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The Expanding
Family of Microbial Rhodopsins
Spudich, John1,
Spudich, Elena1 and Jung, Kwang-Hwan1
Department of Microbiology & Molecular Genetics, University of Texas
Medical School, UT Health Science Center at Houston, TX 77030 USA1
Abstract-
The archaeal rhodopsins, a family of four photoactive seven-transmembrane-helix
retinylidene proteins, have been extensively studied in extremely halophilic
archaea. These pigments exhibit two distinct functions: light-driven
ion transport (transport rhodopsins) and photosensory signaling (sensory
rhodopsins). They all share the seven-helix topology and use of retinal
as a chromophore with visual pigments, but there is no evident primary
sequence homology between the microbial rhodopsin and visual pigment
families, indicating divergence or two independent origins. Genomic
sequencing of cultivated microbes and photobiophysical and genomic analysis
of uncultivated biomass in environmental samples, have recently led
to an unexpected discovery: Members of the archaeal rhodopsin family
are present throughout the microbial world, and appear to make a significant
contribution to phototransduction by the world's biomass. Homologous
genes have been found in bacterial DNA from picoplankton in the oceans,
in the cyanobacterium Anabaena, and in several unicellular eukaryotes,
in particular in fungi and algae. One approach to explore these new
pigments is to study their spectroscopic properties and photochemical
reactions in their native state or in protein heterologously expressed
in the most related laboratory organism. In collaboration with Oded
Beja and Ed DeLong at MBARI in Monterey, we have detected and quantitated
the photochemical activity of the picoplanktonic pigment, designated
proteorhodopsin, in environmental samples. Its rapid photochemical reaction
cycle (15 ms) and the properties of E. coli-expressed proteorhodopsin
genes from diverse regions of the oceans (Monterey Bay, Antarctica,
and Hawaii, both in surface and deep sea plankton) reveal it to be an
abundant light-driven proton pump with an absorption spectrum tuned
to environmental light. On the other hand, we have found that the new
pigment from Anabaena appears rather to be a slower-cycling sensory
rhodopsin, which our evidence suggests signals to a soluble transducer,
as do visual pigments, rather than by protein-protein interaction in
the membrane, as do archaeal sensory rhodopsins. Results from investigation
of homologous genes from eukaryotic microbes will also be presented.
Keywords: photosensory
reception, transport, rhodopsin, proteorhodopsin
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