Altered Bacteriochlorophyll Associations in Combinatorial Mutants of the Rhodobacter Capsulatus Light Harvesting 2 Complex

Abstract

The light-harvesting 2 (LH2, B800–850) complex of the proteobacterium Rhodobacter capsulatus is a bacteriochlorophyll a (BChl) binding, integral membrane protein that serves as a peripheral antenna, directing collected excitations to the cores of photosynthetic units which consist of LH1 and reaction center complexes. The R. capsulatus LH2 complex is a heterodimer of conserved α- and β-polypeptides (7.3 and 5.1 kDa, respectively), which bind a BChl dimer (B850) absorbing near 850 nm, a monomeric BChl (B800) absorbing at 800 nm and one to two carotenoids, as well as a 14-kDa γ-subunit [1]. Recently, considerable interest in the LH2 protein has arisen from the determination of the three-dimensional structure of the homologous LH2 complex of Rhodopseudomonas acidophila by X-ray crystallography [2]. The overall structure forms a cylinder in which the transmembrane helices of nine β- and nine α-apoproteins make up the respective outer and inner walls. A ring of 18 B850 BChls, with overlapping bacteriochlorin moieties, is sandwiched between them, while the nine B800 BChls are positioned between the outer helices. A carotenoid is seen intertwined with the phytol chains of the BChls, nearly spanning the membrane. Despite this major structural advance, the main factors controlling the absorption of the antennae of purple bacteria are not well understood, particularly those distinguishing an LH1-like from an LH2-like complex. Characterization of the set of mutants of the R. capsulatus LH2 protein described here, has provided an opportunity to shed more light on this question.