Tom1 (Target of Myb1) is suggested to be involved in the transport of ubiquitinated proteins, through the interaction of its GAT (GGA and Tom1) domain with ubiquitin. Here, we demonstrate that the three-helix bundle of Tom1-GAT has two ubiquitin-binding sites recognizing the hydrophobic Ile44 surface of ubiquitin. The complex crystal structure demonstrates that the first site is a hydrophobic patch on helices α1 and α2. NMR and biochemical data revealed that the N-terminal half of helix α3 of Tom1-GAT constitutes the second, stronger binding site. The double-sided ubiquitin binding enhances the efficiency of recognition of ubiquitinated proteins by Tom1.
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