Effect of MgCl 2 and Phosphatidylglycerol on CaCl 2 -Mediated Recovery of Oxygen Evolution in a Photosystem II Complex Depleted of the 17 and 24 kDa Extrinsic Proteins

Phosphatidylglycerol (PG) is an anionic lipid of the thylakoid membrane of higher plant chloroplasts. PG was shown previously to stimulate the evolution of oxygen in intact pho­tosystem II (PSII) membranes [Fragata, M., Strzałka, K. and Nénonéné, E. K. (1991) J. Photochem. Photobiol. B: Biol 11, 329-342], In this work, a study was undertaken of the effect of MgCl 2 and PG on the CaCl 2 -mediated recovery of oxygen evolution in a PSII complex depleted of the extrinsic proteins (EP) of molecular masses 17 kDa (EP17) and 24 kDa (EP24), hereunder designated d 17.24 PSII. This molecular system is structurally close to the PSII core complex of cyanobacteria and is therefore useful in the comparative analysis of PSII-PG relationships in cyanobacteria and the higher plants. This work reveals a new aspect of the thylakoid lipids role in the PSII function, namely the PG effect on intact PSII is observed as well in d 17.24 PSII. The results show that phosphatidylglycerol has the ability to compensate for the loss of EP17 and EP24 in the PSII complex. That is, PG restores the oxygen evolution in d 17.24 PSII incubated in the presence of MgCl 2 and/or CaCl 2 to the levels observed in native PSII. Moreover, the site of H 2 O degradation in d 17.24 PSII, including most probably the pool of calcium and chloride ions, would seem to be protected by phosphatidyl­glycerol. This suggests that one of the docking sites of PG in the PSII complex is near EP24, inasmuch as this extrinsic protein participates in the regulation of the affinity of the calcium and chloride ions to the water oxidation site. Furthermore, taking into account that in d 17.24 PSII the PSII core complex is directly exposed to PG, then the phospholipid effect reported here indicates that phosphatidylglycerol might be a functional effector and mem­brane anchor of the D 1 protein in the PSII core complex as was shown recently in the cyanobacterium Oscillatoria chalybea [Kruse, O. and Schmid, G. H. (1995) Z. Naturforsch. 50c, 380-390],