The DNA-binding properties of the ARID-containing subunits of yeast and mammalian SWI/SNF complexes

SWI/SNF complexes are ATP-dependent chromatin remodeling complexes that are highly conserved from yeast to human. From yeast to human the complexes contain a subunit with an ARID (A-T-rich interaction domain) DNA-binding domain. In yeast this subunit is SWI1 and in human there are two closely related alternative subunits, p270 and ARID1B. We describe here a comparison of the DNA-binding properties of the yeast and human SWI/SNF ARID-containing subunits. We have determined that SWI1 is an unusual member of the ARID family in both its ARID sequence and in the fact that its DNA-binding affinity is weaker than that of other ARID family members, including its human counterparts, p270 and ARID1B. Sequence analysis and substitution mutagenesis reveals that the weak DNA-binding affinity of the SWI1 ARID is an intrinsic feature of its sequence, arising from specific variations in the major groove interaction site. In addition, this work confirms the finding that p270 binds DNA without regard to sequence specificity, excluding the possibility that the intrinsic role of the ARID is to recruit SWI/SNF complexes to specific promoter sequences. These results emphasize that care must be taken when comparing yeast and higher eukaryotic SWI/SNF complexes in terms of DNA-binding mechanisms.     

Composition and functional specificity of SWI2/SNF2 class chromatin remodeling complexes

By regulating the structure of chromatin, ATP-dependent chromatin remodeling complexes (remodelers) perform critical functions in the maintenance, transmission and expression of the eukaryotic genome. Although all known chromatin-remodeling complexes contain an ATPase as a central motor subunit, a number of distinct classes have been recognized. Recent studies have emphasized a more extensive functional diversification among closely related chromatin remodeling complexes than previously anticipated. Here, we discuss recent insights in the functional differences between two evolutionary conserved subclasses of SWI/SNF-related chromatin remodeling factors. One subfamily comprises yeast SWI/SNF, fly BAP and mammalian BAF, whereas the other subfamily includes yeast RSC, fly PBAP and mammalian PBAF. We review the subunit composition, conserved protein modules and biological functions of each of these subclasses of SWI/SNF remodelers. In particular, we will focus on the roles of specific subunits in developmental gene control and human diseases. Recent findings suggest that functional diversification among SWI/SNF complexes allows the eukaryotic cell to fine-tune and integrate the execution of diverse biological programs involving the expression, maintenance and duplication of its genome.     

The interactions of yeast SWI/SNF and RSC with the nucleosome before and after chromatin remodeling

Interactions of the yeast chromatin-remodeling complexes SWI/SNF and RSC with nucleosomes were probed using site-specific DNA photoaffinity labeling. 5 S rDNA was engineered with photoreactive nucleotides incorporated at different sites in DNA to scan for the subunits of SWI/SNF in close proximity to DNA when SWI/SNF is bound to the 5 S nucleosome or to the free 5 S rDNA. The Swi2/Snf2 and Snf6 subunits of SWI/SNF were efficiently cross-linked at several positions in the nucleosome, whereas only Snf6 was efficiently cross-linked when SWI/SNF was bound to free DNA. DNA photoaffinity labeling of RSC showed that the Rsc4 subunit is in close proximity to nucleosomal DNA and not when RSC is bound to free DNA. After remodeling, the Swi2/Snf2 and Rsc4 subunits are no longer detected near the nucleosomal DNA and are evidently displaced from the surface of the nucleosome, indicating significant changes in SWI/SNF and RSC contacts with DNA after remodeling.     

Solution structure of SWI1 AT-rich interaction domain from Saccharomyces cerevisiae and its nonspecific binding to DNA

SWI1 is a subunit of the SWI/SNF complex involved in chromatin remodeling. It contains an AT-rich interaction domain (ARID) which has the potential DNA binding activity. In this study, we determined the solution structure of the SWI1 ARID domain from Saccharomyces cerevisiae by nuclear magnetic resonance spectroscopy. Yeast SWI1 ARID domain is composed of seven alpha helices, six of which are conserved among the ARID family. In addition, the DNA-binding activity of the SWI1 ARID domain was confirmed by chemical shift perturbation assay. Similar to its human homolog, the yeast SWI1 ARID domain binds DNA nonspecifically.     

