Chaperone protein guides histone H1 to its nucleosome target, study reveals

Inside every human cell, roughly six feet of DNA must be folded into a nucleus less than one-tenth the width of a human hair. That extraordinary packing feat depends on a family of proteins called histones, around which genomic DNA spools like a garden hose wound around a reel. The basic unit of this packaging, the nucleosome, consists of DNA wrapped around a core of histone proteins. But compaction on this scale requires more than core histones alone. A specialized protein known as linker histone H1 binds to nucleosomes and drives the next level of folding, drawing nucleosomes together into dense chromatin and keeping genes that a cell does not need locked away and silent. Now, new research has revealed a surprising detail about how this essential regulator actually reaches its destination: it cannot get there on its own.

A study published on September 22 in the journal Molecular Cell by researchers at The Ohio State University and the University of Zurich shows that linker histone H1 wanders aimlessly inside the cell even when its target, a nucleosome with spooled-up segments of DNA, is directly in front of it. Although H1 binds readily to two DNA strands that stick out from the nucleosome, it struggles to bind to the nucleosome itself. Only when a chaperone protein escorts it does H1 successfully load onto the nucleosome and carry out its gene-silencing role. The finding fills a long-standing gap in scientists’ understanding of how one of the genome’s most important architectural proteins finds its proper place.

The research team, led by first author Ehsan Akbari, a research scientist in the laboratory of senior author Michael Poirier, professor of physics at The Ohio State University, tackled the question with single-molecule experiments. Rather than averaging the behavior of millions of molecules together, single-molecule methods allow researchers to quantify molecular motions and interactions one molecule at a time, watching individual H1 proteins and their nucleosome targets before, during and after H1 bound to the DNA and to the center of the nucleosome. The team combined optical tweezer instrumentation, which uses highly focused laser light to manipulate and measure single molecules, with single-molecule fluorescence measurements to visualize the process in real time.

“Many proteins are dynamic, and the way you get at mechanistic dynamic information is by doing these single-molecule studies where you literally can watch in real time what individual molecules are doing,” Poirier said. This approach proved essential for a protein whose behavior defied expectations. When the researchers observed H1 in action, they found that the protein continues to move around even after it binds to DNA. More strikingly, instead of loading onto the nucleosome by way of the DNA it had attached to, the protein reflects away from the nucleosome entirely.

“It moves around and doesn’t even go to the nucleosome, which I didn’t believe,” Poirier said. That unexpected observation prompted the team to hypothesize that a chaperone protein might be the missing link in the loading process. Chaperone proteins are known to assist other proteins in folding, targeting and assembly, and three linker histone chaperone proteins were already known to science. When the researchers added these chaperones to their experiments, the results were dramatic.

“Amazingly, the chaperones did a bunch of things to regulate how H1 goes around and how it actually finds and gets onto nucleosomes. And that’s the main point, that people have not understood how H1 loads and gets onto a nucleosome,” Poirier said. “What we found was that it actually will directly load and likes to slide along DNA – we also didn’t even know that it would do that – but it needs a chaperone to help get onto the actual nucleosome.” The experiments revealed that H1 can load directly onto DNA and slide along it, a previously unknown behavior, but the final step of binding to the nucleosome core requires chaperone assistance.

The biological stakes of this process are considerable. Every cell in a plant, animal or human contains the organism’s entire genome, and nucleosomes fold in on each other, clumping together to form chromatin. Linker histone H1’s job is to facilitate chromatin compaction, ensuring that DNA segments containing genes the cell is not using stay wrapped up and unavailable for expression. “We know that H1 is important for compacting the genome, which is important for regulating gene expression,” Poirier said. “In the genome, regions that are going to be compacted are loaded with a lot of H1, while regions that are more open are going to have less H1.”

Most genes in any given cell are turned off at any moment, because only a small fraction of the genome is active in a particular cell type. Maintaining the silent majority in compacted states is therefore a fundamental requirement of cellular life, and H1 is a central player in that maintenance. “H1 is a key regulator of which genes are being used by a cell and which genes are not,” Poirier said. “If you want to understand how disease develops because gene expression is no longer working properly, then you need to understand how H1 works. Once you understand this, then you open up the possibility for new therapies that counteract this cause of disease.”

The connection to disease is not hypothetical. H1 is well known for its role in compacting genes and for its connection to cancer when it undergoes mutation. Mutations in linker histone genes and alterations in how H1 is deployed across the genome can disrupt the careful balance between open, active chromatin and closed, silent chromatin, contributing to the misregulated gene expression that characterizes cancer cells. Understanding the mechanics of how H1 reaches nucleosomes in the first place gives researchers a new handle on where this process could go wrong.

Though the work does not tell the entire H1 story, the step-by-step visualizations offer concrete directions for future research. Poirier’s laboratory has previously studied H1 variants and the post-translational modifications that can change H1 targeting and function after the protein has been fabricated by the cell. “One very natural thing to do next is to look at how these variants, post-translational modifications, and, most importantly, cancer-relevant mutations influence H1 properties and function that we are now in a position to measure,” he said. With single-molecule tools now capable of watching H1’s loading pathway in detail, the effects of disease-linked alterations can be tested directly.

The study, titled “Linker histone H1.0 loads onto nucleosomes through multiple pathways that are facilitated by histone chaperones,” was funded by the National Institutes of Health and the U.S. National Science Foundation. In addition to Akbari and Poirier, the co-authors included Nathaniel Burge of Ohio State and Matti Valdimarsson, Aritra Chowdhury and Benjamin Schuler of the University of Zurich. Together, their findings reframe a seemingly simple question – how does a protein find its binding site? – into a multi-step journey involving DNA sliding, unexpected reflection away from targets, and an essential chaperone handoff, deepening the mechanistic picture of how cells keep their vast genomes organized and their genes properly silenced.

Subject of Research: How linker histone H1 is targeted to nucleosomes with the help of histone chaperone proteins

Article Title: Protein key to cells’ gene regulation needs a chaperone to do its job

Article References: Protein key to cells’ gene regulation needs a chaperone to do its job. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: linker histone H1, nucleosome, chromatin compaction, gene regulation, histone chaperones, single-molecule imaging, optical tweezers, epigenetics, cancer mutations, Molecular Cell, Ohio State University, DNA packaging