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

Oligomerization has also been suggested as a retention mechanism for proteins in the Golgi apparatus (Machamer 1991; Nilsson et al., 1991 Nilsson et al., , 1993 Nilsson et al., , 1994 Swift and Machamer, 1991) . The only example where retention in this organelle has actually been correlated with oligomerization is a chimeric membrane protein in which the membrane-spanning domain of VSV G protein was replaced with the first membrane-spanning domain of the M glycoprotein of avian coronavirus (Weisz et al., 1993) . However, in contrast to the chimera, oligomerization of the intact M glycoprotein could not be demonstrated. Recently, Nilsson et al. (1993 Nilsson et al. ( , 1994 have proposed a model whereby Golgi enzymes form homo-dimers in which the lumenal domains are bound together while the transmembrane domains interact with those of identical or related enzymes of the same Golgi cisternae to form hetero-oligomers. The cytoplasmic domains would be attached to an underlying matrix to stabilize larger arrays. Although similar in the overall architecture, this Golgi model differs from the model supported by the p63 data in several ways. In particular, the domains responsible for the formation of higher order structures (cytoplasmic tail versus transmembrane domain) and the kind of oligomers formed (homo-versus hetero-oligomers) are distinct. Taken together, oligomerization is likely to be a general mechanism that specifies location of proteins in different compartments of the secretory pathway. Oligomerization per se, however, seems to be achievable by different mechanisms and forms of interactions.