T Cell Receptor-induced Nuclear Factor κB (NF-κB) Signaling and Transcriptional Activation Are Regulated by STIM1-and Orai1-mediated Calcium Entry

T cell activation following antigen binding to the T cell receptor (TCR) involves the mobilization of intracellular Ca2+ to activate the key transcription factors nuclear factor of activated T lymphocytes (NFAT) and NF-kappa B. The mechanism of NFAT activation by Ca2+ has been determined. However, the role of Ca2+ in controlling NF-kappa B signaling is poorly understood, and the source of Ca2+ required for NF-kappa B activation is unknown. We demonstrate that TCR- but not TNF-induced NF-kappa B signaling upstream of I kappa B kinase activation absolutely requires the influx of extracellular Ca2+ via STIM1-dependent Ca2+ release-activated Ca2+/Orai channels. We further show that Ca2+ influx controls phosphorylation of the NF-kappa B protein p65 on Ser-536 and that this posttranslational modification controls its nuclear localization and transcriptional activation. Notably, our data reveal that this role for Ca2+ is entirely separate from its upstream control of IB degradation, thereby identifying a novel Ca2+-dependent distal step in TCR-induced NF-kappa B activation. Finally, we demonstrate that this control of distal signaling occurs via Ca2+-dependent PKC-mediated phosphorylation of p65. Thus, we establish the source of Ca2+ required for TCR-induced NF-kappa B activation and define a new distal Ca2+-dependent checkpoint in TCR-induced NF-kappa B signaling that has broad implications for the control of immune cell development and T cell functional specificity.