P23, A MAJOR COPI-VESICLE MEMBRANE-PROTEIN, CONSTITUTIVELY CYCLES THROUGH THE EARLY SECRETORY PATHWAY

A novel type I transmembrane protein of COPI-coated vesicles, p23, has been demonstrated to be localized mainly to the Golgi complex. This p rotein and p24, another member of the p24 family, have been shown to b ind coatomer via their short cytoplasmic tails. Here we demonstrate th at p23 continuously cycles through the early secretory pathway. The cy toplasmic tail of p23 is shown to act as a functional retrieval signal as it confers endoplasmic reticulum (ER) residence to a CD8-p23 fusio n protein. This ER localization is, at least in part, a result of retr ieval from post-ER compartments because CD8-p23 fusion proteins receiv e post-ER modifications. In contrast, the cytoplasmic tail of p24 has been shown not to retrieve a CD8-p24 fusion protein. The coatomer bind ing motifs FF and KK in the cytoplasmic tail of p23 are reported to in fluence the steady-state localization of the CD8-p23 fusion protein wi thin the ER-Golgi recycling pathway. It appears that the steady-state Golgi localization of endogenous p23 is maintained by its lumenal doma in, as a fusion protein with the lumenal domain of CD8, and the membra ne span as well as the cytoplasmic tail of p23 is no longer detected i n the Golgi.