Unanchored K63-linked polyubiquitin chains: a novel second messenger involved in G protein-coupled receptors early signaling events

GPCR rapidly transduced extracellular cues via their ability to activate effector proteins leading to the production of a variety of well-characterized second messenger molecules. We describe here that the E3 ubiquitin ligase tumor necrosis factor receptor associated factor 6 (TRAF6) is required for the rapid and transient production of unanchored lysine (K)63-linked polyubiquitin chains in primary and transformed cells exposed to vasoactive agonists Angiotensin II (Ang II), Lysophosphatidic acid (LPA) and Thrombin (Thr). We further show that unanchored K63-linked polyubiquitin chains accumulate in Transforming Growth Factor beta-activated kinase 1 (TAK1) and IkappaB kinase beta (IKKbeta) immunocomplexes and are required for the T-loop phosphorylation of TAK1 leading to IKKbeta, c- Jun N-terminal kinases (JNK1/2) and p38 mitogen-activated protein kinases (p38) phosphorylation and the induction of transcriptional events. Our results support a novel paradigm in field of GPCR signal transduction, identifying unanchored K63-linked polyubiquitin chains as novel second messenger molecules.