Revelation of a Novel Protein Translocon in Bacterial Plasma Membrane

Many proteins are translocated across biomembranes via protein translocons in targeting to their subcellular destinations. Hitherto, the SecYEG/Sec61 translocon, existing in prokaryotes and eukaryotes, represents the most intensively studied one. According to the current perception, both periplasmic and β-barrel outer membrane proteins (β-barrel OMPs) are translocated via the SecYEG translocon in bacterial cells, although direct living cell evidences remain lacking. Here, mainly viain vivoprotein photo-crosslinking analysis, we revealed that the never reported membrane-integrated SecANprotein apparently functions as the translocon for β-barrel OMPs. Additionally, SecANcontains a GXXXG motif known for mediating protein interactions in biomembranes, and processing of β-barrel OMP precursors was severely affected in cells producing an assembly-defective SecANvariant resulted from the GXXXG motif mutations. Furthermore, SecANwas demonstrated to directly interact with the Bam complex, thus likely be a part of the supercomplex that we revealed earlier to be responsible for β-barrel OMP biogenesis.