Abstract
The TM1088 locus of T. maritima codes for two proteins designated TM1088A and TM1088B, which combine to form the cytosolic portion of a putative Trk K+ transporter. We report the crystal structure of this assembly to a resolution of 3.45 Å. The high resolution crystal structures of the components of the assembly, TM1088A and TM1088B, were also determined independently to 1.50 Å and 1.55 Å, respectively. The TM1088 proteins are structurally homologous to each other and to other K+ transporter proteins, such as TrkA. These proteins form a cytosolic gating ring assembly that controls the flow of K+ ions across the membrane. TM1088 represents the first structure of a two-subunit Trk assembly. Despite the atypical genetics and chain organization of the TM1088 assembly, it shares significant structural homology and an overall quaternary organization with other single-subunit K+ gating ring assemblies. This structure provides the first structural insights into what may be an evolutionary ancestor of more modern single-subunit K+ gating ring assemblies.
Highlights
K+ ions are the major monovalent cation in prokaryotic and eukaryotic cells and stringent regulation of its intracellular concentration is essential for a wide variety of cellular processes [1]
TM1088A:TM1088B complexes (TM1088A) and TM1088B were purified and crystallized as an octameric assembly consisting of 4 units of TM1088A and 4 units of TM1088B (160 kDa octameric complex; Fig 1B–1C)[9]
The TM1088 assembly consists of the same folds, domains and assembly motifs as other K+ transporter assemblies, such as TrkA, GsuK, KtrAB, MthK and BK [16,18,20,21,22,24,25]
Summary
K+ ions are the major monovalent cation in prokaryotic and eukaryotic cells and stringent regulation of its intracellular concentration is essential for a wide variety of cellular processes [1]. Ion transporters that translocate cations, K+, are central to the regulation of cellular pH and osmolarity [2], electrical signal generation [3] and regulation of protein expression and activity [4]. In contrast to the three K+ transporters found in E. coli (Kdp, Kup and Trk), only one K+ transport system, Trk, has been identified in T. maritima [5]. The Trk system is the major constitutive K+ transporter in E. coli and functions as a proton symporter powered by PLOS ONE | DOI:10.1371/journal.pone.0122512. The Trk system is the major constitutive K+ transporter in E. coli and functions as a proton symporter powered by PLOS ONE | DOI:10.1371/journal.pone.0122512 March 31, 2015
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