KtrB, a member of the superfamily of K+ transporters

Abstract

KtrB is the K+-translocating subunit of the K+-uptake system KtrAB from bacteria. It is a member of the (s) under bar uperfamily of (K) under bar (+) (t) under bar ransporters (SKT proteins) with other sub-families occurring in archaea, bacteria, fungi, plants and trypanosomes. SKT proteins may have originated from small K+ channels by at least two gene duplication and two gene fusion events. They contain four covalently linked M1PM2 domains, in which M-1 and M-2 stand for transmembrane stretches, and P for a P-loop, which folds back from the external medium into the membrane. SKT proteins distinguish themselves in two important aspects from K+ channels: first, with just one conserved glycine residue in their P-loops they contain a much simpler K+-selectivity filter sequence than K+ channels with their conserved Thr-Val-Gly-Tyr-Gly sequence. Secondly, the middle part M-2C2 from the long transmembrane stretch M-2C of KtrB from the bacterium Vibrio alginolyticus forms a gate inside the membrane, which prevents K+ permeation to the cytoplasm. Beside the mechanism of K+ transport via KtrB and other SKT proteins existing hypotheses of how the KtrA protein regulates the K+-transport activity of KtrB are discussed. (C) 2011 Elsevier GmbH. All rights reserved.