A Golgi retention signal in a membrane-spanning domain of coronavirus E1 protein

We have been studying the sorting of newly synthesized proteins in the exocytic pathway using a model Golgi protein, the El glycoprotein ofthe avian coronavirus infectious bron-chitis virus (IBV)' . The El protein consists of a short, glycosylated amino-terminal domain, three membrane-spanning domains, and a long carboxy-terminal cytoplasmic domain. The restricted intracellular localization of the El protein in coronavirus-infected cells is believed to direct virus assembly at intracellular membranes (45) . When cDNA is expressed in animal cells in the absence of the other viral proteins, the IBV El protein is targeted to cis-Golgi membranes (29) . Deletion of the first and second, or the second and third ofthe three El membrane-spanning domains showed that the first membrane-spanning domain was apparently required for intracellular retention (26) . We show here that the first membrane-spanning domain is a Golgi retention signal, since it is both necessary and sufficient for Golgi retention . Unlike ER retention signals that have been identified at the carboxy-termini of proteins (on either the lumenal or cytoplasmic side of the ER membrane), this Golgi retention signal is buried in the membrane . Our results suggest the novel possibility that the membrane composition of Golgi subcompartments may play an important role in retaining resident proteins in this organelle.