Crystal structure of murine/human Ubc9 provides insight into the variability of the ubiquitin-conjugating system

Murine/human ubiquitin-conjugating enzyme Ubc9 is a functional homolog of Saccharomyces cerevisiae Ubc9 that is essential for the viability of yeast cells with a specific role in the G(2)-M transition of the cell cycle. The structure of recombinant mammalian Ubc9 has been determined from two crystal forms at 2.0 Angstrom resolution Like Arabidopsis thaliana Ubc1 and S. cerevisiae Ubc4, murine/human Ubc9 was crystallized as a monomer, suggesting that previously reported hetero-and homo-interactions among Ubcs may be relatively weak or indirect. Compared with the known crystal structures of Ubc1 and Ubc4, which regulate different cellular processes, Ubc9 has a 5-residue insertion that forms a very exposed tight beta-hairpin and a a-residue insertion that forms a bulge in a loop close to the active site, Mammalian Ubc9 also possesses a distinct electrostatic potential distribution that may provide possible clues to its remarkable ability to interact with other proteins. The a-residue insertion and other sequence and structural heterogeneity observed at the catalytic site suggest that different Ubcs may utilize catalytic mechanisms of varying efficiency and substrate specificity.