The Role of Phosphatidylglycerol as a Functional Effector and Membrane Anchor of the Dl-Core Peptide from Photosystem II-Particles of the Cyanobacterium Oscillatoria chalybea

Abstract

The intrinsic polypeptide Dl, isolated from photosystem (PS) II-particles of the cyanobacterium Oscillatoria chalybea, was obtained by electroelution and fractionated extraction with organic solvents. Purification was demonstrated by Western blotting and amino acid sequencing. By carrying out Dl-immunization in rabbits a polyclonal monospecific Dl-antiserum was obtained. For the qualitative characterization of Dl as a lipid-binding peptide, the effect of the lipids phosphatidylglycerol (PG), monogalactosyldiacylglyceride (MGDG) and phosphatidylcho-line (PC) on PSII-oxygen evolution was analysed in reconstitution experiments. In these experiments purified photosystem II (PSII)-particle preparations were treated with the enzyme phospholipase A2 and supplemented with lipid emulsions. We were able to show that the inhibition of electron transport, as the consequence of this lipase treatment, was only relieved, if phosphatidylglycerol was added to the preparation. A model was proposed, in which phosphatidylglycerol is a functional effector for the optimal conformation of Dl in the PSII core complex. Phosphatidylglycerol molecules are unusually tightly bound to the Dl peptide by hydrophobic interactions. A covalent binding seems not probable. The localisation of phosphatidylglycerol binding sites was found by trypsin treatment of Dl and analysis of the obtained oligopeptides with HPLC and immunoblotting. The binding sites could be confined to the hydrophobic amino acid section between arginine 27 and arginine 225, which is known to be the membrane anchor of Dl. This has led us to the conclusion that the phospholipid phosphatidylglycerol plays an important role for anchoring the Dl-peptide and for its orientation in the thylakoid membrane. Phosphatidylglycerol with its high amount of palmitic acid has in prokaryotic cyanobacteria apparently a role in stabilization and orientation. The high turn-over of Dl and the spatial separation of the synthesis- and incorporationsite in the membrane, developed during evolution in eukaryotic organisms, might have changed the requirement on the mobility and the orientation of Dl in photosynthetic membranes. © 1995, Verlag der Zeitschrift für Naturforschung. All rights reserved.