Structural Studies of the E. coli Methionine ABC Transporter and Its Cognate Binding Protein

Abstract

ATP binding cassette (ABC) transporters use ATP hydrolysis to facilitate the transfer of diverse substrates across the membrane. Members of the methionine uptake transporter family, thought to be of considerable biological interest, have not been structurally characterized thus far. The crystal structure of the methionine importer MetNI from Escherichia coli has been solved to 3.7 A resolution. The inward-facing conformation of this transporter adopts a more extreme arrangement than seen previously. While the permease domain consists of just five transmembrane helices per monomer, the ATP-binding cassette domain possesses a C-terminal domain in addition to the conserved architecture shared amongst this family. Analysis of the C-terminal extension has revealed a regulatory domain found in other proteins involved in amino acid metabolism, and further classifies this protein as part of the ACT family. Methionine binding in this region suggests a novel mechanism for regulation of transport that possibly stabilizes the inactive conformation of this family of transporters, as this domain is positioned between the nucleotide binding domains. Additionally, crystallization studies of the cognate binding protein to the MetNI system, MetQ were successful. The structure of the MetQ binding protein from E. coli was solved to 1.8 A resolution, revealing a bi-lobed structure consistent with many other substrate binding proteins, yet possessing a few differences when compared with previously characterized methionine binding proteins from other organisms. The substrate binding pocket revealed a bound L-methionine residue, which shares key features with other methionine binding proteins and appears to be appropriately selective for L-methionine binding. These combined studies have provided insight into the methionine uptake system and into the ABC transporter mechanism of transport.