which left most of the beta subunit undigested. The vesicles were solubilized with either C12E8 (polyoxyethylene 8 lauryl ester) or Nonidet P-40 and then passed over a wheat germ agglutinin column, which retained the beta subunit. Among the fragments of membrane spanning segments obtained by digestion in the absence of K+, only one peptide fragment of the alpha subunit coeluted with the beta subunit, representing only the M7 and M8 membrane spanning segments of the alpha subunit along with their extracytoplasmic connecting loop. With K+ present during digestion, the fragments at 19-21 kDa and at 9.4 kDa were retained on the column, representing the M7 to C-terminal end and the M5/loop/M6 sector of the enzyme. These results provide biochemical evidence that a region of the alpha subunit containing the M7/loop/M8 sector is bound to the beta subunit under both digestion conditions; but when the C-terminal 180-amino acid sector of the alpha subunit remains intact, as found in the presence of K+, there is an additional retention of the M5/loop/M6 sector of the alpha subunit.