Sensing External Metals by Outer Membrane Beta-Barrel Proteins
EMSL Project ID
48487
Abstract
Metal-reducing bacteria drive a variety of environmentally important processes, including the biogeochemical cycling of carbon and metals, the bioremediation of radionuclides and organohalides, and the generation of electricity in microbial fuel cells. Despite the recent advances in our understanding of electron transport pathways in metal-reducing bacteria, the molecular mechanism for sensing and coordinating an adaptive response to external, electron-accepting transition metals remains poorly understood. We recently discovered that a unique, conserved cysteine-X-X-cysteine motif (CXXC, where X is any amino acid) in the N-terminus of transmembrane beta-barrel protein MtrB is required for metal reduction by the metal-reducing gamma-proteobacterium Shewanella oneidensis. This finding was quite unexpected since CXXC motifs are not commonly found in transmembrane beta-barrel proteins, most likely to avoid protein folding problems caused by redox-reactive cysteines during passage across the intermembrane space of eukaryotes or the periplasmic space of bacteria. In bacteria, CXXC motifs are generally found in cytoplasmic and periplasmic proteins where they carry out a diverse array of functions such as catalyzing disulfide bond exchanges, binding transition metals, and acting as the redox-sensing module of transcriptional activators that sense and coordinate adaptive responses to deleterious reactive oxygen (ROS), nitrogen (RNS), and electrophilic (RES) species. Other redox sensing modules include FeS cluster-, heme-, flavin-, NADH-, or quinone-based systems. Transmembrane beta-barrel proteins in the mitochondria and chloroplast of higher eukaryotes and in the outer membrane (OM) of Gram-negative bacteria are generally involved in active ion transport or passive nutrient uptake. In S. oneidensis, MtrB is postulated to function as the structural sheath of an OM-spanning extracellular electron conduit that facilitates electron transfer between metal-reducing c-type cytochromes and external electron-accepting transition metals. Based on our initial findings described below, we propose to demonstrate that S. oneidensis MtrB also functions as a sensor of external, electron-accepting transition metals, and activates a trans-membrane signaling pathway that promotes anaerobic metal reduction activity. Results of our study will therefore demonstrate that S. oneidensis MtrB represents a novel, dual-function transmembrane beta-barrel protein with both structural and external metal-sensing capabilities.
Project Details
Project type
FICUS Research
Start Date
2014-10-01
End Date
2017-05-31
Status
Closed
Released Data Link
Team
Principal Investigator