A Family of Proteins with γ-Adaptin and Vhs Domains That Facilitate Trafficking between the Trans-Golgi Network and the Vacuole/Lysosome

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Abstract
We have cloned and characterized members of a novel family of proteins, the GGAs. These proteins contain an NH2-terminal VHS domain, one or two coiled-coil domains, and a COOH-terminal domain homologous to the COOH-terminal “ear” domain of γ-adaptin. However, unlike γ-adaptin, the GGAs are not associated with clathrin-coated vesicles or with any of the components of the AP-1 complex. GGA1 and GGA2 are also not associated with each other, although they colocalize on perinuclear membranes. Immunogold EM shows that these membranes correspond to trans elements of the Golgi stack and the TGN. GST pulldown experiments indicate that the GGA COOH-terminal domains bind to a subset of the proteins that bind to the γ-adaptin COOH-terminal domain. In yeast there are two GGA genes. Deleting both of these genes results in missorting of the vacuolar enzyme carboxypeptidase Y, and the cells also have a defective vacuolar morphology phenotype. These results indicate that the function of the GGAs is to facilitate the trafficking of proteins between the TGN and the vacuole, or its mammalian equivalent, the lysosome.