Topology of the .alpha.-Subunit of Na,K-ATPase Based on Proteolysis. Lability of the Topological Organization

Topology of the alpha-subunit of Na,K-ATPase has been analyzed utilizing proteolytic digestion. Evidence is presented for a model with 10 transmembrane segments and lability of the C-terminal domain (M7-M10). Using reconstituted proteoliposomes, inside-out oriented pumps were digested with trypsin at the cytoplasmic surface. Evidence was obtained for the M7/M8 pair and cytoplasmic splits between M8 and M9 and between M9 and M10. Because an extracellular split between M9 and M10 was also observed, using right-side-out oriented renal microsomes, we propose that the M9/M10 pair either is destabilized by cytoplasmic digestion or is intrinsically mobile. Using renal microsomes, extracellular digestion of the alpha-subunit by trypsin, chymotrypsin, or an endogenous protease has been observed, after incubation at 55 or at 45 degrees C with beta-mercaptoethanol (beta-ME) and n-butanol. Both perturbations inactivate enzyme activity. Rb ions protect against inactivation and digestion. At 45 degrees C, with beta-ME and n-butanol, trypsin and chymotrypsin cut between M7 and M8 and between M9 and M10, consistent with the 10-segment model. At 55 degrees C, the topological organization is altered, the M8/M9 connecting loop is exposed at the extracellular surface, and an additional split between M8 and M9 is observed. Extracellular digestion of the alpha-subunit is associated with digestion of the beta-subunit near the first extracellular S-S bridge. Rb ions protect the beta-subunit. Exposure to proteases of extracellular domains of both subunits appears to be caused by disruption of subunit interactions.