APOPROTEIN STRUCTURE IN THE LH2 COMPLEX FROM RHODOPSEUDOMONAS-ACIDOPHILA STRAIN-10050 - MODULAR ASSEMBLY AND PROTEIN PIGMENT INTERACTIONS

The refined structure of the peripheral light-harvesting complex from Rhodopseudomonas acidophila strain 10050 reveals a membrane protein wi th protein-protein interactions in the trans-membrane region exclusive ly of a van der Waals nature. The dominant factors in the formation of the complex appear to be extramembranous hydrogen bonds (suggesting t hat each apoprotein must achieve a fold close to its final structure i n order to oligomerize), protein-pigment and pigment-pigment interacti ons within the membrane-spanning region, The pigment molecules are kno wn to play an important role in the formation of bacterial light-harve sters, and their extensive mediation of structural contacts within the membrane bears this out. Amino acid residues determining the secondar y structure of the apoproteins influence the oligomeric state of the c omplex. The assembly of the pigment array is governed by the apoprotei ns of LH2. The particular environment of each of the pigment molecules is, however, influenced directly by few protein contacts. These conta cts produce functional effects that are not attributable to a single c ause, e.g. the arrangement of an overlapping cycle of chromophores not only provides energy delocalisation and storage properties, but also has consequences for oligomer size, pigment distortion modes and pigme nt chemical environment, all of which modify the precise function of t he complex. The evaluation of site energies for the pigment array requ ires the consideration of a number of effects, including heterogeneous pigment distortions, charge distributions in the local environment an d mechanical interactions. (C) 1997 Academic Press Limited.