ATP-driven sodium pump in Streptococcus faecalis.

Abstract

Na+ extrusion by bacteria is generally attributed to secondary antiport of Na+ for H+ energized by the proton circulation. S. faecalis is an exception, in that Na+ expulsion from intact cells requires the generation of ATP but does not depend on the protonmotive force. Unfortunately, studies with everted membrane vesicles failed to reveal the expected Na+ pump; instead, the vesicles contained a conventional secondary Na+/H+ antiporter. Everted membrane vesicles prepared in the presence of protease inhibitors retain an ATP-driven Na+ transport system. Accumulation of 22Na+ by these vesicles is resistant to reagents that dissipate the protonmotive force but requires ATP. The vesicles contain a Na+-stimulated ATPase that is distinct from F1F0 ATPase, and whose presence is correlated with Na+ transport activity. Na+ movements appear to be electroneutral and are accompanied by movement of H+ in the opposite direction. When membranes are incubated in the absence of protease inhibitors, a secondary Na+/H+ antiport activity emerges, possibly by degradation of the Na+ pump. S. faecalis may expel Na+ by means of an ATP-driven primary transport system that mediates exchange of Na+ for H+. The Na+/H+ antiporter seen in earlier membrane preparation is an artefact of proteolytic degradation.