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The components involved in the light reactions of photosynthesis are non-covalently bound to proteins which are specifically arranged anisotropically across the lipid bilayer of the thylakoid membrane. There is a strict structure-to-function relationship in the membrane. Using proteolytic enzymes it is possible to investigate the functional components associated with proteins which are close to or protruding from the outside of the thylakoid. e.g. Renger (1976) proposed the existence of a proteinaceous component associated with Photosystem I 1 which has since been identified as a 32 kDa herbicidebinding protein (Mullet & Arntzen, 1981). In addition. by using enzymes which have specificity for particular amino acid bonds i t may be possible to identify the bonds within the protein which are of functional significance. In this study. a comparison was made between the effects of trypsin (specificity for bonds which involve lysine and arginine residues) and chymotrypsin (specificity for tryptophan, tyrosine and phenylalanine). The effects of these enzymes on the function of the membrane were studied using the flash-induced electrochromic bandshift at 515nm (AA, l , ) (Junge & Jackson, 1982). Measurements of AA,,, were made after incubating chloroplasts (25 pg/ml) with trypsin ( 5 pg/ml) or chymotrypsin ( 1 5 pg/ml). Two alterations in AA,,, were seen. (a) The rate of decay was increased, with trypsin having a more marked effect than chymotrypsin. This effect has been correlated with changes in function of the ATPase complex due to partial digestion of coupling factor I (Raines & Hipkins, 1984). (b) The extent was reduced by 80% in the case of trypsin (10 min), but only by 20% with chymotrypsin ( IOmin). The effects of trypsin and chymotrypsin on the amplitude of A A , , , , in chloroplasts suspended in lowor high-salt, are shown in Fig. 1. The Figure shows that a difference existed between highand low-salt controls; this could be due to a direct reduction in efficiency of Photosystem I 1 (Bose & Arntzen, 1978) or to a redistribution of energy in favour of Photosystem I. Fig. I(u) shows that trypsin caused a greater reduction in A A , , , of thylakoids suspended in high-salt than in similar low-salt samples. In contrast, Fig. l (h ) shows that chymotrypsin had a similar effect in both media. This difference may be due to the ability of trypsin to remove ;I portion of the light-harvesting chlorophyll a/h protein (LHCP) of Photosystem 11, which is involved in the maintenance of granal stacks, and in the control of energy redistribution between the two photosystems (Steinbeck et d., 1979). Measurements were made of A A , , , induced by Photosystem I alone, and i t was seen

Comparison of the effects of trypsin and chymotrypsin on thylakoid membrane function