Freezing of Isolated Thylakoid Membranes in Complex Media. IV. Stabilization of CF 1 by ATP and Sulfate

Abstract

Summary Freeze-thaw treatment of isolated spinach thylakoid membranes in buffered media containing Cl-, NO3-, K+, Na+, and Mg2+ at concentrations close to those found in the stroma of intact chloroplasts resulted in inactivation of photophosphorylation. This membrane damage was mainly due to dissociation of CF1, which caused an increase in H+ permeability of the thylakoid membrane and stimulation of linear photosynthetic electron transport; simultaneously, uncoupled electron flow decreased, reflecting a partial inhibition in the capacity of the electron transport chain. The addition of ATP or sulfate at concentrations comparable to those in the chloroplast stroma in situ provided marked protection of photophosphorylation against freeze-thaw inactivation. Under these conditions, the ability to form a transthylakoid proton gradient in the light was largely maintained and uncoupling of photophosphorylation from electron transport and dissociation of peripheral membrane proteins did not occur. Obviously, both ATP and sulfate were highly effective in stabilizing CF1 binding during freezing. The lack of complete cryoprotection of photophosphorylation of isolated thylakoids by ATP and sulfate was likely due to the inability of these solutes to prevent partial damage of the electron transport chain. As an efficient cryopreservation of photophosphorylation was also reached in the presence of other essential compounds of the chloroplast stroma (sugars, amino acids, phosphate), it is concluded that in intact leaves CF1 may be sufficiently protected in the course of freeze-thawing.