No abstract

CKLF-like MARVEL transmembrane domain-containing protein 6 (CMTM6) is a master regulator of PD-L1. By binding PD-L1 at the plasma membrane and recycling endosomes, CMTM6 prevents the lysosomal degradation of PD-L1 and maintains its cell surface expression, thus stabilizing the inhibitory PD-1/PD-L1 axis. However, the mechanism of CMTM6/PD-L1 interaction is unknown. Here we report the first experimentally determined structure of CMTM6 from bovine. Combined with a low-resolution cryo-EM map, computational docking analysis and a protein binding assay an interaction model between CMTM6 and PD-L1 was proposed, providing a structural framework for the CMTM6 regulation on PD-L1.

Zinc transporter 8 (ZnT8) is mainly expressed in pancreatic islet β cells and is responsible for H+-coupled uptake (antiport) of Zn2+ into insulin secretory granules. Structures of human ZnT8 and its prokaryotic homolog YiiP have provided structural basis for constructing a plausible transport cycle for Zn2+. However, the mechanistic role that H+ plays in the transport process remains elusive. Here we present two cryo-EM structures of ZnT8 from Xenopus tropicalis (xtZnT8) captured in the presence of either abundant Zn2+ or abundant H+. Combined with a microscale thermophoresis analysis, our data suggest that binding of Zn2+ to the transmembrane Zn2+-binding site drives xtZnT8 to the outward-facing state. Surprisingly, binding of H+ to xtZnT8 is not sufficient to drive the transporter to an inward-facing state, suggesting that protonation alone is not a determining factor to establish an inward-facing conformation during Zn2+ transport. Instead, the role of protonation appears to unbind and release Zn2+ from the transmembrane site in the outward-facing state of xtZnT8, thus allowing an inward-facing isomerization to occur for the next cycle.