Role of the core region of the PufX protein in inhibition of reconstitution of the core light-harvesting complexes of Rhodobacter sphaeroides and Rhodobacter capsulatus

PufX the protein encoded by the pufX gene of Rhodobacter capsulatus and Rho dobacter sphaeroides, has been further characterized, The mature forms of t hese proteins contain 9 and 12 fewer amino acids, respectively, at the C-te rminal end of the protein than are encoded by their pufX genes. To identify the portion of PufX responsible for inhibition of LH1 formation in reconst itution experiments, different regions (N-terminus and several core regions containing different lengths of the C-terminus) of Rb, sphaeroides and Rb. capsulatus PufX were chemically synthesized. Neither the N- nor C-terminal polypeptides of Rb. sphaeroides were inhibitory to LH1 reconstitution. How ever, all core segments were active, causing 50% inhibition at a concentrat ion ratio of between 3:1 and 6:1 relative to the LH1 alpha -polypeptides wh ose concentrations were 3-4 muM. CD measurements indicated that the core se gment containing 39 amino acids of Rb. sphaeroides PufX exhibited 47% alpha -helix in trifluoroethanol while the core segment containing 43 amino acid s of Rb, capsulatus PufX exhibited 59 and 55% alpha -helix in trifluoroetha nol and in 0.80% octylglucoside in water, respectively. Approximately 50% a lpha -helix was also indicated by a PHD (Burkhard-Rost) structure predictio n. Binding of bacteriochlorophyll to these PufX core segments is implicated .