| The Conserved Oligomeric Golgi (COG) complex is a widely conserved eight-subunit tethering complex required for retrograde Golgi transport, and hence for proper maintenance of Golgi protein localization (1). The complex has been suggested to be related to several other complexes, including the exocyst and the Golgi-associated retrograde protein (GARP) complex, all proposed to facilitate early contact between vesicles and their target membranes (2). However, the similarities do not include significant sequence homology and the mechanism of action is too poorly characterized to support a strong argument for functional equivalence. Mutations in several subunits of COG, including Cog1, Cog7, Cog8, and most recently a missense mutation, R729W in Cog4, have been shown to cause congenital disorders of glycosylation (CDG) in humans (3). This thesis reports the 1.9 A crystal structure of a C-terminal fragment of H. sapiens Cog4 encompassing the identified mutation and a region of conserved surface residues. Cog4 shares a helical bundle motif with the other solved COG and exocyst subunits, with R729 serving a structural role at the boundary between two such bundles. Beyond this shared motif, Cog4 is newly revealed to share a nearly identical fold with S. cerevisiae Sec6p of the exocyst, providing the strongest evidence to date of a common evolutionary origin of the two complexes. While high-resolution structural information is otherwise scarce, biochemical data presented here support a further similarity in other subunits on the basis of similarly elongated subunit shapes and suggest that one of the two proposed "lobes" of the COG complex may itself be a fundamentally bipartite structure. |
