Arabidopsis thaliana MTP1 is a vacuolar membrane Zn 2+ ?H + antiporter of the cation diffusion facilitator family. Here we present a structure-function analysis of AtMTP1-mediated transport and its remarkable Zn 2+ selectivity by functional complementation tests of more than 50 mutant variants in metal-sensitive yeast strains. This was combined with homology modeling of AtMTP1 based on the crystal structure of the Escherichia coli broad-specificity divalent cation transporter YiiP. The Zn 2+-binding sites of EcYiiP in the cytoplasmic C-terminus, and the pore formed by transmembrane helices TM2 and TM5, are conserved in AtMTP1. Although absent in EcYiiP, Cys31 and Cys36 in the extended N-terminal cytosolic domain of AtMTP1 are necessary for complementation of a Zn-sensitive yeast strain. On the cytosolic side of the active Zn 2+-binding site inside the transmembrane pore, Ala substitution of either Asn258 in TM5 or Ser101 in TM2 non-selectively enhanced the metal tolerance conferred by AtMTP1. Modeling predicts that these residues obstruct the movement of cytosolic Zn 2+ into the intra-membrane Zn 2+-binding site of AtMTP1. A conformational change in the immediately preceding His-rich cytosolic loop may displace Asn258 and permit Zn 2+ entry into the pore. This would allow dynamic coupling of Zn 2+ transport to the His-rich loop, thus acting as selectivity filter or sensor of cytoplasmic Zn 2+ levels. Individual mutations at diverse sites within AtMTP1 conferred Co and Cd tolerance in yeast, and included deletions in N-terminal and His-rich intra-molecular cytosolic domains, and mutations of single residues flanking the transmembrane pore or participating in intra- or inter-molecular domain interactions, all of which are not conserved in the non-selective EcYiiP.
All Science Journal Classification (ASJC) codes