title: Crystal structure of a folate energy-coupling factor transporter from Lactobacillus brevis.
abstract: ATP-binding cassette (ABC) transporters, composed of importers and exporters, form one of the biggest protein superfamilies that transport a variety of substrates across the membrane, powered by ATP hydrolysis. Most ABC transporters are composed of two transmembrane domains and two cytoplasmic nucleotide-binding domains. Also, importers from prokaryotes usually have extra solute-binding proteins in the periplasm that are responsible for the binding of substrates. Structures of importers have been reported that suggested a two-state model for the transport mechanism. Energy-coupling factor (ECF) transporters belong to a new class of ATP-binding cassette importers. Each ECF transporter comprises an energy-coupling module consisting of a transmembrane T protein (EcfT), two nucleotide-binding proteins (EcfA and EcfA'), and another transmembrane substrate-specific binding S protein (EcfS). Despite the similarities with ABC transporters, ECF transporters have different organizational and functional properties. The lack of solute-binding proteins in ECF transporters differentiates them clearly from the canonical ABC importers. Previously reported structures of the EcfS proteins RibU and ThiT clearly demonstrated the binding site of substrate riboflavin and thiamine, respectively. However, the organization of the four different components and the transport mechanism of ECF transporters remain unknown. Here we present the structure of an intact folate ECF transporter from Lactobacillus brevis at a resolution of 3 Å. This structure was captured in an inward-facing, nucleotide-free conformation with no bound substrate. The folate-binding protein FolT is nearly parallel to the membrane and is bound almost entirely by EcfT, which adopts an L shape and connects to EcfA and EcfA' through two coupling helices. Two conserved XRX motifs from the coupling helices of EcfT have a vital role in energy coupling by docking into EcfA-EcfA'. We propose a transport model that involves a substantial conformational change of FolT.
authors.affiliation: National Key Laboratory of Plant Molecular Genetics, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, 300 Fenglin Road, Shanghai 200032, China.
authors.forename: Ke
authors.initials: K
authors.lastname: Xu
authors.forename: Minhua
authors.initials: M
authors.lastname: Zhang
authors.forename: Qin
authors.initials: Q
authors.lastname: Zhao
authors.forename: Fang
authors.initials: F
authors.lastname: Yu
authors.forename: Hui
authors.initials: H
authors.lastname: Guo
authors.forename: Chengyuan
authors.initials: C
authors.lastname: Wang
authors.forename: Fangyuan
authors.initials: F
authors.lastname: He
authors.forename: Jianping
authors.initials: J
authors.lastname: Ding
authors.forename: Peng
authors.initials: P
authors.lastname: Zhang
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000737:chemistry; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D020816:Amino Acid Motifs; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D058972:Folic Acid Transporters / Q000737:chemistry* / Q000378:metabolism; D052196:Levilactobacillus brevis / Q000737:chemistry*; D008958:Models, Molecular; D011487:Protein Conformation; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism; D011510:Proteolipids / Q000378:metabolism
chemical_list: D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D058972:Folic Acid Transporters; D021122:Protein Subunits; D011510:Proteolipids; C020485:proteoliposomes; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases