Basis of specificity for a conserved and promiscuous chromatin remodeling protein
This article has been Reviewed by the following groups
Listed in
- Evaluated articles (eLife)
Abstract
Eukaryotic genomes are organized dynamically through the repositioning of nucleosomes. Isw2 is an enzyme that has been previously defined as a genome-wide, nonspecific nucleosome spacing factor. Here, we show that Isw2 instead acts as an obligately targeted nucleosome remodeler in vivo through physical interactions with sequence-specific factors. We demonstrate that Isw2-recruiting factors use small and previously uncharacterized epitopes, which direct Isw2 activity through highly conserved acidic residues in the Isw2 accessory protein Itc1. This interaction orients Isw2 on target nucleosomes, allowing for precise nucleosome positioning at targeted loci. Finally, we show that these critical acidic residues have been lost in the Drosophila lineage, potentially explaining the inconsistently characterized function of Isw2-like proteins. Altogether, these data suggest an ‘interacting barrier model,’ where Isw2 interacts with a sequence-specific factor to accurately and reproducibly position a single, targeted nucleosome to define the precise border of phased chromatin arrays.
Article activity feed
-
-
Reviewer #3:
This manuscript is a detailed analysis of the molecular mechanism for ISW2 recruitment in yeast and delineates not only the binding interface between ISW2 and the transcription factor Ume6, but also finds similar interactions between ISW2 and Swi6. The authors take a systematic and rigorous approach in finding that a 27 amino acid region of Ume6 and the WAC domain of Itc1, accessory subunit in ISW2, are responsible for recruiting ISW2 to Ume6 binding sites. The strength of this paper is that they focus on examining these interactions in vivo and using MNase-seq to show changes in nucleosome positioning upon mutation of Itc1, Ume6 and Swi6. The data is well supported and the conclusions are compelling. In addition, they use the Spytag approach to show these regions alone are capable of recruiting Isw2 to genomic target …
Reviewer #3:
This manuscript is a detailed analysis of the molecular mechanism for ISW2 recruitment in yeast and delineates not only the binding interface between ISW2 and the transcription factor Ume6, but also finds similar interactions between ISW2 and Swi6. The authors take a systematic and rigorous approach in finding that a 27 amino acid region of Ume6 and the WAC domain of Itc1, accessory subunit in ISW2, are responsible for recruiting ISW2 to Ume6 binding sites. The strength of this paper is that they focus on examining these interactions in vivo and using MNase-seq to show changes in nucleosome positioning upon mutation of Itc1, Ume6 and Swi6. The data is well supported and the conclusions are compelling. In addition, they use the Spytag approach to show these regions alone are capable of recruiting Isw2 to genomic target sites. They also show that amino acids 1-73 of Itc1 alone are sufficient for binding to the correct genomics sites and is compelling evidence of their specificity. The authors, by comparing the sequence composition of the WAC domain in ISW2 orthologs from flies to humans, are able to explain a contradiction that has been in this field for a long time about the apparent different role of yeast ISW2 and its Drosophila homolog ACF/ISWI. The Drosophila ISWI complex appears to have a more global role in chromatin organization; whereas yeast ISW2 is more specialized or targeted. The WAC domain in ISWI is defective for recruitment by such transcription factors like Ume6 and Swi6, unlike that observed for ISW2. The other interesting finding or correlation that is derived from their findings is that the recruitment of ISW2 by Ume6 and Swi6 may not only work to recruit ISW2 but may also regulate ISW2 activity as the same region of Itc1 shown to bind to these transcription factors is also shown to regulate the activating function of the H4 tail on Isw2. The paper is well written, clear and nicely organized. I did have one question for the authors, as it seems that this type of recruitment may not be universal as there are only a subset of Ume6 sites that behave as expected in their mutational analysis. Do the authors have any idea why that is the case and what makes this subset of sites behave differently?
