Abstract
Sulfoquinovosyl-diacylglycerol (SQDG) is one of the four lipids present in the thylakoid membranes. Depletion of SQDG causes different degrees of effects on photosynthetic growth and activities in different organisms. Four SQDG molecules bind to each monomer of photosystem II (PSII), but their role in PSII function has not been characterized in detail, and no PSII structure without SQDG has been reported. We analyzed the activities of PSII from an SQDG-deficient mutant of the cyanobacterium Thermosynechococcus elongatus by various spectroscopic methods, which showed that depletion of SQDG partially impaired the PSII activity by impairing secondary quinone (QB) exchange at the acceptor site. We further solved the crystal structure of the PSII dimer from the SQDG deletion mutant at 2.1 Å resolution and found that all of the four SQDG-binding sites were occupied by other lipids, most likely PG molecules. Replacement of SQDG at a site near the head of QB provides a possible explanation for the QB impairment. The replacement of two SQDGs located at the monomer-monomer interface by other lipids decreased the stability of the PSII dimer, resulting in an increase in the amount of PSII monomer in the mutant. The present results thus suggest that although SQDG binding in all of the PSII-binding sites is necessary to fully maintain the activity and stability of PSII, replacement of SQDG by other lipids can partially compensate for their functions.
Highlights
Sulfoquinovosyl-diacylglycerol (SQDG) is one of the four lipids present in the thylakoid membranes
Analyses on the content of photosystem II (PSII) dimer and oxygen evolution. It was shown by blue native-PAGE analysis that inactivation of the sqdB gene in the thermophilic cyanobacterium remarkably affected the stability of the PSII dimer, resulting in a large increase in the content of the PSII monomer and an almost disappearance of the PSII dimer in the mutant [3]
We examined the effects of inactivation of the sqdB gene that results in a mutant unable to synthesize SQDG, on the stability and activity of PSII dimer from the thermophilic cyanobacterium T. elongatus
Summary
It was shown by blue native-PAGE analysis that inactivation of the sqdB gene in the thermophilic cyanobacterium remarkably affected the stability of the PSII dimer, resulting in a large increase in the content of the PSII monomer and an almost disappearance of the PSII dimer in the mutant [3]. The results showed that the oxygen-evolving activity of the mutant samples decreased by about 19 –32% in the thylakoid membranes and purified PSII dimers This decrease is comparable with those observed with the mutant cells of T. elongatus reported previously [3] and with the previous report in the C. reinhardtii mutant with the SQDG synthesis gene inactivated [15,16,17]. The damping of the FTIR difference spectra is attributed to the misses in the S-state transitions, which increase the mixture of other transitions at increased flash numbers [25] All of these spectral features were found to be similar in the ⌬sqdB mutant PSII (Fig. 4A), suggesting that absence of SQDG did not affect the structures and reactions of the water-oxidizing center significantly. Unit cell (Å) Resolution (Å) Unique reflections Redundancy Completeness Rmerge I/(I) CC(1/2)
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