Loss of sensitivity to diuron after trypsin digestion of chloroplast photosystem II particles

The membranes of the chloroplast contain numerous polypeptides, most of which have not yet been assigned to any photosynthetic function. Only the components of photosystem I [ 11, the chlorophyll a/b light harvesting protein [3,4] and of the coupling factor [2] have been clearly resolved. Most other photosynthetic functions have yet to be associated with any of the remaining poly~ptides. One approach to relating indi~du~ polypeptides to function is to use selective trypsin digestion. When used on membrane systems, trypsin digestion has the added advantage of allowing exploration of the topographical organization of the membrane; only those functions that involve components exposed to the outer membrane surface would be expected to be affected by trypsin. When this approach is applied to chloroplast membranes, a variety of alterations in photosynthetic function result. These include a loss of Mg2*-sensitive c~orophy~ fluorescence [S-7], a loss of sensiti~ty to i~bition by diuron [Sj, a drop in the Em, of cytochrome b-559 [7,9] and inhibition of water oxidation 191. Apart from cataloging these effects, it is very difficult to draw further conclusions because of the multitude of trypsin-induced changes. One approach to refining this technique has been to use a very brief exposure to low trypsin concentration such