Localization of ribophorin II to the endoplasmic reticulum involves both its transmembrane and cytoplasmic domains

Proteins that are concentrated in specific compartments of the endomembrane system in order to exert their organelle-specific function must possess sp ecific localization signals that prevent their transport to distal regions of the exocytic pathway, Some resident proteins of the endoplasmic reticulu m (ER) that are known to escape with low efficiency from this organelle to a post ER compartment are recognized by a recycling receptor and brought ba ck to their site of residence. Other ER proteins, however, appear to be ret ained in the ER by mechanisms that operate in the organelle itself. The mam malian oligosaccharyltransferase (OST) is a protein complex that effects th e cotranslational N-glycosylation of newly synthesized polypeptides, and is composed of at least four rough ER-specific membrane proteins: ribophorins I and II (RI and RII), OST48, and Dad1. The mechanism(s) by which the subu nits of this complex are retained in the ER are not well understood. In an effort to identify the domains within RII responsible for its ER localizati on we have studied the fate of chimeric proteins in which one or more RII d omains were replaced by the corresponding ones of the Tac antigen, the latt er being a well characterized plasma membrane protein that lacks intrinsic ER retention signals and serves to provide a neutral framework for the iden tification of retention signals in other proteins. We found that the lumina l domain of RII by itself does not contain retention information, while the cytoplasmic and transmembrane domains contain independent ER localization signals. We also show that the retention function of the transmembrane doma in is strengthened by the presence of a flanking luminal region consisting of 15 amino acids.