Dynamic Interaction of the Sec Translocon with the Chaperone PpiD [Microbiology]

June 20th, 2014 by Sachelaru, I., Petriman, N. A., Kudva, R., Koch, H.-G.

The Sec translocon constitutes a ubiquitous protein transport channel that consists in bacteria of the three core components SecY, SecE and SecG. Additional proteins interact with SecYEG during different stages of protein transport. During targeting, SecYEG interacts with SecA, the SRP receptor or the ribosome. Protein transport into or across the membrane is then facilitated by the interaction of SecYEG with YidC and the SecDFYajC complex. During protein transport, SecYEG is likely to interact also with the protein quality control machinery, but details about this interaction are missing. By in vivo and in vitro site-directed cross-linking we show here that the periplasmic chaperone PpiD is located in front of the lateral gate of SecY, through which transmembrane domains exit the SecY channel. The strongest contacts were found to helix 2b of SecY. Blue-native PAGE analyses verify the presence of a SecYEG-PpiD complex in native E. coli membranes. The PpiD-SecY interaction was not influenced by the addition of SecA and only weakly influenced by binding of non-translating ribosomes to SecYEG. In contrast, PpiD lost contact to the lateral gate of SecY during membrane protein insertion. These data identify PpiD as an additional and transient subunit of the bacterial SecYEG translocon. The data furthermore demonstrate the highly modular and versatile composition of the Sec translocon, which is probably essential for its ability to transport a wide range of substrates across membranes in bacteria and eukaryotes.