Spatial heterogeneity of dynamics of H1 linker histone.

Linker histone H1 participates in maintaining higher order chromatin structures. It is a dynamic protein that binds to DNA and exchanges rapidly with a mobile pool. Therefore, the dynamics of H1 were probed in the nuclei of intact, live cells, using an array of microscopy techniques: fluorescence recovery after photobleaching (FRAP), raster image correlation spectroscopy (RICS), fluorescence correlation spectroscopy (FCS), pair correlation functions (pCF) and fluorescence anisotropy. Combination of these techniques yielded information on H1 dynamics at small (1-100 μs: FCS, RICS, anisotropy), moderate (1-100 ms: FCS, RICS, pCF) and large (1-100 s: pCF and FRAP) time scales. These results indicate that the global movement of H1 in nuclei (at distances >1 µm) occurs at the time scale of seconds and is determined by processes other than diffusion. Moreover, a fraction of H1, which remains immobile at the time scale of tenths of seconds, is detectable. However, local (at distances <0.7 µm) H1 dynamics comprises a process occurring at a short (~3 ms) time scale and multiple processes occurring at longer (10-2,500 ms) scales. The former (fast) process (corresponding probably to H1 diffusion) is more pronounced in the nuclear regions characterized by low H1 concentration, but the latter (slow, attributable to H1 binding) in the regions of high H1 concentration. Furthermore, some regions in nuclei (possibly containing dense chromatin) may constitute barriers that impair or block movement of H1 histones within short (<1 µm) distances.