Understanding the function and impact of histone H3 as a copper reductase enzyme

Abstract The research in our laboratory centers on our recent discovery that histone H3 is an oxidoreductase enzyme, catalyzing the reduction of cupric (Cu2+) to cuprous (Cu1+) ions. Historically, histones have been considered as DNA packaging proteins that regulate gene expression through epigenetic mechanisms. However, considering that ancestral histones existed in simple organisms lacking nuclei and epigenetic capabilities, we thought it would be plausible that histones may have a different function predating their current epigenetic roles. It is also noteworthy that a histone-containing archaeon was the host for the first endosymbiotic event leading to the mitochondria, raising questions about the role of ancestral histones in eukaryogenesis. Inspired by geochemical events surrounding the appearance of the first eukaryotic cell, we have discovered a novel function for histone H3 as a copper reductase enzyme. This activity is important because copper must be in its reduced, Cu1+ state to be effectively transported and utilized by copper-dependent proteins and enzymes. Over the last five years, we have shown that histone H3 binds a Cu2+ ion at the H3-H3’ interface, reconstituted the copper reductase activity of recombinant histone H3 in vitro, confirmed this activity within nucleosomes, and provided genetic and molecular evidence that this activity regulates cellular Cu1+ levels, impacting copper- dependent activities like mitochondrial respiration. Additionally, our research has provided the first example of how the copper reductase activity of histone H3 may contribute to the pathology of a human disease, namely, Friedreich’s ataxia, a neurodegenerative disease. Our data have established histone H3 within nucleosomes as the first known protein-based mechanism for regulating copper oxidation state inside the cell. Our overarching future goal is to expand our understanding of chromatin structure and function by deeply exploring the enzymatic activity of the nucleosome. We aim to investigate both the immediate questions about the mechanism and regulation of enzyme activity, and the broader questions about how this enzymatic activity influences cellular metabolism. Over the next few years, we plan to develop a detailed understanding of nucleosome enzyme activity through structural and functional studies, examine how this activity integrates with broader cellular metabolism, including copper homeostasis, and investigate potential biological connections between the lysosome—the main copper repository—and the nucleosome. We will also explore how copper homeostasis could facilitate a previously unrecognized connection between the mitochondria and the nucleus. Armed with a deeper understanding, we will then begin to extend our investigations to mammalian systems. We envision our work will provide a new metabolic context for understanding the eukaryotic nucleosome, forming a common thread from eukaryogenesis to human disease.