Substrate specificity of nickel/cobalt permeases: insights from mutants altered in transmembrane domains I and II.

HoxN, a high-affinity, nickel-specific permease of Ralstonia eutropha H16, and NhlF, a nickel/cobalt permease of Rhodococcus rhodochrous J1, are structurally related members of the nickel/cobalt transporter (NiCoT) family. These transporters have an eight-helix structure and are characterized by highly conserved segments with polar or charged amino acid residues in transmembrane domains (TMDs) II, III, V, and VI. Two histidine residues in a Ni2+ binding motif, the signature sequence of NiCoTs, in TMD II of HoxN have been shown to be crucial for activity. Replacement of the corresponding His residues in NhlF affected both Co2+ and Ni2+ uptake, demonstrating that NhlF employs a HoxN-like mechanism for transport of the two cations. Multiple alignments of bacterial NiCoT sequences identified a striking correlation between a hydrophobic residue (Val or Phe) in TMD II and a position in the center of TMD I occupied by either an Asn (as in HoxN) or a His (as in NhlF). Introducing an isoleucine residue at the latter position strongly reduced HoxN activity and abolished NhlF activity, suggesting that a Lewis base N-donor moiety is important. The Asn-to-His exchange had no effect on HoxN, whereas the converse replacement reduced NhlF-mediated Ni2+ uptake significantly. Replacement of the entire TMD I of HoxN by the respective NhlF segment resulted in a chimera that transported Ni2+ and Co2+ with low capacity. The Val-to-Phe exchange in TMD II of HoxN led to a considerable rise in Ni2+ uptake capacity and conferred to the variant the ability to transport Co2+. NhlF activity dropped in response to the converse mutation. Our data predict that TMDs I and II in NiCoTs spatially interact to form a critical part of the selectivity filter. As seen for the V64F variant of HoxN, modification of this site can increase the velocity of transport and concomitantly reduce the specificity.