Light-activated amino acid transport systems in Halobacterium halobium envelope vesicles: role of chemical and electrical gradients

Abstract

The accumulation of 20 commonly occurring L-amino acids by cell envelope vesicles of H. halobium, in response to light-induced membrane potential and an artificially created Na gradient, was studied. Nineteen of these amino acids are actively accumulated under either or both of these conditions. Glutamate is unique in that its uptake is driven only by a chemical gradient for Na. Amino acid concentrations at half-maximal uptake rates (Km) and maximal transport rates (Vmax) were determined for the uptake of all 19 amino acids. The transport systems were partially characterized with respect to groups of amino acids transported by common carriers, cation effects and relative response to electrical and chemical components of the Na gradient, the driving forces for uptake. The carrier systems, which are responsible for uptake of individual amino acids, are apparently as variable in their properties as those found in other organisms, i.e., some are highly specific for individual amino acids, some transport several amino acids competitively, some are activated by a chemical gradient of Na only and some function also in complete absence of such a gradient. For all amino acids, Na+ and K+ are both required for maximal rate of uptake. The carriers for L-leucine and L-histidine are symmetrical in that these amino acids are transported in both directions across the vesicle membrane. Coupling of substrate transport to metabolic energy via transient ionic gradients may be a general phenomenon in procaryotes.