Crystal Structure Analysis of a Mutant PhotosystemII Complex Lacking PsbM from Thermosynechococcus vulcanus.

Abstract

PsbM is one of the low-molecular mass subunits of PSII with an approximate molecular mass of 4 kDa and a single trans-membrane helix, and is conserved from cyanobacteria to higher plants. PsbM is located in the central part of PSII dimer in the crystal structure, thus may function in joining the two PSII monomers to form the dimer. In order to study the role of PsbM, we purified PSII from a PsbM-deletion mutant of Thermosynechococcus vulcanus, and crystallized it. We collected the diffraction data to a 4.2 Å resolution, and solved the structure of the mutant PSII. Difference Fourier map between the wild type and mutant PSII indicated that the electron density corresponding to PsbM was lacking in the mutant. The ratio of monomer to dimer was increased in the mutant than that in the wile type, indicating that PsbM is indeed required for the stabilization of PSII dimer.