EFFECT OF MGCL2 AND PHOSPHATIDYLGLYCEROL ON CACL2-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 membran e of higher plant chloroplasts. PG was shown previously to stimulate t he evolution of oxygen in intact photosystem II (PSII) membranes [Frag ata, M., Strzalka, K. and Nenonene, E. K. (1991) J. Photochem. Photobi ol. B: Biol 11, 329-342]. In this work, a study was undertaken of the effect of MgCl2, and PG on the CaCl2-mediated recovery of oxygen evolu tion in a PSII complex depleted of the extrinsic proteins (EP) of mole cular masses 17 kDa (EP17) and 24 kDa (EP24), hereunder designated d(1 7,24)PSII. This molecular system is structurally close to the PSII cor e complex of cyanobacteria and is therefore useful in the comparative analysis of PSII-PG relationships in cyanobacteria and the higher plan ts. 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 wel l 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 comple x. That is, PG restores the oxygen evolution in d(17,24)PSII incubated in the presence of MgCl2 and/or CaCl2 to the levels observed in nativ e PSII. Moreover, the site of H2O degradation in d(17,24)PSII, includi ng most probably the pool of calcium and chloride ions, would seem to be protected by phosphatidylglycerol. This suggests that one of the do cking sites of PG in the PSII complex is near EP24, inasmuch as this e xtrinsic protein participates in the regulation of the affinity of the calcium and chloride ions to the water oxidation site. Furthermore, t aking into account that in d(17,24)PSII the PSII core complex is direc tly exposed to PG, then the phospholipid effect reported here indicate s that phosphatidylglycerol might be a functional effector and membran e anchor of the D1 protein in the PSII core complex as was shown recen tly in the cyanobacterium Oscillatoria chalybea [Kruse, O. and Schmid, G. H. (1995) Z. Naturforsch. 50c, 380-390].