Cedric R. Clapier

The biology of chromatin remodeling complexes.

The packaging of chromosomal DNA by nucleosomes condenses and organizes the genome, but occludes many regulatory DNA elements. However, this constraint also allows nucleosomes and other chromatin components to actively participate in the regulation of transcription, chromosome segregation, DNA replication, and DNA repair. To enable dynamic access to packaged DNA and to tailor nucleosome composition in chromosomal regions, cells have evolved a set of specialized chromatin remodeling complexes (remodelers). Remodelers use the energy of ATP hydrolysis to move, destabilize, eject, or restructure nucleosomes. Here, we address many aspects of remodeler biology: their targeting, mechanism, regulation, shared and unique properties, and specialization for particular biological processes. We also address roles for remodelers in development, cancer, and human syndromes.

ISWI Is an ATP-Dependent Nucleosome Remodeling Factor

The ATPase ISWI is a subunit of several distinct nucleosome remodeling complexes that increase the accessibility of DNA in chromatin. We found that the isolated ISWI protein itself was able to carry out nucleosome remodeling, nucleosome rearrangement, and chromatin assembly reactions. The ATPase activity of ISWI was stimulated by nucleosomes but not by free DNA or free histones, indicating that ISWI recognizes a specific structural feature of nucleosomes. Nucleosome remodeling, therefore, does not require a functional interaction between ISWI and the other subunits of ISWI complexes. The role of proteins associated with ISWI may be to regulate the activity of the remodeling engine or to define the physiological context within which a nucleosome remodeling reaction occurs.

Modulation of ISWI function by site-specific histone acetylation.

Mutations in Drosophila ISWI, a member of the SWI2/SNF2 family of chromatin remodeling ATPases, alter the global architecture of the male X chromosome. The transcription of genes on this chromosome is increased 2‐fold relative to females due to dosage compensation, a process involving the acetylation of histone H4 at lysine 16 (H4K16). Here we show that blocking H4K16 acetylation suppresses the X chromosome defects resulting from loss of ISWI function in males. In contrast, the forced acetylation of H4K16 in ISWI mutant females causes X chromosome defects indistinguishable from those seen in ISWI mutant males. Increased expression of MOF, the histone acetyltransferase that acetylates H4K16, strongly enhances phenotypes resulting from the partial loss of ISWI function. Peptide competition assays revealed that H4K16 acetylation reduces the ability of ISWI to interact productively with its substrate. These findings suggest that H4K16 acetylation directly counteracts chromatin compaction mediated by the ISWI ATPase.

Crystal Structure and Functional Analysis of a Nucleosome Recognition Module of the Remodeling Factor ISWI

Energy-dependent nucleosome remodeling emerges as a key process endowing chromatin with dynamic properties. However, the principles by which remodeling ATPases interact with their nucleosome substrate to alter histone-DNA interactions are only poorly understood. We have identified a substrate recognition domain in the C-terminal half of the remodeling ATPase ISWI and determined its structure by X-ray crystallography. The structure comprises three domains, a four-helix domain with a novel fold and two alpha-helical domains related to the modules of c-Myb, SANT and SLIDE, which are linked by a long helix. An integrated structural and functional analysis of these domains provides insight into how ISWI interacts with the nucleosomal substrate.

Critical Role for the Histone H4 N Terminus in Nucleosome Remodeling by ISWI

ABSTRACT The ATPase ISWI can be considered the catalytic core of several multiprotein nucleosome remodeling machines. Alone or in the context of nucleosome remodeling factor, the chromatin accessibility complex (CHRAC), or ACF, ISWI catalyzes a number of ATP-dependent transitions of chromatin structure that are currently best explained by its ability to induce nucleosome sliding. In addition, ISWI can function as a nucleosome spacing factor during chromatin assembly, where it will trigger the ordering of newly assembled nucleosomes into regular arrays. Both nucleosome remodeling and nucleosome spacing reactions are mechanistically unexplained. As a step toward defining the interaction of ISWI with its substrate during nucleosome remodeling and chromatin assembly we generated a set of nucleosomes lacking individual histone N termini from recombinant histones. We found the conserved N termini (the N-terminal tails) of histone H4 essential to stimulate ISWI ATPase activity, in contrast to other histone tails. Remarkably, the H4 N terminus, but none of the other tails, was critical for CHRAC-induced nucleosome sliding and for the generation of regularity in nucleosomal arrays by ISWI. Direct nucleosome binding studies did not reflect a dependence on the H4 tail for ISWI-nucleosome interactions. We conclude that the H4 tail is critically required for nucleosome remodeling and spacing at a step subsequent to interaction with the substrate.

dMi‐2 and ISWI chromatin remodelling factors have distinct nucleosome binding and mobilization properties

Mi‐2 and ISWI, two members of the Snf2 superfamily of ATPases, reside in separate ATP‐dependent chromatin remodelling complexes. These complexes differ in their biochemical properties and are believed to perform distinct functions in the cell. We have compared the remodelling activity of recombinant Drosophila Mi‐2 (dMi‐2) with that of recombinant ISWI. Both proteins are nucleosome‐stimulated ATPases and promote nucleosome mobilization. However, dMi‐2 and ISWI differ in their interaction with nucleosome core particles, in their substrate requirements and in the direction of nucleosome mobilization. We have used antibodies to immobilize a complex containing dMi‐2 and the dRPD3 histone deacetylase from Drosophila embryo extracts. This complex shares the nucleosome‐stimulated ATPase and nucleosome mobilization properties of recombinant dMi‐2, demonstrating that these activities are maintained in a physiological context. Its functional properties distinguish dMi‐2 from both SWI2/SNF2 and ISWI, defining a new family of ATP‐dependent remodelling machines.

