Interferon-stimulated Gene 15 (ISG15) and ISG15-linked Proteins Can Associate with Members of the Selective Autophagic Process, Histone Deacetylase 6 (HDAC6) and SQSTM1/p62

The ubiquitin-like interferon (IFN)-stimulated gene 15 (ISG15) and its specific E1, E2, and E3 enzymes are transcriptionally induced by type I IFNs. ISG15 conjugates newly synthesized proteins. ISG15 linkage to proteins appears to be an important downstream IFN signaling event that discriminates cellular and pathogenic proteins synthesized during IFN stimulation from existing proteins. This eliminates potentially pathogenic proteins as the cell attempts to return to normal homeostasis after IFN stressed conditions. However, the molecular events that occur in this process are not well known. Here, we show that the C-terminal LRLRGG of ISG15 interacts with the binder of ubiquitin zinc finger (BUZ) domain of histone deacetylase 6 (HDAC6). Because HDAC6 is involved in the autophagic clearance of ubiquitinated aggregates during which SQSTM1/p62 plays a major role as a cargo adapter, we also were able to confirm that p62 binds to ISG15 protein and its conjugated proteins upon forced expression. Both HDAC6 and p62 co-localized with ISG15 in an insoluble fraction of the cytosol, and this co-localization was magnified by the proteasome inhibitor MG132. In addition, ISG15 was degraded via the lysosome. Overexpression of ISG15, which leads to an increased conjugation level of the cellular proteome, enhanced autophagic degradation independently of IFN signaling transduction. These results thus indicate that ISG15 conjugation marks proteins for interaction with HDAC6 and p62 upon forced stressful conditions likely as a step toward autophagic clearance.