The Pellino E3 Ubiquitin Ligases Recognize Specific Phosphothreonine Motifs and Have Distinct Substrate Specificities

The four mammalian Pellinos (Pellinos 1, 2, 3a, and 3b) are E3 ubiquitin ligases that are emerging as critical mediators for a variety of immune signaling pathways, including those activated by Toll-like receptors, the T-cell receptor, and NOD2. It is becoming increasingly clear that each Pellino has a distinct role in facilitating immune receptor signaling. However, the underlying mechanisms by which these highly homologous proteins act selectively in these signaling pathways are not clear. In this study, we investigate whether Pellino substrate recognition contributes to the divergent functions of Pellinos. Substrate recognition of each Pellino is mediated by its noncanonical forkhead-associated (FHA) domain, a well-characterized phosphothreonine-binding module. Pellino FHA domains share very high sequence identity, so a molecular basis for differences in substrate recognition is not immediately apparent. To explore Pellino substrate specificity, we first identify a high-affinity Pellino2 FHA domain-binding motif in the Pellino substrate, interleukin-1 receptor-associated kinase 1 (IRAK1). Analysis of binding of the different Pellinos to a panel of phosphothreonine-containing peptides derived from the IRAK1-binding motif reveals that each Pellino has a distinct phosphothreonine peptide binding preference. We observe a similar binding specificity in the interaction of Pellinos with a number of known Pellino substrates. These results argue that the nonredundant roles that Pellinos play in immune signaling are in part due to their divergent substrate specificities. This new insight into Pellino substrate recognition could be exploited for pharmacological advantage in treating inflammatory diseases that have been linked to the aberrant regulation of Pellinos.