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 {beta}-barrel outer membrane proteins ({beta}-barrel OMPs) are translocated via the SecYEG translocon in bacterial cells, although direct living cell evidences remain lacking. Here, mainly via in vivo protein photo-crosslinking analysis, we revealed that the never reported membrane-integrated SecAN protein apparently functions as the translocon for {beta}-barrel OMPs. Additionally, SecAN contains a GXXXG motif known for mediating protein interactions in biomembranes, and processing of {beta}-barrel OMP precursors was severely affected in cells producing an assembly-defective SecAN variant resulted from the GXXXG motif mutations. Furthermore, SecAN was demonstrated to directly interact with the Bam complex, thus likely be a part of the supercomplex that we revealed earlier to be responsible for {beta}-barrel OMP biogenesis.