What we think about

BRCA1/BARD1 and Protein Ubiquitylation

Our interest in protein ubiquitylation and how it works arose from our interest in understanding how BRCA1 works and why inherited mutations in the BRCA1 gene and its sister gene, BARD1, are associated with extremely high risk for breast and ovarian cancer. BRCA1 and BARD1 form an obligate heterodimer that functions as a Ubiquitin E3 ligase. Despite solving its structure by NMR twenty years ago (Brzovic NSB 2001) and identifying its cognate E2s (Christensen NSMB 2007) over ten years ago, how loss of E3 function is associated with the development of cancer remains enigmatic. We believe this situation is due to a lack of validated cellular substrates that could lead to the missing insight. Our current efforts include structural and biochemical investigation of BRCA1/BARD1 function on nucleosomes, as the flexible C-terminal tail of histone H2A is a bona fide BRCA1/BARD1 substrate.

More generally, we are interested in addressing fundamental questions that underlie protein ubiquitylation, a central regulatory mechanism. Our current focus is on mono-ubiquitylation, which is by far the most prevalent type of ubiquitylation but the least studied of all types of ubiquitylation. BRCA1/BARD1/nucleosome falls into this category, as does the RING-Between-RING E3, HHARI, and certain E2s including Ube2W, all of which are under investigation in the Klevit lab.

Small Heat Shock Proteins

Small heat shock proteins (sHSPs) are among the most enigmatic proteins in the proteome. Generally thought of as “housekeepers,” sHSPs can serve as passive protein chaperones, meaning that they have the capacity to delay the onset of protein aggregation without using an energy source such as ATP. Our overarching goal is to provide insights into these important components of human proteostasis and a new roadmap for future investigations into this key class of underappreciated chaperones.

There are ten sHSPs in the human proteome and inherited mutations in these are associated with numerous protein-folding diseases such as cataract and Alzheimer’s disease. sHSPs are notoriously difficult to study—their “clients” (i.e., aggregate-prone proteins) are themselves difficult to study and sHSPs themselves are more than 50% disordered. We have recently devised an approach that utilizes several types of NMR, HDX/MS, computational modeling, and biochemistry to define an activated state of one of the most ubiquitous human sHSPs, HSPB1 (Clouser eLife 2019). We are using a similar approach to define the mechanism by which HSPB1 delays the onset of Tau aggregation, thought to be the root cause of numerous neurodegenerative diseases known collectively as Tauopathies (Baughman JBC 2018Baughman PNAS 2020).

 

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