COLUMBUS, Ohio – A protein that’s essential to every cell’s ability to turn off unnecessary genes is well known for its key function of compacting genes and its connection to cancer when it undergoes mutation. But until now, scientists weren’t quite sure how the protein gets where it needs to go to keep genes bound up so they aren’t activated.
In a new study, researchers found that this protein, called linker histone H1 , initially flits around inside a cell with no clear aim even in the presence of its target: a nucleosome containing spooled-up segments of DNA. H1 binds directly to two DNA strands sticking out from the nucleosome, but struggles to bind to the nucleosome itself until a chaperone protein escorts it there.
The team used single-molecule experiments to visualize individual H1 proteins and their nucleosome targets before, during and after H1 bound to the DNA and to the center of the nucleosome. Single-molecule methods provide direct understanding of the target processes by quantifying molecular motions and interactions one molecule at a time, said senior author Michael Poirier , professor of physics at The Ohio State University .
“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.
These insights are especially important when studying a protein with so much influence over cellular functions, he said.
“H1 is a key regulator of which genes are being used by a cell and which genes are not. 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.”
First author Ehsan Akbari, a research scientist in Poirier’s lab, led the team’s work. The research was published Sept. 22 in Molecular Cell .
Every cell in a plant, animal or human contains the organism’s entire DNA. In human cells, our 6-foot-long genome must fit inside the nucleus, which is less than 1/10th the width of a human hair. To do this, sections of genomic DNA spool around a collection of histone proteins to form nucleosomes – similarly to how a watering hose might be organized in a backyard.
Nucleosomes then fold in on each other, clumping together to form chromatin . The linker histone H1’s job is to facilitate chromatin compaction, ensuring DNA segments containing genes the cell isn’t 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 of your genes are turned off – you only use a small fraction of your genes for any particular cell, so most of the genes need to be maintained in these compacted states so their expression is turned off.”
Poirier and colleagues used optical tweezer instrumentation and single-molecule fluorescence measurements to visualize H1 interactions with DNA and a nucleosome. They observed that H1 continues to move around even after binding to DNA and, instead of loading onto the nucleosome by way of the DNA, the protein reflects away from the nucleosome.
“It moves around and doesn’t even go to the nucleosome, which I didn’t believe,” Poirier said. That’s when the team, predicting a chaperone protein might be the missing link, added three known linker histone chaperone proteins to the experiments.
“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,” he 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.”
Though this work doesn’t tell the entire H1 story, the step-by-step visualizations offer new hints about where the process could go wrong. Poirier’s lab has previously studied H1 variants and their post-translational modification that can occur – changing H1 targeting and function after it’s 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.
This work was funded by the National Institutes of Health and the U.S. National Science Foundation.
Additional co-authors were Nathaniel Burge of Ohio State and Matti Valdimarsson, Aritra Chowdhury and Benjamin Schuler of the University of Zurich.
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Contact: Michael Poirier, Poirier.18@osu.edu
Written by Emily Caldwell, Caldwell.151@osu.edu
Molecular Cell
Linker histone H1.0 loads onto nucleosomes through multiple pathways that are facilitated by histone chaperones
22-Sep-2026