Snapshots of Bacterial and Mammalian Co-translational Membrane-Targeting Complexes Visualized Using Electron Cryomicroscopy

Targeting of nascent proteins to the membrane is a universal process in all forms of life. This process either commences while the protein is still being made by the ribosome (co-translational) or can take place after the protein is fully translated but not yet folded (post-translational). Co-translational targeting is mediated by the signal recognition particle (SRP) and its receptor (SR). In contrast, post-translational targeting is mediated by SecA/SecB and Hsp70/Sec62-Sec63 machineries in bacteria and eukaryotes, respectively. Both pathways first require cargo recognition on the translating ribosome and then terminate by delivering cargo to the translocation machinery on the membrane, the Sec translocon. Disruption of protein targeting leads to the attenuation of bacterial virulence and has also been linked to the etiology of diseases in humans, such as congenital neutropenia with Shwachman-Diamond-like features. Using electron cryomicroscopy, we determined the structures of snapshots of bacterial and mammalian SRP-mediated targeting pathway, in addition to a recently discovered SecA-mediated co-translational pathway. The structures reveal the molecular interactions between different components of this process and highlight both bacterial- and eukaryotic-specific key events in cargo recognition and handover. Our work underscores an evolutionary divergence of co-translational protein targeting despite it being a fundamental process in all forms of life and provide the molecular basis for further development of therapeutic strategies related to this pathway.