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Reference #: SPU-1017-183806
Submit Date: 03/26/2002 16:37:31-0500
Presentation Type: platform
CONTACT: John Spudich
Dept. Microbiology & Molecular Genetics University of Texas Medical School
Houston, TX 77030
Microbial Rhodopsins: Genome-mining and
Structure/Function Relationships in Transport and Signal Transduction
AUTHOR GROUP:
John Spudich 1
University of Texas Health Science Center, Houston,, Texas 77030 1
ABSTRACT:
Microbial rhodopsins, photoactive, visual pigment-like, 7-transmembrane helix proteins that use retinal as their chromophore, were observed initially in the Archaea and appeared to be restricted to extreme halophilic environments. Our understanding of the abundance and diversity of this family has been radically transformed by findings from genomics over the past three years. Cloning, heterologous expression, and functional analysis of homologous genes from cultivated microbe genome projects as well as environmental genomics of uncultivated microbes have revealed photoactive Archaeal rhodopsin homologs in the other two domains of life as well (i.e. Bacteria and Eucarya). Organisms containing these proteins inhabit such diverse environments as soil, freshwater, and surface and deep ocean waters, and they comprise a broad phylogenetic range of microbial life, including proteobacteria, cyanobacteria, fungi, and algae. Analysis of the sequences of the new microbial rhodopsins, their heterologous expression and study, and biophysical and functional characterization, reveal that they fulfill both ion transport and sensory functions in various organisms and they are spectrally tuned to absorb throughout the visible spectrum. Of special importance to bioenergetics are the proteorhodopsins, light-driven proton pumps that are ubiquitous in planktonic bacteria throughout the world's ocean. Rather than photoenergy transducers, sensory rhodopsins I and II are phototaxis receptors controlling motility of haloarchaeal cells in light gradients. The haloarchaeal cells are able to discriminate color by a unique photochromic mechanism using sensory rhodopsin I, based on one-photon versus two-photon-driven photochemical reactions. Recently we have shown that two sensory rhodopsins also mediate phototaxis in the unicellular eukaryote Chlamydomonas by modulating transmembrane ion currents across the plasma membrane. The crystallographic structure of one of the sensory rhodopsins we obtained recently from X-ray diffraction reveals key features responsible for its spectral tuning and its sensory function. The molecular mechanisms of microbial rhodopsin function will be discussed in terms of a unified model for energy and sensory transduction by this widespread family of photoactive proteins.
Keywords: rhodopsins, signal transduction, ion transport, phototaxis
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