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Publications of the Week

Interplay of Disordered and Ordered Regions of a Human Small Heat Shock Protein Yields an Ensemble of ‘Quasi-Ordered’ States

By October 17, 2019No Comments

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This week we profile a recent publication in eLife from the laboratory of Dr. Rachel Klevit (pictured) at UW.

Can you provide a brief overview of your lab’s current research focus?

An overarching theme of my lab is to understand the guardians of the cell. We focus on two broad topics: guardians of the genome and guardians of the proteome. In the former category, the modification of nucleosomal histones with ubiquitin is emerging as a mark that is involved in the DNA damage response, transcriptional regulation, and maintenance of epigenetic marks. Our current focus in this realm is the breast cancer susceptibility protein, BRCA1, and its “sister gene,” BARD1. In the second category, the small heat shock proteins (sHSPs) play key roles both in cellular housekeeping and as a cell’s first responders in times of stress. These enigmatic protein chaperones are involved in a growing number of human diseases and syndromes, especially neurodegenerative disease, cardiomyopathies, and cataract, but their mechanism of action remains virtually entirely undefined.

What is the significance of the findings in this publication?

A central dogma of structural biology is that protein structure begets function, and much of our understanding of the protein structure-function relationship is derived from characterization of stable, well-ordered structures. However, up to 40% of the human proteome, including human small heat shock proteins and their clients, exist in flexible, disordered states that have defied efforts to characterize experimentally. Powerful techniques such as x-ray crystallography or cryo-electron microscopy cannot be used to characterize proteins that don’t adopt fixed structures. We applied a combination of NMR spectroscopy, Hydrogen-Deuterium Exchange/Mass Spectroscopy (HDXMS), and computational modeling to gain unprecedented insight into the disordered regions of HSPB1, a ubiquitously-expressed sHSP known to delay aggregation of proteins, including tau involved in neurodegenerative disease. The results revealed a novel type of intrinsic disorder that we have termed “quasi-order” that explains the observed heterogeneity and plasticity of sHSPs known to be critical to their function. The insights provide the first residue-level information about the disordered regions of sHSPs and a new way to approach their study. For example, we are able to provide a structural rationale for the known effects of stress-induced phosphorylation on HSPB1 structure and function. We also uncovered unexpected non-local effects from known disease-associated mutations at residues within the disordered region. Missense mutations of residues within disordered regions of human proteins remain enigmatic and our novel approach may provide a new way to understand and interrogate such mutations in the future. Altogether, the publication provides new insights specifically into an important component of human proteostasis, and more generally, a new roadmap for future investigations into this key class of underappreciated chaperones.

What are the next steps for this research?

We envision several directions for this research in the future. First, to assess the generality of quasi-order in human sHSPs, our experimental strategy can be applied to other members of the protein chaperone family. Second, the insights regarding HSPB1 structure will serve as a foundation on which to investigate and understand HSPB1 chaperone activity with its cellular clients, including tau, alpha-synuclein, and other proteins involved in neurodegenerative disease. Third, the notion of quasi-ordered proteins and the approaches we developed that gave rise to it may unlock mysteries of other intrinsically disordered systems in the future.

This research was funded by:

This work was funded by NIH NEI and NIA (National Eye Institute and National Institute for Aging).

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