Aberrant Assembly Complexes of the Reaction Center Light-harvesting 1 PufX (RC-LH1-PufX) Core Complex of Rhodobacter sphaeroides Imaged by Atomic Force Microscopy

作者：Olsen John D; Adams Peter G; Jackson Philip J; Dickman Mark J; Qian Pu; Hunter C Neil^*

来源：JOURNAL OF BIOLOGICAL CHEMISTRY, 2014, 289(43): 29927-29936.

DOI：10.1074/jbc.M114.596585

• Atomic Force Microscopy (AFM)
• Bacteria
• Membrane Biogenesis
• Membrane Protein
• Photosynthesis
• Light Harvesting
• Reaction Center

Background: Photosynthesis relies upon connected pigment protein complexes to capture, transfer, and utilize light energy. Results: Aberrant forms of light-harvesting 1 (LH1) complex occur without the reaction center (RC); complete RC-LH1 complexes and isolated RCs were found when LH1 levels were reduced. Conclusion: The subunits of the RC-LH1 complex of Rhodobacter sphaeroides are assembled cooperatively. Significance: This represents a paradigm for assembly of other purple bacterial photosystems. %26lt;br%26gt;In the purple phototrophic bacterium Rhodobacter sphaeroides, many protein complexes congregate within the membrane to form operational photosynthetic units consisting of arrays of light-harvesting LH2 complexes and monomeric and dimeric reaction center (RC)-light-harvesting 1 (LH1)-PufX core complexes. Each half of a dimer complex consists of a RC surrounded by 14 LH1 subunits, with two bacteriochlorophylls (Bchls) sandwiched between each pair of transmembrane helices. We used atomic force microscopy (AFM) to investigate the assembly of single molecules of the RC-LH1-PufX complex using membranes prepared from LH2-minus mutants. When the RC and PufX components were also absent, AFM revealed a series of LH1 variants where the repeating (11)(Bchl)(2) units had formed rings of variable size, ellipses, and spirals and also arcs that could be assembly products. The spiral complexes occur when the LH1 ring has failed to close, and short arcs are suggestive of prematurely terminated LH1 complex assembly. In the absence of RCs, we occasionally observed captive proteins enclosed by the LH1 ring. When production of LH1 units was restricted by lowering the relative levels of the cognate pufBA transcript, we imaged a mixture of complete RC-LH1 core complexes, empty LH1 rings, and isolated RCs, leading us to conclude that once a RC associates with the first (11)(Bchl)(2) subunit, cooperative associations between subsequent subunits and the RC tend to drive LH1 ring assembly to completion.

• 出版日期2014-10-24