Ubiquitin-mediated activation of TAK1 and IKK.

Transforming growth factor beta activated kinase-1 (TAK1), a member of the mitogen-activated protein kinase kinase kinase family, has emerged as a key regulator of signal transduction cascades leading to the activation of the transcription factors nuclear factor-kappa B (NF-kappaB) and activator protein-1 (AP-1). Stimulation of cells with cytokines and microbial pathogens results in the activation of TAK1, which subsequently activates the I-kappa B kinase complex (IKK) and mitogen-activated protein (MAP) kinases, culminating in the activation of NF-kappaB and AP-1, respectively. Recent studies have shown that polyubiquitination of signalling proteins through lysine (Lys)-63-linked polyubiquitin chains plays an important role in the activation of TAK1 and IKK. Unlike Lys-48-linked polyubiquitination, which normally targets proteins for degradation by the proteasome, Lys-63-linked polyubiquitin chains act as scaffolds to assemble protein kinase complexes and mediate their activation through proteasome-independent mechanisms. The concept of ubiquitin-mediated activation of protein kinases is supported by the discoveries of ubiquitination and deubiquitination enzymes as well as ubiquitin-binding proteins that function upstream of TAK1 and IKK. Recent biochemical and genetic studies provide further insights into the mechanism and function of ubiquitin signalling and these advances will be the focus of this review.