Rapid exchange of histone H1.1 on chromatin in living human cells

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Abstract
The considerable length of DNA in eukaryotic genomes requires packaging into chromatin to fit inside the small dimensions of the cell nucleus. Histone H1 functions in the compaction of chromatin into higher order structures derived from the repeating ‘beads on a string’ nucleosome polymer. Modulation of H1 binding activity is thought to be an important step in the potentiation/depotentiation of chromatin structure for transcription1,2,3,4. It is generally accepted that H1 binds less tightly than other histones to DNA in chromatin and can readily exchange in living cells5,6,7,8. Fusion proteins of Histone H1 and green fluorescent protein (GFP) have been shown9 to associate with chromatin in an apparently identical fashion to native histone H1. This provides a means by which to study histone H1–chromatin interactions in living cells. Here we have used human cells with a stably integrated H1.1–GFP fusion protein to monitor histone H1 movement directly by fluorescence recovery after photobleaching in living cells. We find that exchange is rapid in both condensed and decondensed chromatin, occurs throughout the cell cycle, and does not require fibre–fibre interactions. Treatment with drugs that alter protein phosphorylation significantly reduces exchange rates. Our results show that histone H1 exchange in vivo is rapid, occurs through a soluble intermediate, and is modulated by the phosphorylation of a protein or proteins as yet to be determined.