Cryo-EM structure of a eukaryotic zinc transporter at a low pH suggests its Zn 2+ -releasing mechanism

Zinc transporter 8 (ZnT8) is mainly expressed in pancreatic islet β cells and is responsible for H + -coupled uptake (antiport) of Zn 2+ into the lumen of insulin secretory granules. Structures of human ZnT8 and its prokaryotic homolog YiiP have provided structural basis for constructing a plausible transport cycle for Zn 2+. However, the mechanistic role that protons play in the transport process remains unclear. Here we present a lumen-facing cryo-EM structure of ZnT8 from Xenopus tropicalis (xtZnT8) in the presence of Zn 2+ at a luminal pH (5.5). Compared to a Zn 2+ -bound xtZnT8 structure at a cytosolic pH (7.5), the low-pH structure displays an empty transmembrane Zn 2+ -binding site with a disrupted coordination geometry. Combined with a Zn 2+ -binding assay our data suggest that protons may disrupt Zn 2+ coordination at the transmembrane Zn 2+ -binding site in the lumen-facing state, thus facilitating Zn 2+ release from ZnT8 into the lumen.