Subunit composition, function, and spatial arrangement in the Ca2+-and Mg2+-activated adenosine triphosphatases of Escherichia coli and Salmonella typhimurium.

The Ca²⁺- and Mg²⁺-activated ATPases of Escherichia coli NRC 482 and Salmonella typhimurium LT2 were purified to homogeneity. Both enzymes consisted of five polypeptides (α-?). The molecular weights of the α, β, and ? polypeptides were 56,800, 51,800 and 13,200 for both enzymes. The molecular weights of the γ and δ polypeptides of the E. coli and S. typhimurium ATPases were 32,000 and 20,700, and 30,900 and 21,500, respectively. In both ATPases the stoichiometry of the subunits was α₃β₃γδ? as determined with the ¹⁴C-labeled enzymes. The ATPases of either organism reacted with equal effectiveness with ATPase-deficient particles of the other organism to reconstitute energy-dependent transhydrogenase activity. Treatment of the homogeneous ATPases of both organisms with TPCK-trypsin stimulated ATPase activity but resulted in destruction of coupling factor activity. Trypsin treatment completely digested the δ and ? polypeptides, and removed up to 70% of the γ polypeptide. In the presence of the bifunctional cross-linking reagent dithiobis(succinimidyl propionate) ATPase activity was lost and cross-linking of α to β polypeptides occurred. Crosslinking of α to α or β to β polypeptides was not detected. The function of the individual polypeptides of the ATPase is discussed and a model for their spatial arrangement in the enzyme is presented.