Howard Hughes Medical Institute and Department of Physiology, University of Texas Southwestern Medical Center, Dallas, United States; Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States
Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States; Department of Cell Biology, University of Texas Southwestern Medical Center, Dallas, United States
Weizhong Zeng
Howard Hughes Medical Institute and Department of Physiology, University of Texas Southwestern Medical Center, Dallas, United States; Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States
Howard Hughes Medical Institute and Department of Physiology, University of Texas Southwestern Medical Center, Dallas, United States; Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States
Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States; Department of Cell Biology, University of Texas Southwestern Medical Center, Dallas, United States
ZnT8 is a Zn2+/H+ antiporter that belongs to SLC30 family and plays an essential role in regulating Zn2+ accumulation in the insulin secretory granules of pancreatic β cells. However, the Zn2+/H+ exchange mechanism of ZnT8 remains unclear due to the lack of high-resolution structures. Here, we report the cryo-EM structures of human ZnT8 (HsZnT8) in both outward- and inward-facing conformations. HsZnT8 forms a dimeric structure with four Zn2+ binding sites within each subunit: a highly conserved primary site in transmembrane domain (TMD) housing the Zn2+ substrate; an interfacial site between TMD and C-terminal domain (CTD) that modulates the Zn2+ transport activity of HsZnT8; and two adjacent sites buried in the cytosolic domain and chelated by conserved residues from CTD and the His-Cys-His (HCH) motif from the N-terminal segment of the neighboring subunit. A comparison of the outward- and inward-facing structures reveals that the TMD of each HsZnT8 subunit undergoes a large structural rearrangement, allowing for alternating access to the primary Zn2+ site during the transport cycle. Collectively, our studies provide the structural insights into the Zn2+/H+ exchange mechanism of HsZnT8.
