D1-CP43 Aggregates in Photosystem II Formed by Acceptor-Side Photoinhibition are Substrates for an SDS-Resistant Protease in Stroma

Abstract

Strong illumination acts primarily to damage photosystem (PS) II in thylakoid membranes. After the initial damage to the reaction-center binding D1 protein, D1 is degraded and cross-linked to the surrounding polypeptides in PS II. These events can be detected by SDS/urea-PAGE, Western blotting with antibodies specific to D1 and subsequent high-sensitivity fluorography with ECL (1). The relationship between the degradation and aggregation of D1 is not clear. It is possible that the protein-aggregation step represents a mechanisms for circumventing protein degradation and that the D1 aggregates, once formed, are not processed any further by a specific protease or proteases. If such is the case, the formation of D1 aggregates should, itself, compete with the degradation of D1, decreasing the efficiency of turnover of D1 molecules. In the present report, we show that D1-CP43 aggregates are generated both by donor-side and by acceptor-side photoinhibition. The D1-CP43 aggregates, formed on the stromal side by acceptor-side photoinhibition, are efficiently digested by an SDS-resistant protease (or proteases) that is located in the stroma.