Role of PM-ATPase, amino acid transport and free amino acid pool in the salt stress of, Candida membranefaciens.

The salt tolerant yeast Candida membranefaciens exhibited a pleiotropic modification in response to high NaCl stress. The in vivo specific activity of Plasma Membrane-ATPase (PM-ATPase) of 1.35 M NaCl adapted cells was enhanced at the mid-log phase. The enhancement in the PM-ATPase activity was NaCl specific as cells stressed with identical concentration of KCl did not have any effect on PM-ATPase. The NaCl specific enhancement in the PM-ATPase activity was associated with decreased Km. Studies on H+ efflux correlated with the results of PM-ATPase. However, in vitro incubation of the enzyme with exogenously added salts like NaCl and KCl invariably inhibited enzyme activity by 70-90% in a dose dependent manner to suggest that in vivo effects of the salts on PM-ATPase were different from the in vitro effects. C. membranefaciens showed a higher intracellular levels of glutamate and aspartate in presence of 1.35 M NaCl which may impart osmoprotection to the stressed cells. It was interesting to observe that the transport activities of aspartate and glutamate were not enhanced according to their relative proportion in the total pool of free amino acids. Instead, transport of these and other amino acids (except lysine and arginine) showed a drastic reduction (upto 90%) in the 1.35 M NaCl grown cells.