PROBING PROTEIN STRUCTURAL REQUIREMENTS FOR FORMATION OF THE CORE LIGHT-HARVESTING COMPLEX OF PHOTOSYNTHETIC BACTERIA USING HYBRID RECONSTITUTION METHODOLOGY

The alpha- and beta-polypeptides of LH1 isolated from four different p hotosynthetic bacteria (Rhodospirillum rubrum, Rhodobacter sphaeroides , Rhodobacter capsulatus and Rhodopseudomonas viridis) were used for h omologous and hybrid reconstitution experiments with bacteriochlorophy ll a. Formation of B820-type subunit complexes and LH1-type complexes were evaluated. The beta-polypeptides of R. rubrum, Rb. sphaeroides an d Rb. capsulatus behaved similarly and formed B820-type subunit comple xes in the absence of an alpha-polypeptide. The alpha- and beta-polype ptides were both required to form a LH1-type complex with each of thes e three homologous systems. In hybrid experiments where the beta-polyp eptides were tested for reconstitution with alpha-polypeptides other t han their homologous partners, half of the twelve possible combination s resulted in formation of both B820- and LH1-type complexes. Three of the combinations that did not result in formation of a LH1-type compl ex involved the beta-polypeptide of R. rubrum. It is suggested that th ese latter results can be explained by charge repulsion between the Ly s at position -17 (assigning the conserved His located nearest to the C-terminus as position 0) in the beta-polypeptide of R. rubrum and eac h of the heterologous alpha-polypeptides tested, all of which have an Arg at this location. Conclusions that can be derived from these exper imental results include: (1) the experimental data support the idea th at a central core region of approximately 40 amino acids exists in eac h of the polypeptides, which contains sufficient information to allow formation of both the B820- and LH1-type complexes and (2) a specific portion of the N-terminal hydrophilic region of each polypeptide was f ound in which ion pairs between oppositely charged groups on the alpha - and beta-polypeptides seem to stabilize complex formation.