The correct sorting of proteins is an important challenge for all eukaryotic cells. Problems in protein sorting are known to lead to various developmental defects or acquired diseases in humans. In the secretory pathway, whose main sorting hub is the Golgi apparatus, vesicular transport is used to sort proteins. To maintain the unique composition of different Golgi cisternae for example, resident Golgi-enzymes are continuously sorted away from their substrates, the secreted proteins. This is mostly mediated by COPI dependent retrograde vesicle transport, during which vesicles of different compositions are targeted to the various cisternae. The COG complex, several Rab GTPases and golgins, all function during the process referred to as vesicle tethering, to mediate the targeting of COPI vesicles. The molecular details of vesicle tethering, which is an essential process during every vesicle transport step, are unknown. It is believed to be the first encounter between vesicle and target membrane, and to encompass a series of protein-protein interactions leading to membrane fusion. This proposal aims to determine the interaction-topology of the involved proteins, and the identities of hitherto unknown factors, to establish a molecular mechanism of vesicle tethering at the Golgi. For this purpose we will use a combination of biochemical and genetic methods. Using in vivo immunoprecipitations and in vitro interaction assays we will reveal the interaction map of tethering. To uncover novel factors involved in the process we will take advantage of the known glycan processing defects associated with COG deficiencies. We will devise an RNAi based screen selecting for phenocopies of the glycosylation defects by using a specific combination of toxic lectins that selectively bind to the surface glycans of wild type cells. This work should expand our knowledge about both vesicle tethering and the organization of glycan chain synthesis in the Golgi.