Radiation inactivation analysis of oligomeric structure of the H,K-ATPase

The oligomeric size of the H,K-ATPase was determined in frozen gastric microsomal vesicles irradiated with high energy electrons. Target sizes of various catalytic activities associated with H,K-ATPase function fell into two distinct groups. The lower group of target sizes described the radiation-induced loss of steady-state phosphoenzyme and structural monomer: the MgATP-dependent formation of a beta-aspartyl phosphate exhibited a size range of 133-147 kDa; the size range for the structural measurement (i.e. loss of H,K-ATPase monomer on sodium dodecyl sulfate-polyacrylamide gels) was 92-143 kDa. In contrast, a larger group of target sizes described the loss of full cycle catalytic activities (i.e. K+-dependent stimulation of p-nitrophenyl phosphate and ATP hydrolysis). The K+-phosphatase and K+-stimulated ATPase exhibited target sizes fo 200 +/- 13 and 232 +/- 23 kDa, respectively. The lower target size group represents the first evidence that a monomer of the catalytic subunit maintains partial enzyme function. The larger group of target sizes describing K+-phosphatase and ATPase activities suggest that subunit interactions contribute to full cycle catalytic activity. Subunit interactions appear to be involved in all ion transport activities. Passive Rb+ exchange and active H+ transport in reconstituted proteoliposomes exhibited target sizes of 233n = 2 and 388 +/- 48 kDa, respectively. H+ transport appears to require a subunit arrangement more complex than that associated with catalytic activity or passive ion transport.