TRAM REGULATES THE EXPOSURE OF NASCENT SECRETORY PROTEINS TO THE CYTOSOL DURING TRANSLOCATION INTO THE ENDOPLASMIC-RETICULUM

Translocational pausing is a mechanism used by certain specialized sec retory proteins whereby discrete domains of a nascent chain destined f or the endoplasmic reticulum lumen are transiently exposed to the cyto sol. Proteoliposomes reconstituted from total endoplasmic reticulum pr oteins properly assemble translocationally paused intermediates. The c apacity of the translocon to correctly pause the nascent chain is depe ndent on a glycoprotein fraction whose active component is TRAM. In th e absence of TRAM, the normally sealed ribosome-membrane junction stil l opens in response to a pause transfer sequence. However, nascent cha in domains that are not exposed to the cytosol in the presence of TRAM are so exposed in its absence. Thus, TRAM regulates which domains of the nascent chain are visible to the cytosol during a translocational pause.