Retention of p63 in an ER-Golgi intermediate compartment depends on the presence of all three of its domains and on its ability to form oligomers

In contrast to the considerable progress that has been made in understanding protein localization in the ER and the Golgi apparatus, it is presently unclear how the recently identified ER-Golgi intermediate compartment (ERGIC) 1 maintains its resident population of proteins (for a review see Hauri and Schweizer, 1992) . The ERGIC comprises a membrane system interposed between the rough ER and the cis side of the Golgi apparatus that has been shown to mediate ER-to-Golgi exocytic protein transport (Schweizer et al., 1990; Lotti et al., 1992) . The ERGIC may correspond to the "budding compartment" of mouse coronavirus (Tooze et al., 1988; Krijnse-Locker et al., 1994) and to the "pre-Golgi vacuoles" of Semliki Forest virus-infected cells (Saraste and Kuismanen, 1984) . Isolation of the ERGIC from Vero cells indicated that this compartment has unique properties since it does not share major marker proteins with its direct neighbor organelles, the rough ER and cis-Golgi apparatus (Schweizer et al., 1991) . Currently, a limited number of marker proteins are available for the ERGIC. The ERGIC was originally defined by a 53-kD type I transmembrane protein (Schweizer et al., 1988) termed ERGIC-53. ERGIC-53 is specifically localized in the ERGIC at 37°C but undergoes a temperature-dependent redistribution (Lippincott-Schwartz et al., 1990; Schweizer et al., 1990) which suggested that it may follow a recycling pathway (Hauri and Schweizer, 1992) . Another marker for the ERGIC is the small GTP-binding protein rab2p (Chavrier et al., 1990) . Beta-COP, a major component of non-clathrin-coated vesicles, also associates with the ERGIC at 15°C (Duden et al., 1991) . The p58-positive pre-Golgi elements described by Saraste and Svenson (1991) may also be part of the ERGIC.