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MBE Advance Access originally published online on August 3, 2006
Molecular Biology and Evolution 2006 23(11):2001-2007; doi:10.1093/molbev/msl079
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© The Author 2006. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Research Articles

Conservation of Distantly Related Membrane Proteins: Photosynthetic Reaction Centers Share a Common Structural Core

Sumedha Sadekar*, Jason Raymond{dagger} and Robert E. Blankenship{ddagger}

* Computational Biosciences Program, Arizona State University
{dagger} Microbial Systems Division, Biosciences Directorate, Lawrence Livermore National Laboratory, Livermore, California
{ddagger} Departments of Biology and Chemistry, Washington University

E-mail: blankenship{at}wustl.edu.

Photosynthesis was established on Earth more than 3 billion years ago. All available evidences suggest that the earliest photosynthetic organisms were anoxygenic and that oxygen-evolving photosynthesis is a more recent development. The reaction center complexes that form the heart of the energy storage process are integral membrane pigment proteins that span the membrane in vectorial fashion to carry out electron transfer. The origin and extent of distribution of these proteins has been perplexing from a phylogenetic point of view mostly because of extreme sequence divergence. A series of integral membrane proteins of known structure and varying degrees of sequence identity have been compared using combinatorial extension–Monte Carlo methods. The proteins include photosynthetic reaction centers from proteobacteria and cyanobacterial photosystems I and II, as well as cytochrome oxidase, bacteriorhodopsin, and cytochrome b. The reaction center complexes show a remarkable conservation of the core structure of 5 transmembrane helices, strongly implying common ancestry, even though the residual sequence identity is less than 10%, whereas the other proteins have structures that are unrelated. A relationship of sequence with structure was derived from the reaction center structures; with characteristic decay length of 1.6 Å. Phylogenetic trees derived from the structural alignments give insights into the earliest photosynthetic reaction center, strongly suggesting that it was a homodimeric complex that did not evolve oxygen.

Key Words: photosynthesis • evolution • reaction center • protein structure


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