Change to alanine of one out of four selectivity filter glycines in KtrB causes a two orders of magnitude decrease in the affinities for both K+ and Na+ of the Na+ dependent K+ uptake system KtrAB from Vibrio alginolyticus

KtrAB from Vibrio alginolyticus is a recently described new type of high af finity bacterial K+ uptake system. Its activity assayed in an Escherichia c oli K+ uptake negative mutant depended on Na+ ions (K-m of 40 mu M). Subuni t KtrB contains four putative P-loops, The selectivity filter from each P-l oop contains a conserved glycine residue. Residue Gly-290 from the third P- loop selectivity filter in KtrB mas exchanged for Ala, Ser or Asp. KtrB var iants Ser-290 and Asp-290 were without activity. In contrast, KtrB variant Ala-290 was still active. This variant transported K+ with a two orders of magnitude decrease in apparent affinity for both K+ and Na+ with little eff ect on V-max. (C) 1999 Federation of European Biochemical Societies.