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The high-affinity uptake of methionine by Escherichia coli is mediated by MetNI, a member of the methionine uptake transporter family of ATP-binding cassette (ABC) transporters. We previously reported the crystal structure of MetNI at 3.7 A resolution and a brief analysis of the methionine mediated trans-inhibition of the transporters' ATPase activity1. Here, we report two new crystal structures of the MetNI transporter, solved at 2.8 and 4.0 A resolution. While both structures of MetNI reveal the transporter adopting an inward-facing conformation, significant changes are observed in the conformation of the MetI transmembrane, MetN nucleotide binding, and MetN-C2 carboxyl-terminal regulatory domains. The conformational changes can be described primarily as rigid-body movements which result in a partial closing of the MetN nucleotide binding domains, and a simultaneous rotational rearrangement of the MetN-C2 regulatory domain. The kinetic properties of trans-inhibition have also been characterized by an analysis of ligand binding on the ATPase activity of wild type and specific MetN-C2 regulatory domain mutants.1. Kadaba N, Kaiser J, Johnson E, Lee A, and Rees D C. The high affinity E. coli methionine ABC transporter: structure and allosteric regulation. Science. 2008 Jul 11;321(5886):250-3.

ATP binding cassette (ABC) transporters constitute a ubiquitous super-family of integral membrane proteins that translocate a diverse array of substrates across cell membranes. Studies of several well-characterized systems suggested a mechanistic similarity between different members of this large family of transporters. However, more recent reports pointed out significant differences at the genetic and structural levels. We report here a functional comparison between several ABC transporters of different substrate specificities and find fundamental differences between them. Type I ABC transporters, exemplified by the arch typical maltose transporter, are characterized by an inherent instability of the transporter-receptor complex. In these systems, ATP binding promotes complex formation, and binding of substrate-loaded receptor accelerates the rate of ATP hydrolysis. In contrast, in type II ABC transporters (the metal-chelate transporters), the “default” complex is extremely stable. However, for productive transport to occur, the complex must dissociate, an event mediated by both substrate and ATP binding. Relative to type I transporters, high basal ATPase rates are measured with modest to negligible stimulation by substrate-loaded receptors. These and other findings presented here highlight significant mechanistic differences between ABC transport systems, indicating that considerable mechanistic diversity exists within this large super-family of proteins.