Archaeal photoreceptors consist of sensory rhodopsins in complex with their cognate transducers. After light excitation, a two-component signaling chain is activated, which is homologous to the chemotactic signaling cascades in enterobacteria. The latter system has been studied in detail. From structural and functional studies, a picture emerges which includes stable signaling complexes, which assemble to receptor arrays displaying hexagonal structural elements. At this higher order structural level, signal amplification and sensory adaptation occur. Here, we describe electron microscopy data, which show that also the archaeal phototaxis receptors sensory rhodopsin I and II in complex with their cognate transducers can form hexagonal lattices even in the presence of a detergent. This result could be confirmed by molecular dynamics calculations, which revealed similar structural elements. Calculations of the global modes of motion displayed one mode, which resembles the "U"-"V" transition of the NpSRII:NpHtrII complex, which was previously argued to represent a functionally relevant global conformational change accompanying the activation process [Ishchenko et al. (2013) J. Photochem. Photobiol. B 123, 55-58]. A model of cooperativity at the transmembrane level is discussed.
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Article Synopsis
- - Sensory rhodopsin II (SRII) is a light-sensitive protein that transforms when it absorbs light, leading to signaling changes through photointermediates: M and O.
- - The M intermediate features a deprotonated chromophore, while the O intermediate has a protonated chromophore with an unusual 15- (C═N) configuration.
- - Using Raman spectroscopy, the study reveals that the chromophore in the O intermediate actually shows a 13-, 15- (C═N) configuration, challenging the previously accepted structural changes associated with SRII's signaling.
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