Abstract
Proton influx was measured after imposition of an electrochemical potential difference for protons (delta muH+) across the cell membrane of the anaerobe, Streptococcus lactis. As delta muH+ was increased, there was an approximately parallel increase in proton entry, until delta muH+ attained 175 to 200 mV. At this point, a new pathway became available for proton entry, allowing an abrupt increase in both the rate and extent of H+ influx. This gated response depended upon the value of delta muH+ itself, and not upon the value of either the membrane potential or the pH gradient. For delta muH+ above 175 to 200 mV, elevated proton entry occurred only in cells having a functional membrane-bound Ca2+-stimulated, Mg2+stimulated adenosine 5'-triphosphatase (EC 3.6.1.3). When present, elevated proton entry coincided with the appearance of net synthesis of adenosine 5'-triphosphate catalyzed by this adenosine 5'-triphosphatase. These observations demonstrate that membrane-bound adenosine 5'-triphosphatase catalyzes an obligatory coupling between the inward movement of protons and synthesis of adenosine 5'-triphosphate.