Abstract
The Anabaena sensory rhodopsin transducer (ASRT) is a small protein that has been claimed to function as a signaling molecule downstream of the cyanobacterial sensory rhodopsin. However, orthologs of ASRT have been detected in several bacteria that lack rhodopsin, raising questions about the generality of this function. Using sequence profile searches we show that ASRT defines a novel superfamily of β-sandwich fold domains. Through contextual inference based on domain architectures and predicted operons and structural analysis we present strong evidence that these domains bind small molecules, most probably sugars. We propose that the intracellular versions like ASRT probably participate as sensors that regulate a diverse range of sugar metabolism operons or even the light sensory behavior in Anabaena by binding sugars or related metabolites. We also show that one of the extracellular versions define a predicted sugar-binding structure in a novel cell-surface lipoprotein found across actinobacteria, including several pathogens such as Tropheryma, Actinomyces and Thermobifida. The analysis of this superfamily also provides new data to investigate the evolution of carbohydrate binding modes in β-sandwich domains with very different topologies.Reviewers: This article was reviewed by M. Madan Babu and Mark A. Ragan.
Highlights
In the past decade membrane-embedded photoactive retinylidene-containing rhodopsins have been identified in several bacteria [1]
As a consequence we show that Anabaena sensory rhodopsin transducer (ASRT) defines a novel family of prokaryotic small-molecule-binding domains that function in a wide range of contexts, predominantly as a carbohydrate sensor
Detection of novel ASRT homologs The 3D structure of ASRT (PDB: 2II7) shows that the entire protein is comprised of a single globular domain with an eight-stranded β-sandwich fold [5]
Summary
In the past decade membrane-embedded photoactive retinylidene-containing rhodopsins have been identified in several bacteria [1] One such rhodopsin from the cyanobacterium Anabaena, which utilizes a chlorophyll-based photosynthetic apparatus, was proposed to function as a sensory rhodopsin, as opposed to being a proton pump [2,3,4]. Inference of a sensory function for the Anabaena sensory rhodopsin (ASR) was based on several features [2], including its co-transcription with a 14 kDa protein with which it was shown to physically interact This cotranscribed 14 kDa protein has been proposed to function as an analog of the G-protein signal transducers associated with the animal visual rhodopsins [2,5] and was named ASRT ("Anabaena sensory rhodopsin transducer"). As a consequence we show that ASRT defines a novel family of prokaryotic small-molecule-binding domains that function in a wide range of contexts, predominantly as a carbohydrate sensor
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