Abstract
The halotolerant alga, D. salina, offsets the high (up to 5 M) external NaCl concentration in its environment by maintaining a nearly equivalent level of intracellular glycerol. Cells experiencing sudden hypoosmotic shock respond initially by volume expansion and then convert glycerol to osmotically inactive starch. Within 2 min following dilution of cells from 1.7 M NaCl to 0.85 M NaCl, their volume and surface area increase by 50%. Concurrently, the plasma membrane phosphatidylinositol 4,5‐bisphosphate (PIP~2~) level falls by 30% due to hydrolysis by a GTP‐binding, protein‐activated phospholipase C that produces inositol trisphosphate and diacylglycerol (DAG). Following the short‐lived rise of PIP~2~‐derived DAG, a second and larger peak of DAG is produced by the hydrolysis of phosphatidylcholine and through de novo synthesis. Since protein kinase C is not detectably stimulated, DAG may act through the alternative pathway of increasing actin nucleation sites, thereby facilitating expansion of the cell's cytoskeleton. D. salina contains a family of at least 4 low molecular weight (28–30 kDa) GTP‐binding proteins. The major one of these (28 kDa) is immunologically cross‐reactive with antibodies to the yeast ypt1 protein that is involved in intracellular vesicle trafficking. The D. salina 28 kDa protein may play a similar role in facilitating the plasma membrane expansion triggered by hypoosmotic shock. During this expansion, which features a massive fusion of cytoplasmic vesicles with the plasma membrane, there is significant translocation of the 28 kDa GTP‐binding protein from the ER‐enriched microsomal fraction to the plasma membrane, chloroplast, and cytosol. Factors regulating these translocations are under study. © 1994 Wiley‐Liss, Inc.