Yeast Gga coat proteins function with clathrin in Golgi to endosome transport

Gga proteins represent a newly recognized, evolutionarily conserved protein family with homology to the ear domain of the clathrin adaptor AP-1 gamma subunit. Yeast cells contain two Gga proteins, Gga1p and Gga2p, that have been proposed to act in transport between the trans-Golgi network and endosomes. Here we provide genetic and physical evidence that yeast Gga proteins function in trans-Golgi network clathrin coats. Deletion of Gga2p (gga2 Delta), the major Gga protein, accentuates growth and a-factor maturation defects in cells carrying a temperature-sensitive allele of the clathrin heavy chain gene. Cells carrying either gga2 Delta or a deletion of the AP-1 beta subunit gene (apl2 Delta) alone are phenotypically normal, but cells carrying both gga2 Delta and apl2 Delta are defective in growth, a-factor maturation, and transport of carboxypeptidase S to the vacuole. Disruption of both GGA genes and APL2 results in cells so severely compromised in growth that they form only microcolonies. Gga proteins can bind clathrin in vitro and cofractionate with clathrin-coated vesicles. Our results indicate that yeast Gga proteins play an important role in cargo-selective clathrin-mediated protein traffic from the trans-Golgi network to endosomes.