Many membrane proteins initially take the same route, but they are inserted into the lipid bilayer rather than being transported all the way across the membrane. Further signals in the polypeptide chain determine whether the protein stays in the ER or is transported to other destinations. We are interested in the decisive first step during which proteins are translocated across or inserted into the ER membrane. Specifically, we would like to understand how signal sequences are recognized, how proteins are able to cross the membrane barrier, and how membrane proteins are integrated into the membrane and achieve their different topologies. Finally, we are interested in how membrane-bound organelles are transported in cells by molecular motors.
Related pathways exist in bacteria.
Our studies on the cotranslational pathway concentrate on the mammalian system. In the past, we have used photocrosslinking to identify components that interact with the signal sequence of a nascent polypeptide chain and that are in proximity to the chain when it is transferred through the membrane. With the purified components, we have established a reconstituted system that reproduces the cotranslational mode of transport. Proteoliposomes reconstituted from phospholipids, the Sec61p complex, the TRAM protein, and the signal recognition particle receptor not only transport secretory proteins but also incorporate membrane proteins in the correct topology. Our studies indicate that the Sec61p complex is the major component of the translocation machinery in the membrane.