Characterization of Herc5: the major ligase for ISG15, an antiviral ubiquitin-like protein
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Human ISG15 is a 17 kDa ubiquitin-like protein (Ubl) that is induced by type I interferons (interferons [alpha] and [beta]) and plays a role in antiviral responses. ISG15 is conjugated via its C-terminus to more than 150 cellular proteins, and like ubiquitin, an E1-E2-E3 enzymatic cascade is required for conjugation. Ube1L and UbcH8 were previously identified as the E1 and E2 enzymes for this pathway. My experiments identified Herc5, a HECT domain E3, as the major ligase for ISG15. Like ISG15, Ube1L, and UbcH8, expression of Herc5 is transcriptionally induced by type I interferons. siRNAs against Herc5 abrogated ISG15 conjugation to the vast majority of target proteins in interferon-treated cells. Wild type Herc5, but not the catalytically inactive C994A mutant, supported conjugation of ISG15 in non-interferon-treated cells co-transfected with Ube1L, UbcH8 and ISG15. IQGAP1, a scaffold protein, was identified as another essential component of the ISG15 system. IQGAP1 was discovered to interact with Herc5, and this interaction was mediated by the C-terminal domain of IQGAP1 and the N-terminal RCC1-like repeats of Herc5. IQGAP1 was required for auto-conjugation of ISG15 to Herc5, and I propose a model where IQGAP1 functions, at least in part, by relieving an auto-inhibitory conformation of Herc5. Thus, I have identified two factors that are critical for ISG15 conjugation and my discoveries have increased our understanding of the ISG15 pathway. Identification and characterization of the conjugation apparatus will aid in establishing an in vitro biochemical system for ISG15 conjugation, which in turn, will be important to decipher the biological function of ISG15 modification.