Evidence for M1-Linked Polyubiquitin-Mediated Conformational Change in NEMO.

The NF-κB essential modulator (NEMO) is the scaffolding subunit of the inhibitor of κB kinase (IKK) holocomplex and is required for the activation of the catalytic IKK subunits, IKKα and IKKβ, during the canonical inflammatory response. Although structures of shorter constructs of NEMO have been solved, efforts to elucidate the full-length structure of NEMO have proved difficult due to its apparent high conformational plasticity. To better characterize the gross dimensions of full-length NEMO, we employed in-line size exclusion chromatography-small-angle X-ray scattering. We show that NEMO adopts a more compact conformation (Dmax=320Å) than predicted for a fully extended coiled-coil structure (>500Å). In addition, we map a region of NEMO (residues 112-150) in its coiled-coil 1 domain that impedes the binding of linear (M1-linked) di-ubiquitin to its coiled-coil 2-leucine zipper ubiquitin binding domain. This ubiquitin binding inhibition can be overcome by a longer chain of linear, but not K63-linked polyubiquitin. Collectively, these observations suggest that NEMO may be auto-inhibited in the resting state by intramolecular interactions and that during signaling, NEMO may be allosterically activated by binding to long M1-linked polyubiquitin chains.