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With the original cloning of hMATE1, it was predicted that the secondary structure contained 12 transmembrane helices with internal‐facing NH₂‐ and COOH‐termini [5]. Subsequent predictions pointed to a 13th transmembrane domain for hMATE and rbMate1 that would result in extracellular localization of the carboxy‐terminus. This additional domain was not anticipated for mMate1 protein and instead the additional amino acids were thought to form a long cytoplasmic COOH terminus [20]. These predictions were supported by experimental evidence that showed extracellular accessibility of the carboxy‐terminus of rbMate1, but not mMate1 [20]. Epitope tagging and cysteine accessibility scanning affirmed that rbMate1 protein includes 13 transmembrane domains with an intracellular NH₂‐terminus and extracellular COOH‐terminus (Fig. 3.3) [61]. Subsequently, a functionally active variant of mMate1, mMate1b, was identified and revealed to contain a long hydrophobic tail, similar to other MATE transporters. This region of mMate1b encodes a 13th transmembrane domain that results in an extracellular carboxy terminus [62]. The exact purpose of the 13th transmembrane domain has remained unclear as the first 12 domains form the “functional core” of the rbMate1, hMATE1, and mMate1b orthologs [63]. Truncated mutant forms of MATE1/Mate1 proteins retain functional activity, ligand binding, and multi‐selectivity of substrates, leading researchers to posit other potential roles for this domain in substrate translocation, stabilization in the membrane bilayer, oligomerization, or protein–protein interactions [61, 62].

Using the X‐ray structure of the NorM transporter (3.65 Å) [64], a homology model for hMATE1 was developed [63]. This model positions hMATE1 in two bundles of six transmembrane helices (N lobe: transmembrane domains 1–6, C lobe: transmembrane domains 7–12) with an internal cavity of ~4,000 Å that is open to the extracellular space [63]. A relatively short cytoplasmic loop between domains 6 and 7 is positioned to connect the two halves and is consistent with hydropathy plots for N or M. Evaluation of the crystal structure of a MATE transporter from Arabidopsis thaliana (2.6 Å), known as AtDTX14, has also provided insights into hMATE1 structure and function [65]. The amino acid sequence identity between hMATE1 and AtDTX14 is 32%. A key hydrogen bonding network in the C‐lobe demonstrated in AtDTX14 is conserved in hMATE1 and is considered to be the substrate‐binding site [65]. While key insights into the likely structure of hMATE1 (and orthologs) have been made, there is little insight into similarities and differences for MATE2/2‐K, as well as a need for a MATE structure from crystallography or cryo‐electron microscopy.

FIGURE 3.3 Predicted structure of the human MATE1 transporter. The membrane topology of the human MATE1 transporter has been adapted from [128].