-
Reviewer #2:
Chromatin remodelers use the energy derived from ATP hydrolysis to reposition or evict nucleosomes, thus shaping the chromatin landscape of the cell. In this study, the McKnight lab use creative genetic and genomic approaches to understand how the apparently nonspecific biochemical activity of one such chromatin remodeler, Isw2, is targeted to specific nucleosomes in the budding yeast genome. The use of an isw1/chd1 mutant is a nice approach to remove the effects of spacing factors, and the SpyTag/SpyCatcher approach is a novel idea for artificial recruitment of factors. The bottom line of the study is that small, conserved epitopes in transcription factors act as recruiting elements for Isw2, allowing precise targeting of a nonspecific biochemical activity to specific genomic loci. From a larger perspective, the results …
Reviewer #2:
Chromatin remodelers use the energy derived from ATP hydrolysis to reposition or evict nucleosomes, thus shaping the chromatin landscape of the cell. In this study, the McKnight lab use creative genetic and genomic approaches to understand how the apparently nonspecific biochemical activity of one such chromatin remodeler, Isw2, is targeted to specific nucleosomes in the budding yeast genome. The use of an isw1/chd1 mutant is a nice approach to remove the effects of spacing factors, and the SpyTag/SpyCatcher approach is a novel idea for artificial recruitment of factors. The bottom line of the study is that small, conserved epitopes in transcription factors act as recruiting elements for Isw2, allowing precise targeting of a nonspecific biochemical activity to specific genomic loci. From a larger perspective, the results lend support to an interacting barrier model of nucleosome positioning, wherein positioning of specific nucleosomes defines the borders of nucleosomal arrays. The data appear to be of high quality and soundly interpreted, and I believe that the results will be of great interest to those interested in chromatin and transcription. There are many questions raised by the results that I believe will drive further investigation into specificity in chromatin remodeling. My one major criticism (not that major in the scheme of things) is that the authors analyze the interesting subsets of their sites, as detailed below. One example is the analysis of the Isw2/Itc2 co-bound sites to the exclusion of the Isw2-alone sites. I think some exploration of these sites would be warranted, as discussed below.
In Fig. S1C, there is nice correspondence between strong Isw2 K215R binding and Isw2-dependent nucleosome remodeling. However, at PICs where there is no apparent Isw2 remodeling, there does seem to be some Isw2 K215R ChIP-seq signal, albeit at a lower level. Does this potentially represent capture of transient sampling-type interactions, or something else?
In Fig. S1D, Ume6 ChIP (WT and DBD alone) is shown at 202 intergenic Ume6 motifs. It is stated that the rows are linked with Fig. 1B - it would be nice to see the nucleosome data next to the ChIP data in this panel, as it appears that Ume6 is bound to at some level to the majority of these 202 sites, while Isw2 seems only to be active at the 58 sites of cluster 1. Germane to this point, I of course understand why the authors focused on the cluster 1 sites, but it would be nice to have some speculation on why Isw2 only seems to function at a fraction of Ume6-bound loci. Also, the lengths of the cluster-denoting bars appear to be off here relative to Fig. 1B.
In Fig. 5C, it appears that only a subset of Isw2 sites are bound by Itc1 as well. Again, as with the selection of the 58 Ume6 sites, I understand why the Isw2/Itc1 co-bound sites are selected for further analysis, but the Isw2 sites without Itc1 could be discussed as well. Are these sites non-functional? How does Itc1 ChIP-seq data compare to the Isw2 remodeling activity shown in Fig. 1A? How does it compare to Ume6 binding? Does it specify the Isw2-remodeled nucleosomes?
Did the authors perform western blots to ensure that their various truncation constructs were stable? This is important for interpretation of the results vs deletions.
To summarize the above points, a major thing missing from the discussion is why only subsets of TF binding sites recruit Isw2. For instance, as mentioned above, 58 Ume6 sites seem to specific Isw2 remodeling - what is special about those sites versus the other ~150 sites that appear to be bound by Ume6? It's mentioned briefly in the discussion that only three Swi6 sites were identified as Isw2-recruiting and that this may be tuned by cellular context, but this is quite vague and superficial. More speculation on what differentiates these sites from the TF-bound but non-Isw2 recruiting sites could be included.
-
Reviewer #1 (Jerry Workman):
This is a paradigm shifting study which demonstrates targeting of the Isw2 complex by a sequence-specific DNA binding protein Ume6. Previously the Isw2 complex was thought to be a promiscuous nucleosome sliding ATPase that would globally space nucleosomes like Chd1 or Isw1. However, the current study demonstrates the Isw2 primarily targets a single nucleosome adjacent to Ume6 binding sites.
-