SWI/SNF protein component BAF250a regulates cardiac progenitor cell differentiation by modulating chromatin accessibility during second heart field development

ATP-dependent SWI/SNF chromatin remodeling complexes alter the structure of chromatin at specific loci and facilitate tissue-specific gene regulation during development. Several SWI/SNF subunits are required for cardiogenesis. However, the function and mechanisms of SWI/SNF in mediating cardiac progenitor cell (CPC) differentiation during cardiogenesis are not well understood. Our studies of the SWI/SNF chromatin remodeling complex identified that BAF250a, a regulatory subunit of the SWI/SNF, plays a key role in CPC differentiation. BAF250a ablation in mouse second heart field (SHF) led to trabeculation defects in the right ventricle, ventricular septal defect, persistent truncus arteriosus, reduced myocardial proliferation, and embryonic lethality around E13. Using an embryonic stem cell culture system that models the formation and differentiation of SHF CPCs in vivo, we have shown that BAF250a ablation in CPCs specifically inhibits cardiomyocyte formation. Moreover, BAF250a selectively regulates the expression of key cardiac factors Mef2c, Nkx2.5, and Bmp10 in SHF CPCs. Chromatin immunoprecipitation and DNase I digestion assays indicate that BAF250a regulates gene expression by binding selectively to its target gene promoters and recruiting Brg1, the catalytic subunit of SWI/SNF, to modulate chromatin accessibility. Our results thus identify BAF250a-mediated chromatin remodeling as an essential epigenetic mechanism mediating CPC differentiation.     

Catalytic activity of the yeast SWI/SNF complex on reconstituted nucleosome arrays.

A novel, quantitative nucleosome array assay has been developed that couples the activity of a nucleosome 'remodeling' activity to restriction endonuclease activity. This assay has been used to determine the kinetic parameters of ATP-dependent nucleosome disruption by the yeast SWI/SNF complex. Our results support a catalytic mode of action for SWI/SNF in the absence of nucleosome targeting. In this quantitative assay SWI/SNF and ATP lead to a 100-fold increase in nucleosomal DNA accessibility, and initial rate measurements indicate that the complex can remodel one nucleosome every 4.5 min on an 11mer nucleosome array. In contrast to SWI/SNF action on mononucleosomes, we find that the SWI/SNF remodeling reaction on a nucleosome array is a highly reversible process. This result suggests that recovery from SWI/SNF action involves interactions among nucleosomes. The biophysical properties of model nucleosome arrays, coupled with the ease with which homogeneous arrays can be reconstituted and the DNA accessibility analyzed, makes the described array system generally applicable for functional analysis of other nucleosome remodeling enzymes, including histone acetyltransferases.     

The Drosophila Brahma (SWI/SNF) chromatin remodeling complex exhibits cell-type specific activation and repression functions

The Brahma (Brm) complex of Drosophila melanogaster is a SWI/SNF-related chromatin remodeling complex required to correctly maintain proper states of gene expression through ATP-dependent effects on chromatin structure. The SWI/SNF complexes are comprised of 8-11 stable components, even though the SWI2/SNF2 (BRM, BRG1, hBRM) ATPase subunit alone is partially sufficient to carry out chromatin remodeling in vitro. The remaining subunits are required for stable complex assembly and/or proper promoter targeting in vivo. Our data reveals that SNR1 (SNF5-Related-1), a highly conserved subunit of the Brm complex, is required to restrict complex activity during the development of wing vein and intervein cells, illustrating a functional requirement for SNR1 in modifying whole complex activation functions. Specifically, we found that snr1 and brm exhibited opposite mutant phenotypes in the wing and differential misregulation of genes required for vein and intervein cell development, including rhomboid, decapentaplegic, thick veins, and blistered, suggesting possible regulatory targets for the Brm complex in vivo. Our genetic results suggest a novel mechanism for SWI/SNF-mediated gene repression that relies on the function of a 'core' subunit to block or shield BRM (SWI2/SNF2) activity in specific cells. The SNR1-mediated repression is dependent on cooperation with histone deacetylases (HDAC) and physical associations with NET, a localized vein repressor.