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J Biol Chem. 2019 May 17;294(20):8046-8063. doi: 10.1074/jbc.RA118.006113. Epub 2019 Mar 26.

Unraveling the structural elements of pH sensitivity and substrate binding in the human zinc transporter SLC39A2 (ZIP2).

Author information

1
From the Institute of Biochemistry and Molecular Medicine, University of Bern, Bühlstrasse 28, 3012 Bern.
2
the National Center of Competence in Research, NCCR TransCure, Bühlstrasse 28, 3012 Bern, and.
3
the Department of Nephrology and Hypertension, Inselspital, Bern University Hospital, University of Bern, Freiburgstrasse 18, 3010 Bern, Switzerland.
4
From the Institute of Biochemistry and Molecular Medicine, University of Bern, Bühlstrasse 28, 3012 Bern, matthias.hediger@ibmm.unibe.ch.
5
From the Institute of Biochemistry and Molecular Medicine, University of Bern, Bühlstrasse 28, 3012 Bern, jonai.pujol@dbmr.unibe.ch.

Abstract

The transport and ion-coupling mechanisms of ZIP transporters remain largely uncharacterized. Previous work in our laboratory has revealed that the solute carrier family 39 member A2 (SLC39A2/ZIP2) increases its substrate transport rate in the presence of extracellular H+ Here, we used a combination of in silico and in vitro techniques involving structural modeling, mutagenesis, and functional characterization in HEK293 cells to identify amino acid residues potentially relevant for both the ZIP2-H+ interaction and substrate binding. Our ZIP2 models revealed a cluster of charged residues close to the substrate-translocation pore. Interestingly, the H63A substitution completely abrogated pH sensitivity, and substitutions of Glu-67 and Phe-269 altered the pH and voltage modulation of transport. In contrast, substitution of Glu-106, which might be part of a dimerization interface, altered pH but not voltage modulation. Substitution of Phe-269, located close to the substrate-binding site, also affected substrate selectivity. These findings were supported by an additional model of ZIP2 that was based on the structure of a prokaryotic homolog, Bordetella bronchiseptica ZrT/Irt-like protein (bbZIP), and in silico pKa calculations. We also found that residues Glu-179, His-175, His-202, and Glu-276 are directly involved in the coordination of the substrate metal ion. We noted that, unlike bbZIP, human ZIP2 is predicted to harbor a single divalent metal-binding site, with the charged side chain of Lys-203 replacing the second bound ion. Our results provide the first structural evidence for the previously observed pH and voltage modulation of ZIP2-mediated metal transport, identify the substrate-binding site, and suggest a structure-based transport mechanism for the ZIP2 transporter.

KEYWORDS:

SLC; ZIP; homology modeling; membrane transport; metal homeostasis; site-directed mutagenesis; structure–function; zinc

PMID:
30914478
PMCID:
PMC6527156
DOI:
10.1074/jbc.RA118.006113
[Indexed for MEDLINE]
Free PMC Article

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