Why H1 matters

Here is the problem H1 solves. Each human cell contains a genome roughly 6 feet (1.8 meters) long, yet it must fit inside a nucleus less than one-tenth the width of a human hair. To manage this, DNA wraps around clusters of histone proteins, forming nucleosomes—much like a garden hose coiled for storage. The nucleosomes then fold together into a denser structure called chromatin.

H1's task is to push this compaction further, sealing off DNA regions the cell does not currently need. "H1 is a key regulator of which genes are being used by a cell and which genes are not," Poirier explained. Regions destined for compaction carry a lot of H1, while more open regions carry little. Since most genes in any given cell stay silent, H1 must maintain vast stretches of the genome in a closed state.

"If you want to understand how disease develops because gene expression is no longer working properly, then you need to understand how H1 works," Poirier said. "Once you understand this, then you open up the possibility for new therapies that counteract this cause of disease."

An unexpected detour

The surprise came when the researchers watched H1 in action. Instead of traveling along the protruding DNA strands to reach the nucleosome, as many had assumed, H1 simply bounced off its target.

"It moves around and doesn't even go to the nucleosome, which I didn't believe," Poirier said.

Suspecting a missing helper, the team added three known linker histone chaperone proteins to their experiments. The chaperones changed everything. They regulated how H1 moves, how it finds nucleosomes, and how it loads onto them.

"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."

What comes next

The study does not tell the complete story of H1 biology. The experiments were conducted in controlled single-molecule setups, and the findings describe the H1.0 variant specifically. Still, the step-by-step visualizations give researchers a framework for probing where the process breaks down.

Poirier's lab has previously studied H1 variants and post-translational modifications—chemical changes made to the protein after the cell manufactures it—that alter H1's targeting and function. "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.

Additional co-authors include Nathaniel Burge of Ohio State and Matti Valdimarsson, Aritra Chowdhury and Benjamin Schuler of the University of Zurich.