Pellino proteins are more than scaffold proteins in TLR/IL-1R signalling: a role as novel RING E3-ubiquitin-ligases

Members of the Toll-like receptor (TLR) and IL-1 receptor (IL-1R) family initiate signalling pathways that shape innate immunity. Pellino proteins have recently been implicated as evolutionary conserved scaffold proteins in TLR/IL-1R signalling leading to nuclear factor-kappaB and mitogen activated protein kinase-dependent gene expression. We found that Pellino proteins contain a new RING-like motif. Because RING motifs are a feature of a subclass of E3-ubiquitin-ligases that target specific proteins for ubiquitination, we suggest that Pellino proteins are involved in TLR/IL-1R signalling not only as scaffold proteins but also as RING E3-ubiquitin-ligases. In support of this hypothesis we show that Pellino proteins induce IRAK-1 polyubiquitination in a RING-dependent manner. We further propose a model in which Pellino-mediated IRAK-1 polyubiquitination regulates TLR/IL-1R signalling.