Uncovering the membrane-integrated SecAN protein that plays a key role in translocating nascent outer membrane proteins.

A large number of nascent polypeptides have to get across a membrane in targeting to the proper subcellular locations. The SecYEG protein complex, a homolog of the Sec61 complex in eukaryotic cells, has been viewed as the common translocon at the inner membrane for targeting proteins to three extracytoplasmic locations in Gram-negative bacteria, despite the lack of direct verification in living cells. Here, via unnatural amino acid-mediated protein-protein interaction analyses in living cells, in combination with genetic studies, we unveiled a hitherto unreported SecAN protein that seems to be directly involved in translocationg nascent outer membrane proteins across the plasma membrane; it consists of the N-terminal 375 residues of the SecA protein and exists as a membrane-integrated homooligomer. Our new findings place multiple previous observations related to bacterial protein targeting in proper biochemical and evolutionary contexts. [Display omitted] • A hitherto unreported SecAN protein directly interacts with nascent OMPs. • SecAN seems to exist as a membrane-integrated homooligomer. • Precursor processing of OMPs is severely retarded when the SecAN assembly becomes defective due to mutations introduced in its GXXXG motif. • SecAN directly interacts with BamA in living cells. • SecAN shares the N-terminal 375 residues of SecA. [ABSTRACT FROM AUTHOR]