29th Annual Meeting of the American Society of Photobiology

Downtown Marriot

Chicago, Il.

Interaction of the Core Light-Harvesting Complex with the Reaction Center in Photosynthetic Bacteria: Role of PufX

Abstract-
Because the structures of the reaction center (RC) and accessory light-harvesting (LH2) complexes have been solved and a low resolution structure of the core light-harvesting complex (LH1) has been obtained using electron cryomicroscopy, the field of bacterial photosynthesis has matured to the level where the main goal is to establish structure-function relationships of this supramolecular complex as a whole. It is likely that other as yet undiscovered proteins play an important role in the structure and function of this complex. One such protein, PufX, is found in Rhodobacter species and has been implicated to interact with LH1 to allow reducing equivalents from the RC in the form of ubiquinol to reach the bc1 complex. This protein has been isolated by this laboratory from Rb. sphaeroides and Rb. capsulatus and shown to inhibit the reconstitution of LH1 complexes The mature forms of these proteins contain twelve and nine fewer amino acids, respectively, at the C-terminal end of the protein than are encoded by their pufX genes. To identify the portion of PufX responsible for inhibition of LH1 formation in reconstitution experiments, different regions (N-terminus and several core regions containing different lengths of the C-terminus) of PufX were chemically synthesized. Neither the N-terminal nor C-terminal polypeptides were inhibitory to LH1 reconstitution. However, all core segments were active, causing 50% inhibition at a concentration ratio of between 3:1 and 6:1 relative to the LH1 alpha-polypeptides. CD measurements indicated that the core segment containing 39 amino acids of Rb. sphaeroides PufX exhibited 47% alpha-helix in trifluoroethanol while the core segment containing 43 amino acids of Rb. capsulatus PufX exhibited 59% and 55% alpha-helix in trifluoroethanol and in 0.80% octylglucoside in water at pH 7.5, respectively. Approximately 50% alpha-helix was also indicated by a PHD (Burkhard-Rost) structure prediction. By covalent attachment of a fluorescent probe to the core segments, specific binding of bacteriochlorophyll was demonstrated.

Keywords: reaction center, core light-harvesting complex, bacterial photosynthesis, photoreceptor complex