Dunaliella salina Hsp90 is halotolerant

Dunaliella salina is a unicellular green alga with exceptional halotolerance. Although the D. salina cells are capable to proliferate in hypersaline medium, the intracellular salt concentrations are maintained at a low level. Thus the extracellular but not intracellular Dunaliella proteins are expected to be highly halotolerant. In this research, we compared the salt-dependence of the activity and stability of Hsp90s from the halotolerant alga D. salina (dsHsp90) and the mesophilic alga Chlamydomonas reinhardtii (crHsp90). We found that the ATPase activity of crHsp90 could be enhanced about six-fold by 2 M NaCl, while the activity of dsHsp90 showed a much weaker dependence on salinity. When denatured by urea, both crHsp90 and dsHsp90 exhibited an apparent three-state unfolding with the population of an unfolding intermediate. High salinity significantly decreased the Gibbs free energy change of crHsp90 but not dsHsp90 for the transition from the native state to the intermediate. The little dependence of dsHsp90 activity and folding on salinity suggests that dsHsp90 is halotolerant though it is an intracellular protein. We propose that the halotolerance of intracellular Dunaliella proteins might play a role in fighting against the transient intracellular salt fluctuations during hyperosmotic or hypoosmotic shock.