The Anabaena sensory rhodopsin transducer defines a novel superfamily of prokaryotic small-molecule binding domains

Unusual Stability of Anabaena Sensory Rhodopsin Transducer from Anabaena PCC7120

Advances in biotechnology generated wide range of microbial genome and their related protein database. Freshwater cyanobacterium Anabaena PCC7120 sensory rhodopsin, ASR in contrast to classical haloarchaeal sensory rhodopsins interacts with putative soluble transducer, ASRT. The 125 amino acid transducer exists as a soluble protein and is involved in photoreceptor binding. Recombinant DNA tools in biotechnology conventionally support the use of affinity tags for ease of protein purification and subsequent studies. The ASRT exists as a stable tetramer. Both X-ray crystal structure and solution NMR results with ASRT utilizing hexa-histidine affinity tag reveal it as a primarily β-stranded protein We have observed that the affinity tagged ASRT exhibits altered oligomeric stability. In this communication we outlined the effect of commonly used denaturant, Sodium Dodecyl Sulfate (SDS) on the tetrameric packing of ASRT. Our results support that N-terminus hexa-histidine tagged ASRT displayed unusual SDS-resistant structure. The unusual stability of ASRT and its homologues present in other microbial population could provide further insight towards their role in receptor, other ligand binding and signaling.

DNA binding activity of Anabaena sensory rhodopsin transducer probed by fluorescence correlation spectroscopy

Anabaena sensory rhodopsin transducer (ASRT) is believed to be a major player in the photo-signal transduction cascade, which is triggered by Anabaena sensory rhodopsin. Here, we characterized DNA binding activity of ASRT probed by using fluorescence correlation spectroscopy. We observed clear decrease of diffusion coefficient of DNA upon binding of ASRT. The dissociation constant, K(D), of ASRT to 20 bp-long DNA fragments lied in micro-molar range and varied moderately with DNA sequence. Our results suggest that ASRT may interact with several different regions of DNA with different binding affinity for global regulation of several genes that need to be activated depending on the light illumination.

A role of Anabaena sensory rhodopsin transducer (ASRT) in photosensory transduction

In 2003, Anabaena sensory rhodopsin (ASR), a membrane-bound light sensor protein, was discovered in cyanobacteria. Since then, a large number of functions have been described for ASR, based on protein biochemical and biophysical studies. However, no study has determined the in vivo mechanism of photosensory transduction for ASR and its transducer protein (ASRT). Here, we aimed to determine the role of ASRT in physiological photo-regulation. ASRT is known to be related to photochromism, because it regulates the expression of phycocyanin (cpc-gene) and phycoerythrocyanin (pec gene), two major proteins of the phycobilisome in cyanobacteria. By examining wild type and knockout mutant Anabaena cells, we showed that ASRT repressed the expression of these two genes. We also demonstrated physical interactions between ASRT, ASR, and the promoter regions of cpc, pec, kaiABC (circadian clock gene) and the asr operon, both in vitro and in vivo. Binding assays indicated that ASRT had different sites of interaction for binding to ASR and DNA promoter regions. ASRT also influenced the retinal re-isomerization rate in dark through a physical interaction with ASR, and it regulated reporter gene expression in vivo. These results suggested that ASRT relayed the photosignal from ASR and directly regulated gene expression.

Eubacterial rhodopsins - unique photosensors and diverse ion pumps

Since the discovery of proteorhodopsins, the ubiquitous marine light-driven proton pumps of eubacteria, a large number of other eubacterial rhodopsins with diverse structures and functions have been characterized. Here, we review the body of knowledge accumulated on the four major groups of eubacterial rhodopsins, with the focus on their biophysical characterization. We discuss advances and controversies on the unique eubacterial sensory rhodopsins (as represented by Anabaena sensory rhodopsin), proton-pumping proteorhodopsins and xanthorhodopsins, as well as novel non-proton ion pumps. This article is part of a Special Issue entitled: Retinal Proteins - You can teach an old dog new tricks.

Xenorhodopsins, an enigmatic new class of microbial rhodopsins horizontally transferred between archaea and bacteria

Based on unique, coherent properties of phylogenetic analysis, key amino acid substitutions and structural modeling, we have identified a new class of unusual microbial rhodopsins related to the Anabaena sensory rhodopsin (ASR) protein, including multiple homologs not previously recognized. We propose the name xenorhodopsin for this class, reflecting a taxonomically diverse membership spanning five different Bacterial phyla as well as the Euryarchaeotal class Nanohaloarchaea. The patchy phylogenetic distribution of xenorhodopsin homologs is consistent with historical dissemination through horizontal gene transfer. Shared characteristics of xenorhodopsin-containing microbes include the absence of flagellar motility and isolation from high light habitats.