ATP-linked sodium transport in Streptococcus faecalis. II. Energy coupling in everted membrane vesicles.

Everted membrane vesicles of Streptococcus faecalis accumulated “Na+ to a nominal concentration gradient of about 100 when incubated with ATP and M&+. The characteristics of “Na+ accumulation varied in a complex manner as a function of the pH and of the ionic environment. To illustrate, when the vesicles were prepared in potassium ma1eate:Hepes:sucrose buffer (0.45 N K+) and incubated in the same buffer at pH 7.4, uptake of ”Na+ was specifically dependent upon ATP and inhibited by N, N”dicyc1ohexylcarbodiimide (DCCD) and by proton conductors; when the same vesicles were incubated at pH 8, uptake of ”Na+ was resistant to the inhibitors. When the vesicles were incubated at pH 7.4 in Tris/maleate:sucrose buffer, ”Na’ uptake was again unaffected by the inhibitors; moreover, ADP and UTP now supported uptake almost as well as did ATP. The results suggest that there are two modes of ‘*Na+ uptake, one dependent upon the proton-motive force (interior positive and acid) and the other upon ATP. Independent support comes from studies on revertants of mutant 7683, known to lack Na+ transport in both intact cells and membrane vesicles. Revertants that regained this capacity fell into two classes: in one, Na+ movements appear to depend primarily upon the proton-motive force, in the other upon ATP. The present results, as well as those from intact cells (Heefner, D. L., and Harold, F. M. (1980) J. Biot!. Chem 255, 1139611402) can be rationalized by the hypothesis that S. faecal is contains two transport systems for sodium: one a primary, ATP-driven sodium pump and the other a secondary porter linked to the proton circulation but activated by ATP. However, we consider it more likely that these are two modes of sodium transport mediated by a single, ATP-linked system.