Editors' ChoiceProtein Degradation

Nuclear Shut Off of NF-κB

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Science's STKE  29 May 2007:
Vol. 2007, Issue 388, pp. tw186
DOI: 10.1126/stke.3882007tw186

Shutting off proinflammatory gene expression activated by the transcription factor nuclear factor κB (NF-κB) is an important part of resolving the inflammatory response. Although it is known that termination of NF-κB activity involves proteasomal degradation, Tanaka et al. report identification of the E3 ubiquitin ligase as the PDZ and LIM domain-containing protein PDLIM2 and that degradation occurs in the subnuclear structures called PML bodies. Activation of mouse embryo fibroblasts (MEFs) or a macrophage cell line with lipopolysaccharide (LPS) in the presence of a proteasome inhibitor lead to the accumulation of polyubiquitinated p65 subunit of NF-κB in an insoluble nuclear fraction. PDLIM2 coimmunoprecipitated with p65 when transfected into human embryonic kidney 293 cells and inhibited NF-κB-mediated reporter gene expression. Transfection of cells with both tagged p65 and PDLIM2 lacking the PDZ domain or lacking the LIM domain showed that the LIM domain was required for ubiquitination and degradation but not for trafficking of the p65 to the insoluble fraction. To prevent the rapid degradation of p65 that occurred in cells overexpressing PDLIM2, the authors assessed the subnuclear localization of p65 in cells expressing the LIM-deficient version PDLIM2. p65 accumulated in structures that were stained with antibodies that detected promyelocytic leukemia protein (so-called PML bodies) or components of the proteasome. The PDZ domain-lacking mutant did not alter p65 distribution, and despite efficient polyubiquitination, p65 was not degraded, which suggests that trafficking to the PML bodies was essential for degradation and required the PDZ domain of PDLIM2. In the reporter gene assay, the PDZ domain-lacking mutant partially inhibited NF-κB activity, and the mutant lacking the LIM domain retained full inhibitory activity, which suggests that sequestration of p65 in the PML even in the absence of ubiquitination is sufficient to inhibit NF-κB activity. Finally, compared with wild-type animals, Pdlim2 knockout mice exhibited increased sensitivity to LPS-induced shock. T cells (CD11c+) from the knockout animals showed increased nuclear p65 and increased production of proinflammatory cytokines (IL-6 and IL-12p40).

T. Tanaka, M. J. Grusby, T. Kaisho, PDLIM2-mediated termination of transcription factor NF-κb activation by intranuclear sequestration and degradation of the p65 subunit. Nat. Immunol. 8, 584-591 (2007). [PubMed]

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