CALCIUM-TRANSPORT IN HALOBACTERIUM-HALOBIUM ENVELOPE VESICLES

Calcium transport was demonstrated in envelope vesicles prepared from Halobacterium halobium. Calcium influx is driven by a Na+ gradient (in > out), and the initial transport rates are proportional to log( Nain Naout).The addition of monensin completely inhibits transport, but FCCP has no effect. The results suggest the existence of sodium/ calcium antiport. Calcium appears to be internally bound, since it does not exit upon the addition of monensin, although efflux of 45Ca is achieved when a 20-fold excess of calcium is added to the external medium. Calcium uptake follows Michaelis-Mententype kinetics, with a Km of 71 μm and V of 2.0 nmol · min-1 · mg-1 of membrane protein. Illumination, which provides an electrical potential owing to light-induced proton translocation by bacteriorhodopsin, has no appreciable effect on calcium efflux from preloaded vesicles. The transport carrier requires both NaCl and KCl: Na+ for energetic reasons, required on the trans side of translocation, and K+ for regulatory reasons, required probably on the cis side of translocation. The results provide evidence for a specific sodium/calcium antiporter in these halophilic cells, similar to that found in eukaryotic cells, but unlike other investigated bacterial systems which have proton/ calcium antiporters. © 1978.