A C-TERMINALLY-ANCHORED GOLGI PROTEIN IS INSERTED INTO THE ENDOPLASMIC-RETICULUM AND THEN TRANSPORTED TO THE GOLGI-APPARATUS

Unlike conventional membrane proteins of the secretory pathway, protei ns anchored to the cytoplasmic surface of membranes by hydrophobic seq uences near their C termini follow a posttranslational, signal recogni tion particle-independent insertion pathway. Many such C-terminally-an chored proteins have restricted intracellular locations, but it is not known whether these proteins are targeted directly to the membranes i n which they will ultimately reside. Here we have analyzed the intrace llular sorting of the Golgi protein giantin, which consists of a rod-s haped 376-kDa cytoplasmic domain followed by a hydrophobic C-terminal anchor sequenc. Unexpectedly, we find that giantin behaves like a conv entional secretory protein in that it inserts into the endoplasmic ret iculum (ER) and then is transported to the Golgi. A deletion mutant la cking a portion of the cytoplasmic domain adjacent to the membrane anc hor still inserts into the ER but fails to reach the Golgi, even thoug h this mutant has a stable folded structure. These findings suggest th at the localization of a C-terminally-anchored Golgi protein involves at least three steps: insertion into the ER membrane, controlled incor poration into transport vesicles, and retention within the Golgi.