Complex Structure of OspI and Ubc13: The Molecular Basis of Ubc13 Deamidation and Convergence of Bacterial and Host E2 Recognition

Ubc13 is an important ubiquitin-conjugating (E2) enzyme in the NF-kappa B signaling pathway. The Shigella effector OspI targets Ubc13 and deamidates Gln100 of Ubc13 to a glutamic acid residue, leading to the inhibition of host inflammatory responses. Here we report the crystal structure of the OspI-Ubc13 complex at 2.3 angstrom resolution. The structure reveals that OspI uses two differently charged regions to extensively interact with the alpha 1 helix, L1 loop and L2 loop of Ubc13. The Gln100 residue is bound within the hydrophilic catalytic pocket of OspI. A comparison between Ubc13-bound and wild-type free OspI structures revealed that Ubc13 binding induces notable structural reassembly of the catalytic pocket, suggesting that substrate binding might be involved in the catalysis of OspI. The OspI-binding sites in Ubc13 largely overlap with the binding residues for host ubiquitin E3 ligases and a deubiquitinating enzyme, which suggests that the bacterial effector and host proteins exploit the same surface on Ubc13 for specific recognition. Biochemical results indicate that both of the differently charged regions in OspI are important for the interaction with Ubc13, and the specificity determinants in Ubc13 for OspI recognition reside in the distinct residues in the alpha 1 helix and L2 region. Our study reveals the molecular basis of Ubc13 deamidation by OspI, as well as a convergence of E2 recognition by bacterial and host proteins.