REVERSIBLE CHANGES IN CONFORMATION OF THE 23-KDA PROTEIN OF PHOTOSYSTEM-II AND THEIR RELATIONSHIP TO THE SUSCEPTIBILITY OF THE PROTEIN TO A PROTEINASE FROM PHOTOSYSTEM-II MEMBRANES

An aspartic proteinase was partially purified from PSII membranes of spinach. The 23-kDa protein of PSII was resistant to the proteinase when in its native form but became susceptible upon modification by p-chloromercuriphenylsulfonic acid (pCMPS), upon incubation at pH 3, and upon incubation in dilute SDS. Conformational changes caused by pCMPS seemed to be restricted to a localized region since the modified protein reconstituted the water-oxidizing complex with slightly less efficiency as compared with the untreated protein. Under the three proteolytic conditions, cleavage occurred at the Asn-58-Leu-59 bond, suggesting that the protein underwent similar conformational changes in each case. Dithiothreitol, dialysis at pH 7, and neutral detergents suppressed the facilitative effects of pCMPS, pH 3, and SDS on proteolysis, respectively. With the reversible changes in susceptibility, reversible changes in circular dichroism of the protein also occurred at 200-208 nm. These observations indicate that the conformational changes are reversible and the renaturation of the substrate was associated with the suppression of the susceptibility. These results suggest that the scissile bond becomes reversibly exposed and susceptible to the proteinase in response to environmental changes.