Protein ubiquitination: CHIPping away the symmetry

CHIP is a ubiquitin ligase implicated in the degradation of misfolded proteins. In the November 23 issue of Molecular Cell, Zhang et al. (2005) identified CHIP as a protein that interacts with the ubiquitin E2 complex Ubc13-Uev1A, which catalyzes the synthesis of Lys-63-linked polyubiquitin chains. Although the ubiquitin ligase activity of CHIP requires its dimerization through the U box domain, the crystal structure of the CHIP-E2 complex reveals that the protomers in the CHIP homodimer adopt distinct conformations such that only one U box of CHIP interacts with Ubc13.