2 D1 PROTEIN-DEGRADATION PATTERNS IN ISOLATED PHOTOSYSTEM-2 CORE AND REACTION-CENTER COMPLEXES

A double induction mechanism of D1 protein degradation in isolated pho tosystem 2 (PS2) core complexes and reaction centres is described, sho wing the existence of two potential sites for primary cleavage. D o n o r s i d e i n h i b i t i o n c o n d i t i o n s (presence of elect ron acceptors but no electron donors and pH 8.0) trigger the hydrolysi s of the D1 protein between the putative helices I and II on the lumen al side of the thylakoid membrane. This results in the generation of a C-terminal 24 kDa fragment. However, when the donor-side is active (p resence of electron donors but no electron acceptors and pH 6.0, a c c e p t o r s i d e i n h i b i t i o n conditions) both preparations a re able to produce a N-terminal 23 kDa fragment, indicating cleavage b etween helices IV and V, on the stromal side of the membrane.