Mutual relationships between structural and functional changes in a PsbM-deletion mutant of photosystem II.

Photosystem II (PSII) is a membrane protein complex that performs light-induced electron transfer and oxygen evolution from water. PSII consists of 19 or 20 subunits in its crystal form and binds various cofactors such as chlorophyll a, plastoquinone, carotenoid, and lipids. After initial light excitation, the charge separation produces an electron, which is transferred to a plastoquinone molecule (QA) and then to another plastoquinone (QB). PsbM is a low-molecular-weight subunit with one transmembrane helix, and is located in the monomer-monomer interface of the PSII dimer. The function of PsbM has been reported to be stabilization of the PSII dimer and maintenance of electron transfer efficiency of PSII based on previous X-ray crystal structure analysis at a resolution of 4.2 Å. In order to elucidate the structure-function relationships of PsbM in detail, we improved the quality of PSII crystals from a PsbM-deleted mutant (ΔPsbM-PSII) of Thermosynechococcus elongatus, and succeeded in improving the diffraction quality to a resolution of 2.2 Å. X-ray crystal structure analysis of ΔPsbM-PSII showed that electron densities for the PsbM subunit and neighboring carotenoid and detergent molecules were absent in the monomer-monomer interface. The overall structure of ΔPsbM-PSII was similar to wild-type PSII, but the arrangement of the hydrophobic transmembrane subunits was significantly changed by the deletion of PsbM, resulting in a slight widening of the lipid hole involving QB. The lipid hole-widening further induced structural changes of the bicarbonate ion coordinated to the non-heme Fe(ii) atom and destabilized the polypeptide chains around the QB binding site located far from the position of PsbM. The fluorescence decay measurement indicated that the electron transfer rate from QA to QB was decreased in ΔPsbM-PSII compared with wild-type PSII. The functional change in electron transfer efficiency was fully interpreted based on structural changes caused by the deletion of the PsbM subunit.

Electric-field-temperature phase diagram of an antiferroelectric liquid crystal.

We present a precise electric-field-temperature phase diagram of an antiferroelectric liquid crystal with a short pitch Sm-Calpha* phase. This was obtained by using a photoelastic modulator. A unique field-induced phase was found inside the Sm-Calpha* phase, which displayed low birefringence. Two tricritical points related to the phase were also observed. In addition, numerical calculations were made based on the discrete phenomenological model. The numerical results reproduced the experimental ones and it was clarified that the phase has a three-layer structure without spatial modulation.