The proper glycosylation of proteins trafficking through the Golgi apparatus depends upon the conserved oligomeric Golgi (COG) complex. Defects in COG can cause fatal congenital disorders of glycosylation (CDGs) in humans. The recent discovery of a form of CDG, caused in part by a COG4 missense mutation changing Arg 729 to Trp, prompted us to determine the 1.9 Å crystal structure of a Cog4 C-terminal fragment. Arg 729 is found to occupy a key position at the center of a salt bridge network, thereby stabilizing Cog4's small C-terminal domain. Studies in HeLa cells reveal that this C-terminal domain, while not needed for the incorporation of Cog4 into COG complexes, is essential for the proper glycosylation of cell surface proteins. We also find that Cog4 bears a strong structural resemblance to exocyst and Dsl1p complex subunits. These complexes and others have been proposed to function by mediating the initial tethering between transport vesicles and their membrane targets; the emerging structural similarities provide strong evidence of a common evolutionary origin and may reflect shared mechanisms of action.congenital disorder of glycosylation ͉ Golgi apparatus ͉ multi-subunit tethering complex ͉ vesicle trafficking ͉ X-ray crystallography M ultiple lines of evidence implicate the conserved oligomeric Golgi (COG) complex in retrograde transport within the Golgi apparatus (1, 2). For example, partial knockdown of mammalian COG3 permitted exocytosis, while simultaneously blocking the retrograde transport of Shiga toxin from endosomes to the ER (3). COG was initially isolated from bovine brain on the basis of its ability to stimulate an in vitro Golgi transport assay (4). Two of its 8 subunits, Cog1/ldlB and Cog2/ ldlC, had previously been identified in a genetic screen for compromised cell surface receptor stability (5, 6). In the yeast Saccharomyces cerevisiae, the COG complex was discovered through studies of one of its subunits, Cog8p/Dor1p. Implicated in vesicle targeting to the Golgi apparatus, Cog8p was found to co-immunoprecipitate with 7 other polypeptides (7). The 7 associated polypeptides included two (Cog2p/Sec35p and Cog3p/ Sec34p) that had earlier been identified in genetic screens for temperature-sensitive mutations causing secretion defects (8). In further support of a role in trafficking to and within the Golgi, COG interacts physically with a large number of Golgi-localized trafficking factors, including (in yeast) the Rab family G protein Ypt1p, the SNAREs Sed5p, Ykt6p, Gos1p, and Sec22p, and COPI vesicle coat proteins (9).COG is one of several large protein complexes proposed to function as multisubunit tethering factors, mediating the initial interaction between transport vesicles and their target membranes (2, 10-12). For many of these complexes, a dearth of structural information has slowed the development of mechanistic models and has made it difficult to know how-or whether-to apply lessons learned from studies of one complex to another. Subunits of the COG, exocyst, Dsl1p, and Golgiassoci...