Review

Photosynthesis Research

, Volume 104, Issue 2, pp 233-243

A model of the protein–pigment baseplate complex in chlorosomes of photosynthetic green bacteria

  • Marie Ø. PedersenAffiliated withCenter for Insoluble Protein Structures (inSPIN), Interdisciplinary Nanoscience Center (iNANO), Aarhus UniversityDepartment of Chemistry, Aarhus University
  • , Juha LinnantoAffiliated withDepartment of Chemistry, University of Jyväskylä
  • , Niels-Ulrik FrigaardAffiliated withDepartment of Biology, University of Copenhagen
  • , Niels Chr. NielsenAffiliated withCenter for Insoluble Protein Structures (inSPIN), Interdisciplinary Nanoscience Center (iNANO), Aarhus UniversityDepartment of Chemistry, Aarhus University
  • , Mette MillerAffiliated withDepartment of Biochemistry and Molecular Biology, University of Southern Denmark Email author 

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Abstract

In contrast to photosynthetic reaction centers, which share the same structural architecture, more variety is found in the light-harvesting antenna systems of phototrophic organisms. The largest antenna system described, so far, is the chlorosome found in anoxygenic green bacteria, as well as in a recently discovered aerobic phototroph. Chlorosomes are the only antenna system, in which the major light-harvesting pigments are organized in self-assembled supramolecular aggregates rather than on protein scaffolds. This unique feature is believed to explain why some green bacteria are able to carry out photosynthesis at very low light intensities. Encasing the chlorosome pigments is a protein-lipid monolayer including an additional antenna complex: the baseplate, a two-dimensional paracrystalline structure containing the chlorosome protein CsmA and bacteriochlorophyll a (BChl a). In this article, we review current knowledge of the baseplate antenna complex, which physically and functionally connects the chlorosome pigments to the reaction centers via the Fenna–Matthews–Olson protein, with special emphasis on the well-studied green sulfur bacterium Chlorobaculum tepidum (previously Chlorobium tepidum). A possible role for the baseplate in the biogenesis of chlorosomes is discussed. In the final part, we present a structural model of the baseplate through combination of a recent NMR structure of CsmA and simulation of circular dichroism and optical spectra for the CsmA–BChl a complex.

Keywords

Chlorosome Baseplate CsmA Molecular modeling Photosynthetic antenna