Transition from heterotypic to homotypic PDK1 homodimerization is essential for TCR-mediated NF-κB activation.

Strong NF-κB activation requires ligation of both the CD28 coreceptor and TCR. Phosphoinositide-dependent kinase 1 (PDK1) acts as a scaffold by binding both protein kinase Cθ (PKCθ) and CARMA1, and is therefore essential for signaling to NF-κB. In this article, we demonstrate the importance of PDK1 Thr(513) phosphorylation in regulating the intermolecular organization of PDK1 homodimers. Thr(513) is directly involved in heterotypic PDK1 homodimer formation, in which binding is mediated through the pleckstrin homology (PH) and kinase domains. Upon activation, phosphorylated Thr(513) instead mediates homotypic intermolecular binding through the PH domains. Consequently, cell-permeable peptides with a Thr(513) to Ile derivative (protein transduction domain [PTD]-PDK1-Thr(513)-Ile) bound the kinase domain, whereas a Thr(513)-to-Asp peptide (PTD-PDK1-Thr(513)-Asp) bound the PH domain. PTD-PDK1-Thr(513)-Ile blocked binding between PDK1 and PKCθ, phosphorylation of PKCθ Thr(538), and activation of both NF-κB and AKT. In contrast, PTD-PDK1- Thr(513)-Asp selectively inhibited binding between PDK1 and CARMA1, and blocked TCR/CD28-induced NF-κB activation. Therefore, Thr(513) phosphorylation regulates a critical intermolecular switch governing PDK1 homodimer structure and the capacity to interact with downstream signaling pathway components. Given the pleiotropic functions of PDK1, these data may open the door to the development of immunosuppressive therapies that selectively target the PDK1 to NF-κB pathway in T cell activation.