The major facilitator superfamily (MFS) transporters are an ancient and widespread family of secondary active transporters1. In Escherichia coli, the uptake of l-fucose, a source of carbon for microorganisms, is mediated by an MFS proton symporter, FucP2,3. Despite intensive study of the MFS transporters, atomic structure information is only available on three proteins and the outward-open conformation has yet to be captured4,5,6. Here we report the crystal structure of FucP at 3.1 Å resolution, which shows that it contains an outward-open, amphipathic cavity. The similarly folded amino and carboxyl domains of FucP have contrasting surface features along the transport path, with negative electrostatic potential on the N domain and hydrophobic surface on the C domain. FucP only contains two acidic residues along the transport path, Asp 46 and Glu 135, which can undergo cycles of protonation and deprotonation. Their essential role in active transport is supported by both in vivo and in vitro experiments. Structure-based biochemical analyses provide insights into energy coupling, substrate recognition and the transport mechanism of FucP.
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We thank N. Shimizu and T. Kumasaka at Spring-8 beamline BL41XU in Japan, and J. He and S. Huang at the Shanghai Synchrotron Radiation Facility (SSRF) for on-site assistance. This work was supported by funds from the Ministry of Science and Technology (grant number 2009CB918802) and by Tsinghua University 985 Phase II funds. N.Y. acknowledges support from the Yuyuan Foundation and the Li Foundation.
The authors declare no competing financial interests.
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