A 150 kilodalton cell surface protein is induced by salt in the halotolerant green alga Dunaliella salina

Abstract

Dunaliella salina is an extremely halotolerant, unicellular, green alga lacking a rigid cell wall. Osmotic adaptation to high salinities is based on the accumulation of glycerol. To uncover other functions responsible for halotolerance, protein profiles of algae continuously grown in different salinities were compared. A 150 kilodalton protein (p150) increased in amount with salt concentration. Furthermore, when the cells were subjected to drastic hyperosmotic shocks, p150 started to rise long after completion of the osmotic response but coincident with reinitiation of cell proliferation. Cells with an initially higher level of p150 resumed growth faster than cells with a lower level of the protein. Addition of cycloheximide early after hyperosmotic shock prevented the rise in p150. indicating this rise was due to de novo synthesis of the protein. These observations suggest that p150 is a salt-induced protein required for proliferation of the cells in saline media. p150 was purified to homogeneity and found to be a detergent-soluble glycoprotein. Polyclonal antibodies against p150 recognized a single protein component in D. salina crude extracts. A high Mr cross-reacting protein was also observed in another Dunaliella strain, D. bardawil. Immunoelectron microscopy localized p150 to the cell surface.