FUNCTIONAL RECONSTITUTION OF THE PRIMARY QUINONE ACCEPTOR, QA, IN THE PHOTOSYSTEM-II CORE COMPLEXES

PS II reaction center core complexes consisting of CP47, D1, D2 and I proteins and alpha and beta subunits of cytochrome b-559 were isolated from spinach PS II particles using a detergent solution containing 10 mM n-octyl beta-D-thioglucopyranoside (OTG) and 17.5 mM n-octyl beta-D-glucopyranoside (OG). Retention of plastoquinone-9 (PQ-9) in the particles was controlled at different levels from 0.1 to 0.7 per reaction center on average by changing the conditions of the detergent treatments, but primary quinone acceptor (Q(A)) activity was not detected in every preparation. PQ-9, lipids and proteins extracted from the original PS II particles into the detergent solution were separated by several steps of column chromatography to be reconstituted into the isolated complexes. The purified PQ-9 was reinserted into the vacant Q(A) site of the complex, but the Q(A) activity was not restored by itself. However, when the reinsertion of PQ-9 was carried out together with the protein fraction involving the H, L and nuclear encoding 4.1 kDa proteins and the extracted lipids, Q(A) activity was successfully restored in the resulting complexes. The addition of PQ-9 in the reconstitution medium in a molar ratio of 2:1 with the reaction center is sufficient to restore Q(A) function in the PQ-9-depleted complex, suggesting that the added PQ-9 is reinserted in the native Q(A) site of the complex and functions as the primary quinone acceptor under the influence of the H, L, and/or 4.1 kDa proteins and the lipids.