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Role of the C-Terminal Extrinsic Region of the α Polypeptide of the Light-Harvesting 2 Complex of Rhodobacter sphaeroides:  A Domain Swap Study

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journal contribution
posted on 30.12.2003, 00:00 by John D. Olsen, Bruno Robert, C. Alistair Siebert, Per A. Bullough, C. Neil Hunter
The LH1 and LH2 complexes of Rhodobacter sphaeroides form ring structures of 16 and 9 protomers, respectively, comprising α and β polypeptides, bacteriochlorophylls (Bchl), and carotenoids. Using the LH2 complex as a starting point, two chimeric LH complexes were constructed incorporating the αC-terminal domain of either the Rb. sphaeroides LH1 complex or the Rhodospirillum molischianum LH2 complex. The LH1 domain swap produced a new red-shifted component that comprised ∼30% of the total absorbance. In the LH1α C-terminal mutant this new red-shifted species acts as the terminal emitter, with the new emission maximum located 10 nm further to the red than for the WT. Raman spectroscopy indicates that a fraction of the B850 Bchls is involved in relatively weak H-bonds, possibly involving the αTrp+11 residue within the new αC-terminus, consistent with a more LH1-like character for one of the Bchls. The CD data indicate that the domain swaps have perturbed the native arrangement of the B850 Bchls, including the site energy difference between the α- and β-bound Bchls. Thus, the normal energetic structure of the ring system has been disrupted, with one component blue shifted due to the presumed loss of an H-bond donor and the other red shifted by the influence of the new αC-terminal domain. The dichotomous response of the mutants to the carotenoids incorporated, spheroidenone or neurosporene, strongly suggests that the C-terminal region of the α polypeptide is involved in binding a carotenoid. The projection map of the LH1α C-terminal mutant complex was determined in negative stain at 25 Å resolution, and it shows a diameter of 53 Å, compared to 50 Å for the WT. Hence these new spectral properties have not been accompanied by an alteration in ring size.