Interaction between Ubc13 and the CHIP U-box. Only the main-chain or side-chain atoms of some residues are shown for clarity. Hydrogen bonds and salt bridges are shown as dotted lines. A. Interactions between Ubc13 (white) and the CHIP U-box (cyan), from PDB:2C2V . B. Superimposition of the UbcH5a:CHIP U-box and Ubc13:CHIP U-box complexes (UbcH5a in yellow, Ubc13 in white). As the conformations of the CHIP U-box backbone and interface residues are almost identical between the structures, only the U-box from the Ubc13:CHIP complex is shown. Several key interacting residues are shown explicitly. The red dashed circles indicate two interaction sites that differ between the two E2 enzymes.