Recruitment of NEMO/IKKγ to TCR microclusters during T cell activation

Abstract

Abstract The IκB kinase (IKK) complex mediates the activation of canonical NFκB isoforms following T cell receptor (TCR) ligation. This complex consists of the kinases IKKα and IKKβ and an essential adaptor subunit, the NFκB essential modulator protein (NEMO). Most models suggest that the IKK complex is activated within oligomeric Carma1/Bcl10/Malt1 (CBM) signalosomes. However, we observed that NEMO enters TCR microclusters before CBM complexes are assembled, within ~70 seconds of TCR engagement. NEMO also entered mobile vesicles and in larger membrane-bounded structures (hereafter ‘macroclusters’). The recruitment of NEMO into TCR microclusters is prevented by Src kinase inhibitors and by the catalytic inactivation of ZAP-70, but occurs in the absence of either SLP-76 or Carma1. Further, NEMO fails to co-localize with TCR-induced CBM polymers. Thus, the recruitment of NEMO to the TCR occurs via a CBM-independent mechanism. The deletion the zinc-finger (ZnF) domain of NEMO disables NFκB signaling and eliminates NEMO from microclusters, while preserving NEMO macroclusters. Since the ZnF domain interacts with polyubiquitin chains, we generated point mutations impacting the ubiquitin-binding site in the ZnF domain and two independent sites within the NEMO ubiquitin-binding ‘NUB’ domain. These mutations impair the ability of NEMO to capture K63-linked and/or linear polymers, hinder NFκB signaling, and eliminate NEMO from microclusters without disrupting NEMO macroclusters. These findings suggest that NEMO is rapidly recruited to polyubiquitin chains associated with the TCR, rather than the CBM complex, and that the CBM complex augments IKK-dependent NFκB signaling via a distinct, recruitment-independent mechanism.