Ubiquitin (Ub) is a small protein modifier, regulating diverse biological functions such as signalling, DNA repair and proteasomal degradation. Ub can form polymers via 7 Lys residues of Ub itself. Recently, we have discovered that an E3 ligase complex, Linear Ubiquitin chain Assembly Complex (LUBAC) generates a novel type of Ub polymer linked via Met-1, ‘linear Ub chain’ and regulates NF-kB signalling in mice. Because linear Ub is unique and the study of it is still in infancy, the only E3 ligase known is LUBAC, comprising a catalytic protein HOIP, and two regulatory subunits SHARPIN and HOIL-1L. We have shown that SHARPIN deficiency leads inflammation in mice. A mutation in HOIL-1L gene of human was shown to lead immunodeficiency. Yet, the regulatory mechanisms of HOIP catalytic activity and the biological implications remain poorly understood. Here, we aim to- Elucidate the roles of HOIP in Drosophila- Elucidate the roles of ubiquitination and ligase activities of mammalian HOIP in vivo- Identify novel substrates of human HOIP and clarification of their rolesWe recently identified an orthologue of HOIP in Drosophila, yet its genome does not encode SHARPIN or HOIL-1L. We aim to elucidate how dmHOIP mediates linear ubiquitination in the absence of regulatory subunits and the roles of HOIP in the NF-kB signalling by genetically deleting HOIP in Drosophila. We further aim to elucidate the role of HOIP E3 ligase activity and ubiquitination in inflammation by generating the conditional knockin mice of HOIP mutants. Moreover, we will develop a protein chip assay to identify new substrates of HOIP and determine how they contribute to the biological functions.Since Ub plays such a wide variety of pathological functions including cancer, inflammation and neuronal diseases, I believe the expected results not only will lead to a better understanding of functional role of HOIP but will also identify novel aspects of linear ubiquitination in human diseases.