Electronic Thesis and Dissertation Repository


Doctor of Philosophy


Microbiology and Immunology


Dr. Stephen Barr


Interferon (IFN)-induced proteins serve as one of the first lines of defense against viral pathogens such as Human Immunodeficiency Virus (HIV)-1. IFN treatment has been shown to restrict multiple stages of HIV-1 replication. The identities and functions of IFN-induced proteins involved in the inhibition of HIV-1 and other viruses are not fully understood. Homologous to the E6-AP C-terminus (HECT) and regulator of chromosome condensation 1 (RCC1)-like domain (RLD) containing protein 5 (HERC5) is strongly upregulated by the Type I IFN response. HERC5, a member of the small HERC family, is composed of an N-terminus RLD domain and a C-terminus HECT domain separated by a spacer region. With the discovery that HERC5 is the main E3 ligase for ISG15, and given the well-known antiviral properties of ISG15, I hypothesized that HERC5 is an evolutionarily conserved restriction factor that interferes with HIV-1 particle production. Using an HIV-1 particle release assay, I showed HERC5 inhibits HIV-1 assembly and budding using an ISG15 E3 ligase-dependent mechanism which modifies HIV-1 Gag with ISG15. Next, I determined that HERC5 restricts HIV-1 replication by inhibiting unspliced HIV-1 mRNA export, which is dependent on its RLD domain. Using a similar approach, I characterized the entire small HERC family and demonstrated that the other HERC members exhibit differential abilities in inhibiting HIV-1 replication. My research characterizes HERC5 as an antiviral protein that has the ability to potently restrict HIV-1 replication by two novel mechanisms. I provided new insight into how the innate immune system combats viral pathogens such as retroviruses. My research also lays the groundwork for future research into exploiting the antiviral activity of HERC5 by developing drugs that mimic or enhance the antiviral activity of HERC5.