Loss of membrane transport ability in leaf cells and release of protein as a result of osmotic shock

Abstract

Osmotic shock severely reduces the ability of aged strips of Phaseolus vulgaris leaves to take up alpha-aminoisobutyric acid, an amino acid analogue which is known to be transported by a specific mechanism. Cold osmotic shock, i.e., transfer from 0.5 m sucrose at 25 C to H(2)O at 2 C, decreases alpha-aminoisobutyric acid uptake almost to zero. Substitution of 10(-3)m ethylenediaminetetraacetate for the sucrose, i.e., treatment which does not involve plasmolysis, produces a similar, but less severe, effect.About 3.5% of the total cell protein is released as a result of cold osmotic shock, by far the greater proportion being liberated into the water during the second stage of the shock treatment. Ethylenediaminetetraacetate and other shock treatments also bring about protein release, and the amount released is correlated with degree of depression of subsequent alpha-aminoisobutyric acid uptake.Shock tissue is capable of recovering a large proportion of its uptake capacity during subsequent immersion in 10(-4)m CaSO(4).Separate estimation of alpha-aminoisobutyric acid influx and efflux showed that the marked effect of shock on net flux is largely attributable to a reduction in influx, and not to an increase in efflux. This and other results indicate that the shock effect on net flux is not due to nonspecific damage to membranes bringing about "leakiness."The fact that alpha-aminoisobutyric acid uptake is reduced to near zero by treatment which allows the cells to retain over 95% of their protein suggests that the shock phenomenon is analogous to that in bacteria, and that the small fraction of protein lost may be closely involved in the transport mechanism.