Zinc and cadmium are similar metal ions, but though Zn ²⁺ is an essential nutrient, Cd ²⁺ is a toxic and common pollutant linked to multiple disorders. Faster body turnover and ubiquitous distribution of Zn ²⁺ vs. Cd ²⁺ suggest that a mammalian metal transporter distinguishes between these metal ions. We show that the mammalian metal transporters, ZnTs, mediate cytosolic and vesicular Zn ²⁺ transport, but reject Cd ²⁺ , thus constituting the first mammalian metal transporter with a refined selectivity against Cd ²⁺ . Remarkably, the bacterial ZnT ortholog, YiiP, does not discriminate between Zn ²⁺ and Cd ²⁺ . A phylogenetic comparison between the tetrahedral metal transport motif of YiiP and ZnTs identifies a histidine at the mammalian site that is critical for metal selectivity. Residue swapping at this position abolished metal selectivity of ZnTs, and fully reconstituted selective Zn ²⁺ transport of YiiP. Finally, we show that metal selectivity evolves through a reduction in binding but not the translocation of Cd ²⁺ by the transporter. Thus, our results identify a unique class of mammalian transporters and the structural motif required to discriminate between Zn ²⁺ and Cd ²⁺ , and show that metal selectivity is tuned by a coordination-based mechanism that raises the thermodynamic barrier to Cd ²⁺ binding.

All living cells need zinc ions to support cell growth. Zrt-, Irt-like proteins (ZIPs) represent a major route for entry of zinc ions into cells, but how ZIPs promote zinc uptake has been unclear. Here we report the molecular characterization of ZIPB from Bordetella bronchiseptica, the first ZIP homolog to be purified and functionally reconstituted into proteoliposomes. Zinc flux through ZIPB was found to be nonsaturable and electrogenic, yielding membrane potentials as predicted by the Nernst equation. Conversely, membrane potentials drove zinc fluxes with a linear voltage-flux relationship. Direct measurements of metal uptake by inductively coupled plasma mass spectroscopy demonstrated that ZIPB is selective for two group 12 transition metal ions, Zn(2+) and Cd(2+), whereas rejecting transition metal ions in groups 7 through 11. Our results provide the molecular basis for cellular zinc acquisition by a zinc-selective channel that exploits in vivo zinc concentration gradients to move zinc ions into the cytoplasm.

The COVID-19 pandemic, caused by the SARS-CoV-2 virus, has created global health and economic emergencies. SARS-CoV-2 viruses promote their own spread and virulence by hijacking human proteins, which occurs through viral protein recognition of human targets. To understand the structural basis for SARS-CoV-2 viral-host protein recognition, here we use cryo-electron microscopy (cryo-EM) to determine a complex structure of the human cell junction protein PALS1 and SARS-CoV-2 viral envelope (E) protein. Our reported structure shows that the E protein C-terminal DLLV motif recognizes a pocket formed exclusively by hydrophobic residues from the PDZ and SH3 domains of PALS1. Our structural analysis provides an explanation for the observation that the viral E protein recruits PALS1 from lung epithelial cell junctions. In addition, our structure provides novel targets for peptide- and small-molecule inhibitors that could block the PALS1-E interactions to reduce E-mediated virulence.

In the transport cycle of Yiip, zinc(ii) binding triggers a highly localized, all-or-nothing change of water accessibility to the transport site and an adjacent hydrophobic gate. The Escherichia coli membrane protein YiiP is a proton-coupled transporter that drives Zn(II) efflux. Mammalian homologues are responsible for the sequestration of zinc into secretory vesicles, thereby playing important roles in neurotransmission and hormone secretion. This paper reports the use of X-ray-mediated hydroxyl radical labelling and mass spectrometry to examine the transport cycle of YiiP in the millisecond timeframe. Zinc-binding to YiiP is shown to trigger a concerted conformational change in a transmembrane helix, closing a hydrophobic 'gate' on the cytoplasmic side of the membrane protein and flipping the protein from an intracellular- to an extracellular-facing conformation. The zinc ion is then released into the extracellular space and the transport cycle can start a new. The proton gradient is a principal energy source for respiration-dependent active transport, but the structural mechanisms of proton-coupled transport processes are poorly understood. YiiP is a proton-coupled zinc transporter found in the cytoplasmic membrane of Escherichia coli. Its transport site receives protons from water molecules that gain access to its hydrophobic environment and transduces the energy of an inward proton gradient to drive Zn(ii) efflux1,2. This membrane protein is a well-characterized member3,4,5,6,7 of the family of cation diffusion facilitators that occurs at all phylogenetic levels8,9,10. Here we show, using X-ray-mediated hydroxyl radical labelling of YiiP and mass spectrometry, that Zn(ii) binding triggers a highly localized, all-or-nothing change of water accessibility to the transport site and an adjacent hydrophobic gate. Millisecond time-resolved dynamics reveal a concerted and reciprocal pattern of accessibility changes along a transmembrane helix, suggesting a rigid-body helical re-orientation linked to Zn(ii) binding that triggers the closing of the hydrophobic gate. The gated water access to the transport site enables a stationary proton gradient to facilitate the conversion of zinc-binding energy to the kinetic power stroke of a vectorial zinc transport. The kinetic details provide energetic insights into a proton-coupled active-transport reaction.

Heterodimers of a wild-type soluble lipid desaturase and a substrate-binding mutant demonstrate a reactivity that contrasts with membrane desaturases which requ