Oligomerization of a membrane protein correlates with its retention in the Golgi complex

The transmembrane domains of many proteins have been shown to mediate dimerization (Sternberg and Gullick, 1990; Manolios et al., 1990 ; for review see Bormann and Engelman, 1992) . The best example is glycophorin A, whose single transmembrane domain interacts to form homodimers (Bormann et al., 1989) . This dimer is resistant to solubilization by SDS at ambient temperature, although resistance is overcome by dilution or incubation at higher temperatures (Furthmayr and Marchesi, 1976) . Dimerization ofglycophorin A is highly sequence specific and appears to involve interactions between hydrophobic residues in the transmembrane domain (Lemmon et al., 1992) . The transmembrane domains of the T-cell receptor (Cosson et al., 1991) and of class H major histocompatibility complex molecules also appear to mediate dimer formation (Cosson and Bonifacino, 1992) . In the case of major histocompatibility complex molecules, o~ and/~ chains interact to form heterodimers; dimerization appears to require a pair of oppositely charged amino acids at the lumenal edge of the apposing membrane spans. A series of glycine residues on each membrane span forms a nonpolar face which allows close packing of the transmembrane domains (Cosson and Bonifacino, 1992) . Similarly, the o~ chain of the T-cell receptor assembles with the CD3 6 chain via interaction between charged transmembrane residues (Manolios et al., 1990; Cosson et al., 1991) .