FocA is a representative member of the formate-nitrite transporter family, which transports short-chain acids in bacteria, archaea, fungi, algae and parasites. The structure and transport mechanism of the formate-nitrite transporter family remain unknown. Here we report the crystal structure of Escherichia coli FocA at 2.25 resolution. FocA forms a symmetric pentamer, with each protomer consisting of six transmembrane segments. Despite a lack of sequence homology, the overall structure of the FocA protomer closely resembles that of aquaporin and strongly argues that FocA is a channel, rather than a transporter. Structural analysis identifies potentially important channel residues, defines the channel path and reveals two constriction sites. Unlike aquaporin, FocA is impermeable to water but allows the passage of formate. A structural and biochemical investigation provides mechanistic insights into the channel activity of FocA.
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