GGA proteins bind ubiquitin to facilitate sorting at the trans -Golgi network

Ubiquitination functions as a sorting signal for lysosomal degradation of cell-surface proteins by facilitating their internalization from the plasma membrane and incorporation into lumenal vesicles of multivesicular bodies (MVBs) 1 . Ubiquitin may also mediate sorting of proteins from the trans-Golgi network (TGN) to the endosome, thereby preventing their appearance on the cell surface and hastening their degradation in the lysosome-vacuole 2 - 6 . Substantiation of a direct ubiquitin-dependent TGN sorting pathway relies in part on identifying candidate machinery that may function as a ubiquitin-sorting 'preceptor'at the TGN. Members of the GGA family of coat proteins localize to the TGN and promote the incorporation of proteins into clathrin-coated vesicles destined for transport to endosomes 7 , 8 . We show that the GGA coat proteins bind directly to ubiquitin through their GAT domain and demonstrate that this interaction is required for the ubiquitin-dependent sorting of the Gapl amino acid transporter from the TGN to endosomes. Thus, GGA proteins fulfill the role of ubiquitin sorting receptors at the TGN.