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Amino acid screening based on structural modeling identifies critical residues for function, ion selectivity and structure of Arabidopsis MTP

, : Amino acid screening based on structural modeling identifies critical residues for function, ion selectivity and structure of Arabidopsis MTP.

Arabidopsis thaliana MTP1 is a vacuolar membrane Zn2+/H+ antiporter of the cation diffusion facilitator family. Here we present a structure-function analysis of AtMTP1-mediated transport and remarkable Zn2+ selectivity through functional complementation tests of more than 5 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 Zn2+-binding sites of EcYiiP in the cytoplasmic C-terminus, and the pore formed by transmembrane helices TM2 and TM5, are conserved in AtMTPAlthough missing 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 Zn2+-binding site inside the transmembrane pore, Ala substitution of either Asn258 in TM5 or Ser11 in TM2 non-selectively enhanced metal tolerance conferred by AtMTPModeling predicts these residues to obstruct the movement of cytosolic Zn2+ into the intra-membrane Zn2+-binding site of AtMTPA conformational change in the directly preceding His-rich cytosolic loop could displace Asn258 and permit Zn2+ entry into the pore. This would allow the dynamic coupling of Zn2+ transport to the His-rich loop acting as selectivity filter or Zn2+ sensor. Individual mutations at diverse sites within AtMTP1 conferred Co and Cd tolerance to yeast, including 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 divergent from the non-selective EcYiiP.


DOI: 10.1111/j.1742-4658.2012.08613.x

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