Discrimination between the endoplasmic reticulum and mitochondria by spontaneously inserting tail-anchored proteins

Tail-anchored (TA) proteins insert into their target organelles by incompletely elucidated posttranslational pathways. Some TA proteins spontaneously insert into protein-free liposomes, yet target a specific organelle in vivo. Two spontaneously inserting cytochrome b5 forms, b5-ER and b5-RR, which differ only in the charge of the C-terminal region, target the endoplasmic reticulum (ER) or the mitochondrial outer membrane (MOM), respectively. To bridge the gap between the cell-free and in cellula results, we analyzed targeting in digitonin-permeabilized adherent HeLa cells. In the absence of cytosol, the MOM was the destination of both b5 forms, whereas in cytosol the C-terminal negative charge of b5-ER determined targeting to the ER. Inhibition of the transmembrane recognition complex (TRC) pathway only partially reduced b5 targeting, while strongly affecting the classical TRC substrate synaptobrevin 2 (Syb2). To identify additional pathways, we tested a number of small inhibitors, and found that Eeyarestatin I (ESI) reduced insertion of b5-ER and of another spontaneously inserting TA protein, while not affecting Syb2. The effect was independent from the known targets of ESI, Sec61 and p97/VCP. Our results demonstrate that the MOM is the preferred destination of spontaneously inserting TA proteins, regardless of their C-terminal charge, and reveal a novel, substrate-specific ER-targeting pathway.