Regulation of ISWI involves inhibitory modules antagonized by nucleosomal epitopes

Chromatin-remodelling complexes (CRCs) mobilize nucleosomes to mediate the access of DNA-binding factors to their sites in vivo. These CRCs contain a catalytic subunit that bears an ATPase/DNA-translocase domain and flanking regions that bind nucleosomal epitopes. A central question is whether and how these flanking regions regulate ATP hydrolysis or the coupling of hydrolysis to DNA translocation, to affect nucleosome-sliding efficiency. ISWI-family CRCs contain the protein ISWI, which uses its ATPase/DNA-translocase domain to pump DNA around the histone octamer to enable sliding. ISWI is positively regulated by two ‘activating’ nucleosomal epitopes: the ‘basic patch’ on the histone H4 tail, and extranucleosomal (linker) DNA. Previous work defined the HAND-SANT-SLIDE (HSS) domain at the ISWI carboxy terminus that binds linker DNA, needed for ISWI activity. Here we define two new, conserved and separate regulatory regions on Drosophila ISWI, termed AutoN and NegC, which negatively regulate ATP hydrolysis (AutoN) or the coupling of ATP hydrolysis to productive DNA translocation (NegC). The two aforementioned nucleosomal epitopes promote remodelling indirectly by preventing the negative regulation of AutoN and NegC. Notably, mutation or removal of AutoN and NegC enables marked nucleosome sliding without the H4 basic patch or extranucleosomal DNA, or the HSS domain, conferring on ISWI the biochemical attributes normally associated with SWI/SNF-family ATPases. Thus, the ISWI ATPase catalytic core is an intrinsically active DNA translocase that conducts nucleosome sliding, onto which selective ‘inhibition-of-inhibition’ modules are placed, to help ensure that remodelling occurs only in the presence of proper nucleosomal epitopes. This supports a general concept for the specialization of chromatin-remodelling ATPases, in which specific regulatory modules adapt an ancient active DNA translocase to conduct particular tasks only on the appropriate chromatin landscape.

The RSC chromatin remodelling ATPase translocates DNA with high force and small step size

ATP‐dependent chromatin remodelling complexes use the energy of ATP hydrolysis to reposition and reconfigure nucleosomes. Despite their diverse functions, all remodellers share highly conserved ATPase domains, many shown to translocate DNA. Understanding remodelling requires biophysical knowledge of the DNA translocation process: how the ATPase moves DNA and generates force, and how translocation and force generation are coupled on nucleosomes. Here, we characterize the real‐time activity of a minimal RSC translocase ‘motor’ on bare DNA, using high‐resolution optical tweezers and a ‘tethered’ translocase system. We observe on dsDNA a processivity of ∼35 bp, a speed of ∼25 bp/s, and a step size of 2.0 (±0.4, s.e.m.) bp. Surprisingly, the motor is capable of moving against high force, up to 30 pN, making it one of the most force‐resistant motors known. We also provide evidence for DNA ‘buckling’ at initiation. These observations reveal the ATPase as a powerful DNA translocating motor capable of disrupting DNA–histone interactions by mechanical force.

Structure of the Drosophila nucleosome core particle highlights evolutionary constraints on the H2A-H2B histone dimer

We determined the 2.45 Å crystal structure of the nucleosome core particle from Drosophila melanogaster and compared it to that of Xenopus laevis bound to the identical 147 base‐pair DNA fragment derived from human α‐satellite DNA. Differences between the two structures primarily reflect 16 amino acid substitutions between species, 15 of which are in histones H2A and H2B. Four of these involve histone tail residues, resulting in subtly altered protein–DNA interactions that exemplify the structural plasticity of these tails. Of the 12 substitutions occurring within the histone core regions, five involve small, solvent‐exposed residues not involved in intraparticle interactions. The remaining seven involve buried hydrophobic residues, and appear to have coevolved so as to preserve the volume of side chains within the H2A hydrophobic core and H2A‐H2B dimer interface. Thus, apart from variations in the histone tails, amino acid substitutions that differentiate Drosophila from Xenopus histones occur in mutually compensatory combinations. This highlights the tight evolutionary constraints exerted on histones since the vertebrate and invertebrate lineages diverged. Proteins 2008.

Chromatin Remodeling Complexes

The packaging of chromosomal DNA by nucleosomes not only condenses and organizes the genome but also blocks access of DNA-binding proteins to regulatory DNA elements. However, this process also allows nucleosomes and other chromatin components to actively participate in the regulation of transcription, DNA replication, DNA repair, and other chromosomal processes. To enable dynamic regulation of nucleosomes, cells have evolved a set of specialized chromatin remodeling complexes (termed Remodelers) that use the energy of ATP hydrolysis to slide, destabilize, eject, or edit/restructure nucleosomes. Here, we address many aspects of Remodeler function, including their composition, specialization, mechanisms, and regulation. We also address their roles in embryonic stem cells, differentiation, development, cancer, and human disease syndromes.