PROTEIN TRANSLOCATION INTO PROTEOLIPOSOMES RECONSTITUTED FROM PURIFIED COMPONENTS OF THE ENDOPLASMIC-RETICULUM MEMBRANE

We have reproduced the process of protein transport across and of prot ein integration into the mammalian endoplasmic reticulum membrane by t he use of proteoliposomes reconstituted from pure phospholipids and pu rified membrane proteins. The transport of some proteins requires only two membrane protein complexes: the signal recognition particle recep tor, needed for targeting of a nascent chain to the membrane, and a no vel complex, the Sec61p complex, that consists of Sec61p and two small er polypeptides. The translocation of other proteins also needs the pr esence of the translocating chain-associating membrane (TRAM) protein. The integration of two membrane proteins of different topologies into the membrane does not require additional components. These results in dicate a surprising simplicity of the basic translocation machinery. T hey suggest that the Sec61p complex binds the ribosome during transloc ation and forms the postulated protein-conducting channel.