Kelch-like Protein 21 (KLHL21) Targets IκB Kinase-β to Regulate Nuclear Factor κ-Light Chain Enhancer of Activated B Cells (NF-κB) Signaling Negatively.

Activation of IKKβ is the key step in canonical activation of NF-κB signaling. Extensive work has provided insight into the mechanisms underlying IKKβ activation through the identification of context-specific regulators. However, the molecular processes responsible for its negative regulation are not completely understood. Here, we identified KLHL21, a member of the Kelch-like gene family, as a novel negative regulator of IKKβ. The expression of KLHL21 was rapidly down-regulated in macrophages upon treatment with proinflammatory stimuli. Overexpression of KLHL21 inhibited the activation of IKKβ and degradation of IκBα, whereas KLHL21 depletion via siRNA showed the opposite results. Coimmunoprecipitation assays revealed that KLHL21 specifically bound to the kinase domain of IKKβ via its Kelch domains and that this interaction was gradually attenuated upon TNFα treatment. Furthermore, KLHL21 did not disrupt the interaction between IKKβ and TAK1, TRAF2, or IκBα. Also, KLHL21 did not require its E3 ubiquitin ligase activity for IKKβ inhibition. Our findings suggest that KLHL21 may exert its inhibitory function by binding to the kinase domain and sequestering the region from potential IKKβ inducers. Taken together, our data clearly demonstrate that KLHL21 negatively regulates TNFα-activated NF-κB signaling via targeting IKKβ, providing new insight into the mechanisms underlying NF-κB regulation in cells.