Dissecting the binding mechanism of the linker histone in live cells: an integrated FRAP analysis.

The linker histone H1 has a fundamental role in DNA compaction. Although models for H1 binding generally involve the H1 C-terminal tail and sites S1 and S2 within the H1 globular domain, there is debate about the importance of these binding regions and almost nothing is known about how they work together. Using a novel fluorescence recovery after photobleaching (FRAP) procedure, we have measured the affinities of these regions individually, in pairs, and in the full molecule to demonstrate for the first time that binding among several combinations is cooperative in live cells. Our analysis reveals two preferred H1 binding pathways and we find evidence for a novel conformational change required by both. These results paint a complex, highly dynamic picture of H1-chromatin binding, with a significant fraction of H1 molecules only partially bound in metastable states that can be readily competed against. We anticipate the methods we have developed here will be broadly applicable, particularly for deciphering the binding kinetics of other nuclear proteins that, similar to H1, interact with and modify chromatin.