ARRDC3 is a novel ISG that modulates ubiquitination of IFITM3

Abstract

Innate immune response is the first line of defence against viral infections. Sensing of viral PAMPs by pattern-recognition receptors triggers the production of type I interferons, which further induce the expression of a subset of genes called interferon-stimulated genes (ISGs). Despite the importance of ISGs in combating infections, the identities of ISGs and their corresponding functions have not been fully elucidated. In this study, we aimed at identifying ISGs important for respiratory tract infections. Normal human lung fibroblasts IMR90 and WI38 cells and human bronchial epithelial cells (HBECs) were treated with interferon beta (IFNβ) or infected with Cantell-strain Sendai virus. Extracted RNA was subjected to deep sequencing for identification of up-regulated genes common to all three cell-lines. Apart from some well-recognized ISGs, one candidate known as ARRDC3, which has not been reported as an ISG, stood out as one of the most highly induced genes. To study the possible functions of this new ISG, immunoprecipitation-mass spectrometry (IP-MS) was performed to identify interacting partners of ARRDC3. Interestingly, most interacting proteins are related to the ubiquitination pathway, in particular E3 ligases harbouring a HECT domain. Co-IP experiment further confirmed the association of ARRDC3 with E3 ligase NEDD4, and this interaction is dependent on the PPxY motif of ARRDC3. Previous reports suggested that NEDD4 is a pro-viral E3 ligase that ubiquitinates IFITM3 protein. In our co-expression experiment, ARRDC3 reduced IFITM3 ubiquitination in the presence of NEDD4. This suggests one of the functional role of this interferon-induced ARRDC3 might enhance IFITM3 stability by inhibiting NEDD4-mediated ubiquitination of the latter. Further study is required to investigate any additional roles of ARRDC3 in innate immunity. This study was supported by Research Grant Council - General Research Fund (17153216).