Transient and Selective NF-κB p65 Serine 536 Phosphorylation Induced by T Cell Costimulation Is Mediated by IκB Kinase β and Controls the Kinetics of p65 Nuclear Import

Abstract

Full transcriptional activity of the nuclear, DNA-bound form of NF-κB requires additional posttranslational modifications. In this study, we systematically mapped the T cell costimulation-induced phosphorylation sites within the C-terminal half of the strongly trans-activating NF-κB p65 subunit and identified serine 536 as the main phosphorylation site. The transient kinetics of serine 536 phosphorylation paralleled the kinetics of IκBα and IκB kinase (IKK) phosphorylation and also mirrored the principle of T cell costimulation. The TCR-induced pathway leading to serine 536 phosphorylation is regulated by the kinases Cot (Tpl2), receptor interacting protein, protein kinase Cθ, and NF-κB-inducing kinase, but is independent from the phosphatidylinositol 3-kinase/Akt signaling pathway. Loss-of-function and gain-of-function experiments showed phosphorylation of p65 serine 536 by IKKβ, but not by IKKα. Phosphorylation occurs within the cytoplasmic and intact NF-κB/IκBα complex and requires prior phosphorylation of IκBα at serines 32 and 36. Reconstitution of p65^−/− cells either with wild-type p65 or a p65 mutant containing a serine to alanine mutation revealed the importance of this phosphorylation site for cytosolic IκBα localization and the kinetics of p65 nuclear import.