The Huibregtse lab studies the biochemistry of the ubiquitin proteolysis system and ISG15, a ubiquitin-like modifier involved in innate immune responses to viral and microbial infections.  Our current NIH-funded projects include:

1) The mechanism and function of ISG15 conjugation. ISG15 is an interferon-induced ubiquitin-like protein that is conjugated to cellular proteins through a pathway separate from that for ubiquitin. ISG15 has anti-viral activity against a range of virus types, and a major goal is to understand the biochemical basis of its anti-viral activity and the mechanism of its conjugation.  One of the strongest lines of evidence indicating that ISG15 is anti-viral is that several virus types, including SARS-CoV-2 and influenza viruses, have evolved mechanisms for reversing or overcoming the effect of ISG15 conjugation.  The SARS-CoV-2 PLpro protease is a “de-ISGylase” that catalyzes deconjugation of ISG15, and PLpro also catalyzes viral polyprotein cleavage and is therefore essential for viral replication.  We have identified an FDA-approved drug that inhibits PLpro-catalyzed de-ISGylation and polyprotein cleavage and blocks CoV-2 viral replication at sub-micromolar levels.  This compound is a direct-activing antiviral that can potentially be re-purposed against COVID-19 and future emergent coronaviruses.  

2)  Remarkably, ISG15 has a second biochemical function as an extracellular signaling protein.  Free (unconjugated) ISG15 is released from cells and signals to Natural Killer cells and other cells of the immune system to release pro-inflammatory cytokines, including interferon-gamma and IL-6.  We have identified the receptor on NK cells as LFA-1, an integrin receptor. We are currently characterizing the mechanism of release of ISG15 from cells and the mechanism of ISG15-dependent cytokine secretion from NK cells. 

3)  We are interested in the role of the ubiquitin system in protein quality control, particularly the role of co-Translational Ubiquitination (CTU).  We discovered that a large fraction of polyribosome-associated nascent polypeptides are ubiquitinated during translation.  Based on the factors that influence the extent of CTU, our working model is that this reflects a protein quality control system that monitors protein folding during translation.  This system can trigger the initiation of degradation of a protein before its synthesis is complete.  We are exploring the potential implications of these findings in protein folding diseases and cellular and organismal aging.