Roles of PsbM and PsbI subunits in the formation and stabilization of PSII dimer in Thermosynechococcus vulcanus

Abstract

PsbM and PsbI are two low-molecular mass subunits of PSII, and PsbM is located in the center of PSII dimer, whereas PsbI is located in its periphery. Thease locations were confirmed by crystal structure analysis of their deletion mutants, and suggest that PsbM is required for the stable formation of PSII dimer, whereas PsbI may not be directly related to the dimer formation or stability. Thylakoids isolated from the two deletion mutants, however, showed a similar decrease in PSII dimer, suggesting that both subunits are required for the dimer formation or stability. Further analysis with purified dimers showed that while dimers lacking PsbM were unstable and decomposed into PSII monomer upon appropriate detergent treatment, dimers lacking PsbI were stable against the same treatment, suggesting that PsbI may be required for the assembly of dimer in vivo but may not be needed for the stability of dimer in vitro.