ИСТИНА

Marine unicellular green microalga Tetraselmis (Platymonas) viridis is a convinient model system allowing to study the molecular mechanisms underlying salt adaptation in plant cells. This organism tolerates successfully salt concentration up to 1 M NaCl, but it is able to grow at low NaCl concentrations as well. Salt tolerance, as it often assumed, could be the result of altered gene expression. In this study effects of long-term adaptation to five distinct salt concentrations (10 mM, 50 mM, 500 mM, 900 mM and 1,2 M NaCl in an artificial sea water) on protein composition of plasma membrane (PM) were analysed. Special attention was payed to Na+-transporting ATPase, which has been demonstrated in PM of T. viridis [1]. This primary electrogenic vanadate-sensitive Na+-ATPase supposed to play a key role in regulation of Na+ concentration in cytoplasm and in halotolerance of this alga. The electrophoresis of PM proteins in SDS-PAG revealed that long-term adaptation did not induce synthesis of any unique PM polypeptides or cause disappearance of some polypeptides, but did cause changes in the amounts of several polypeptides. The Na+-ATPase from T. viridis belongs to the family of P-type cation-pumping ATPases and forms the phosphorylated intermediate with the apparent molecular weight of 100 kDa [2]. No consistent changes in the intensity of bands in the 100 kDa region could be observed comparing the control membranes (500 mM NaCl) and those isolated from high salt- or low salt-treated alga after long-term adaptation. This could indicate similar amounts of the plasma membrane P-type ATPases under all conditions which is quite typical for different plant cells [3, 4]. Nevertheless, several slight changes could be detected in this region after short-term (20 h) salt stress. Three bands corresponding to the proteins of 118, 112 and 100 kDa were intensified. More marked differences after long-term adaptation were detected in the lower molecular weight region. Proteins with apparent molecular masses of 80, 73, 70, 50 and 38 kDa were intensified with an increase in external sodium chloride concentration whereas proteins in the 60-70 kDa region were diminished in high salt adapted algae. The most striking change was an increase in polypeptides with apparent molecular mass of 25 and 27 kDa, especially after short-term salt stress. The detailed functions of these proteins are unknown, but they could be useful targets for identifying possible mechanisms of salt tolerance in this alga. The data obtained suggest that adaptation to high salt concentration of T. viridis should be associated with increasing specific activity of the Na+-ATPase which in turn could be the result of post-translation modification of the protein or the result of physiological control on the ATPase. (This work is supported by RFBR, grant No.01-04-49135).