page 405 that three separately studied Golgi protein complexes are one and the same. The mammalian version of the complex has been named the conserved oligomeric Golgi (COG) complex.COG is octameric, and five of its eight subunits have homologues in the octameric Sec34/35 complex from budding yeast, which was recently isolated and characterized (Whyte, J.R.C., and S. Munro. 2001. Dev. Cell. 1:527–537). In addition, some of COG's subunits have previously been identified based on either an in vitro transport assay or studies of cell lines with Golgi glycosylation defects.
A detailed function for COG is not yet apparent, although mutant phenotypes in yeast and mammalian cells suggest that it participates either in forming Golgi structures or in controlling Golgi traffic. The yeast approach has subdivided the Sec34/35 complex into four components whose absence has little effect, and four whose absence is lethal or near lethal. A complete mutational analysis has not been performed in the mammalian system, but Ungar et al. note that COG can be split in half in another way. Under the electron microscope, COG appears as a bilobed structure ∼37 nm in length. Ungar et al. speculate that each lobe may contain homologues of one class of the Sec34/35 proteins. Future work will include detailed structural investigations to test this hypothesis. ▪