Research
The water oxidation reaction that releases molecular oxygen into the atmosphere is catalyzed by the oxygen-evolving complex (OEC) of photosystem II (PSII), a multisubunit enzyme found in all photosynthetic organisms (cyanobacteria, algae, higher plants). In higher plants, the OEC consists of the extrinsic subunits (17, 23, and 33 kDa polypeptides) that surround the OEC active site that binds the inorganic ion (Manganese, Calcium, and Chloride) cofactors. The largest extrinsic protein,
the manganese-stabilizing protein (MSP;
33 kDa), binds to PSII to form the structure that stabilizes the
Mn cluster (consisting of four Mn atoms) and regulates the presence of
Calcium and
Chloride cofactors in the proximity of the Mn cluster.
A model of PSII structure based on the crystal structure from the cyanobacterium T. elongatus at 3.2 Å resolution according to Besiadka et al. (2004) Phys Chem Chem Phys 6, 4733-4736.
Several amino acid residues of MSP that are required for the structural and functional assembly of the protein into PSII have been identified. It has been revealed that the only disulfide bridge in MSP is not required for protein function, while the C-terminal domain was found to be essential for functional assembly of the protein into PSII. The N-terminal domain is a determinant for the binding of MSP to PSII. Truncation of the protein by several N-terminal amino acid residues increases its structural compactness in solution, and also causes the mutant protein to bind non-specifically to PSII. Experimental data also show that wild-type MSP from higher plants binds two copies per PSII reaction center. N-terminal deletions of MSP have been produced that bind two, one or zero copies to specific binding sites on PSII. Biochemically-refined preparations of spinach photosystem II particles can be depleted of all extrinsic proteins. Currently, we reconstitute these depleted particles with MSP deletion mutants to investigate its role in binding of inorganic ion cofactors and maintaining the stability of the Mn cluster.
In solution, MSP behaves as an intrinsically disordered protein. It is thermostable (even without its native disulfide bridge), has a high content of unordered secondary structure, is acidic, and produces exaggerated estimates of size on SDS-PAGE and by gel filtration chromatography. The high charge on MSP prevents protein aggregation in acidic environments like that of the thylakoid lumen, and this may explain part of the rationale for its unusual behavior in solution. Using biochemical and spectroscopic techniques, we are conducting additional explorations of the intrinsically disordered behavior of the protein in connection with its function in PSII.
Crystal structure of the manganese-stabilizing protein from cyanobacteria T. elongatus according to Ferreira et al. (2004) Science 303, 1831-1838.The crystal structure of MSP from higher plants in not available.
