Most eukaryotic proteins are secreted by co-translational translocation at the endoplasmic reticulum (ER). Here, ribosomes associate with the universally conserved Sec61 channel, which translocates newly synthesized polypeptides into the ER lumen.
Opening of the central pore of the ‘translocon’ allows the passage of the growing polypeptide chain, and subsequent opening of its lateral gate ensures the insertion of hydrophobic transmembrane domains and signal peptides into the membrane.
Various inhibitors of this process are thought to share a common mechanism of action. Among them, mycolactone is most potent and central to the pathology caused by Buruli Ulcer.
Using cryo-electron microscopy of ribosomes-translocon complexes in the presence of mycolactone, we solved the first structure of the inhibited mammalian translocon. Here, the central conduit is gated, and the lateral gate is wide open at its cytoplasmic side, with mycolactone acting as a wedge. Its position prevents signal peptide engagement at the lateral gate, thus preventing opening of the channel. We propose an unexpected model, in which translocation inhibitors stabilise a partially activated conformation of Sec61 rather than an inactive state.
This work deepens our understanding of the dynamics of translocation and provides a basis to guide future drug design of Sec61 modulators.