The eukaryotic genome is folded inside the cell in the form of nucleosomes in order to pack the ∼2 m-long DNA into a ∼10 µm-sized nucleus. Each nucleosome consists of a core comprised of 147 bp of DNA wrapped around a histone octamer, characterized by a pseudo-symmetric dyad with 15 superhelical locations (SHLs) distributed symmetrically on both sides. While such packaging provides spatial economy and offers a mean for modulating gene activation or silencing, it also poses an obstacle when access to the underlying DNA is needed for other processes such as transcription, replication, DNA repair, etc. Such obstacle is alleviated inside the cell through chromatin remodeling, a crucial process performed by a family of protein complexes known as chromatin remodelers (Clapier et al., 2017). Divided into four major subfamilies (SWI/SNF, ISWI, CHD and INO80), these multi-subunit remodeler complexes share a homologous ATPase domain responsible for remodeling activity, but each contains distinct regulatory domains (e.g., bromodomain (BD), HAND-SANT-SLIDE (HSS) domain and DNA-binding domain (DBD)) for interacting with various features on the nucleosome (e.g., acidic patches and histone tail modifications), as well as non-catalytic subunits (with the exception of CHD remodelers) harboring distinct properties. These complexes carry out diverse context-specific activities, including DNA unwrapping, nucleosome repositioning/eviction, and histone exchange/ejection.
In recent years, a suite of quantitative imaging approaches, both in vitro and in cellulo, has been increasingly employed to probe this crucial process across different scales, ranging from elucidating the molecular choreography of remodeling at nanoscale to revealing their intranuclear organization and dynamics in single living cells. In this In Focus review, we survey key works over the past decade along both of these directions, paying particular attention to the novel mechanistic insights and previously unknown physical parameters they have revealed, as well as their implications for our understanding of the overarching principles that govern the actions of diverse remodeler subfamilies.