Comparison of the structure of the extrinsic 33 kDa protein from different organisms.

The psbO gene encoding the extrinsic 33 kDa protein of oxygen-evolving photosystem II (PSII) complex was cloned and sequenced from a red alga, Cyanidium caldarium. The gene encodes a polypeptide of 333 residues, of which the first 76 residues served as transit peptides for transfer across the chloroplast envelope and thylakoid membrane. The mature protein consists of 257 amino acids with a calculated molecular mass of 28,290 Da. The sequence homology of the mature 33 kDa protein was 42.9-50.8% between the red alga and cyanobacteria, and 44.7-48.6% between the red alga and higher plants. The cloned gene was expressed in Escherichia coli, and the recombinant protein was purified, subjected to protease-treatments. The cleavage sites of the 33 kDa protein by chymotrypsin or V8 protease were determined and compared among a cyanobacterium (Synechococcus elongatus), a euglena (Euglena gracilis), a green alga (Chlamydomonas reinhardtii) and two higher plants (Spinacia oleracea and Oryza sativa). The cleavage sites by chymotrypsin were at 156F and 190F for the cyanobacterium, 159M, 160F and 192L for red alga, 11Y and 151F for euglena, 10Yand 150F for green alga, and 16Y for spinach, respectively. The cleavage sites by V8 protease were at 181E (cyanobacterium), 182E and 195E (red alga), 13E, 67E, 69E, 153D and 181E (euglena), 176E and 180E (green alga), and 18E or 19E (higher plants). Since most of the residues at these cleavage sites were conserved among the six organisms, the results indicate that the structure of the 33 kDa protein, at least the structure based on the accessibility by proteases, is different among these organisms. In terms of the cleavage sites, the structure of the 33 kDa protein can be divided into three major groups: cyanobacterial and red algal-type has cleavage sites at residues around 156-195, higher plant-type at residues 16-19, and euglena and green algal-type at residues of both cyanobacterial and higher plant-types.

A domain of the manganese-stabilizing protein from Synechococcus elongatus involved in functional binding to photosystem II.

Site-directed mutagenesis was performed to investigate whether the two protease-sensitive sequences Phe(156)-Gly(163) and Arg(184)-Ser(191), of the manganese-stabilizing protein (MSP) from a thermophilic cyanobacterium, Synechococcus elongatus (Motoki, A., Shimazu, T., Hirano, M., and Katoh, S. (1998) Biochim. Biophys. Acta 1365, 492-502), are involved in functional interaction with photosystem II (PSII). The ability of MSP to bind to its functional site on the PSII complex and to reactivate oxygen evolution was dramatically reduced by the substitution of Arg(152), Asp(158), Lys(160), or Arg(162) with uncharged residues, by insertion of a single residue between Phe(156) and Leu(157), or by deletion of Leu(157). Substitution of each of the four charged residues with an identically charged residue showed that the charges at Asp(158), and possibly Lys(160), are important for the electrostatic interaction with PSII. The reactivating ability was also strongly affected by the alteration of Phe(156) to Leu. Replacement of Lys(188), the only strictly conserved charged residue in the Arg(184)-Ser(191) sequence, by Gln had only a marginal effect on the function of MSP. High affinity binding of MSP to PSII was also affected significantly by mutation at Arg(152), which is located in a region (Val(148)-Arg(152)) strictly conserved among the 14 sequences so far reported. These results imply that the Val(148)-Gly(163) sequence, which is well conserved among MSPs from cyanobacteria to higher plants, is a domain of MSP for functional interaction with PSII.