Regulation of H Excretion : ROLE OF PROTEIN RELEASED BY OSMOTIC SHOCK.

When the protoplasts of peeled oat leaf segments (Avena sativa L.) expand after a brief plasmolysis (osmotic shock), fusicoccin-enhanced H(+) excretion is reduced and protein is released to the rehydration medium. This shock protein seems to arise from the cell surface, not from the interior of leaky cells or from broken cells, because (a) the protein differs quantitatively and qualitatively from protein of cell homogenates as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis; (b) peroxidase, phosphatase, and malate dehydrogenase activities, which are associated with the cell surface, are detected in the shock fluids; (c) the specific activities of enzymes in shock fluids are different than those of cell homogenates; (d) the amount of protein released is correlated with tissue mass, not number of cut surfaces and is not diminished by pre-washing the tissue.Some of the shock protein may arise from plasmodesmata; this suggestion is based on (a) the cell surface origin of the protein; (b) the presence in the shock fluid of NADPH-cytochrome c reductase activity, usually associated with the endoplasmic reticulum which traverses plasmodesmata; (c) on the release of smaller amounts of protein after plasmolysis with polyethylene glycol 4000, an osmoticum which may tend to preserve plasmodesmata.The amount of protein released by osmotic shock is correlated with the extent of inhibition of fusicoccin-enhanced H(+) excretion. A specific function for the shock protein is implied by the presence of a component which specifically binds fusicoccin.