PT - JOURNAL ARTICLE AU - Feng Jin AU - Zengyi Chang TI - A shortened version of SecA (SecA<sup>N</sup>) functions as the protein-conducting channel for nascent β-barrel outer membrane proteins AID - 10.1101/121335 DP - 2017 Jan 01 TA - bioRxiv PG - 121335 4099 - http://biorxiv.org/content/early/2017/03/28/121335.short 4100 - http://biorxiv.org/content/early/2017/03/28/121335.full AB - During their biogenesis in cells, many proteins have to be translocated across biological membranes by protein-conducting channels. In Gram-negative bacteria, proteins destined to inner membrane, periplasm or outer membrane are synthesized in the cytoplasm and most of them are believed to be translocated across the inner membrane by the SecYEG translocon. Nevertheless, this perception, largely formed on the basis of genetic and in vitro studies, has hardly been proved by in vivo studies. Here, our in vivo photocrosslinking analysis mediated by genetically incorporated unnatural amino acid pBpA proved a direct interaction between the nascent periplasmic proteins and SecY, but strikingly, not between the nascent β-barrel outer membrane proteins (OMPs) and SecY. We then demonstrated that precursors of OMPs but not of periplasmic proteins are effectively processed in a SecY-defective strain. Subsequently, we discovered that a shortened form of SecA that lacks its C-terminal region, designated as SecAN, apparently functions as the translocon for nascent OMPs. In support of this, we discovered that SecAN directly interacts with BamA protein of the β-barrel assembly machine, and that SecAN is able to form a ternary complex with BamA and nascent OmpF in living cells. These observations strongly implicate that, in contrast to the common perception, the SecYEG only functions as the translocon for the nascent periplasmic proteins, not for the OMPs.