Structure, function and organization of antenna polypeptides and antenna complexes from the three families of Rhodospirillaneae.

Comparative primary structural analysis of polypeptides from antenna complexes from species of the three families of Rhodospirillaneae indicates the structural principles responsible for the formation of spectrally distinct light-harvesting complexes. In many of the characterized antenna systems the basic structural minimal unit is an alpha/beta polypeptide pair. Specific clusters of amino acid residues, in particular aromatic residues in the C-terminal domain, identify the antenna polypeptides to specific types of antenna systems, such as B880 (strong circular dichroism (CD)), B870 (weak CD), B800-850 (high), B800-850 (low) or B800-820. The core complex B880 (B1020) of species from Ectothiorhodospiraceae and Chromatiaceae apparently consists of four (alpha 1 alpha 2 beta 1 beta 2) or three (2 alpha beta 1 beta 2) chemically dissimilar antenna polypeptides respectively. There is good evidence that the so-called variable antenna complexes, such as the B800-850 (high), B800-850 (low) or B800-820 of Rp. acidophila, Rp. palustris and Cr. vinosum, are comprised of multiple forms of peripheral light-harvesting polypeptides. Structural similarities between prokaryotic and eukaryotic antenna polypeptides are discussed in terms of similar pigment organization. The structural basis for the strict organization of pigment molecules (bacteriochlorophyll (BChl) cluster) in the antenna system of purple bacteria is the hierarchical organization of the alpha- and beta-antenna polypeptides within and between the antenna complexes. On the basis of the three-domain structure of the antenna polypeptides with the central hydrophobic domain, forming a transmembrane alpha helix, possible arrangements of the antenna polypeptides in the three-dimensional structure of core and peripheral antenna complexes are discussed. Important structural and functional features of these polypeptides and therefore of the BChl cluster are the alpha/beta heterodimers, the alpha 2 beta 2 basic units and cyclic arrangements of these basic units. Equally important for the formation of the antenna complexes or the entire antenna are polypeptide-polypeptide, pigment-pigment and pigment-polypeptide interactions.