FUNCTIONAL ANALYSIS OF PROTEINS ON THE THYLAKOID MEMBRANE——EFFECTS OF Mg 2+ AND BSA ON THE TRYPSIN-INDUCED CHLOROPHYLL a FLUORESCENCE AND THYLAKOID MEMBRANE SURFACE CHARGE CHANGES IN SPINACH CHLOROPLASTS

Abstract

Trypsin digestion of spinach chloroplasts not only inhibits light-induced variable fluorescence yield (△F/F_0) but also abolishes Mg~(2+) stimulating effect on △F/F_0. If chloroplasts are incubated by Mg~(2+) prior to trypsin digestion, Mg~(2+) stimulating ability remains unchanged whereas the enzyme-induced △F/F_0 decline itself is not affected. The protective effect of pretreatment of chloroplasts with Mg~(2+) against trypsin closely correlated with alteration of the net negative charge on the outer surface of the thylakoid membrane by Mg~(2+). In contrast to this, addition of BSA to the chloroplast digested with trypsin significantly recovers the light-induced variable fluorescence yield but it does not influence Mg~(2+)-induced variable fluorescence change. The recovery effect of BSA incubation is not dependent upon Mg~(2+) concentration. Also, BSA does not alter the net negative charge on the outer surface of the thylakoid membrane.These results demonstrate the existence of two independent proteins or polypeptides on the outer surface of the thylakoid membrane which play an important role for the functional integrity of excitation energy distribution between PS-Ⅱ and PS-Ⅰ. Different mechanisms involved in the Chl a fluorescence change are considered in this context.