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Identification of Novel Domains within the Ioc2 and Ioc3 Subunits of the ISW1 Chromatin Remodeling Complex That Explicitly Distinguishes the Catalytic Activity of ISW1a and ISW1b

In S.cerevisiae, ISW1 forms two distinct chromatin remodeling complexes, ISW1a and ISW1b. ISW1a is composed of Isw1 and Ioc3 while, ISW1b is comprised of Isw1, Ioc2, and Ioc4. Although these complexes share the same ATPase subunit, both exhibit catalytic activity that is distinctive. ISW1a remodels with directionality preferences, spaces nucleosomes, and adapts active/inactive conformational changes dependent on linker DNA length. Meanwhile, ISW1b has substrate preference for short nucleosome arrays, lacks directional remodeling and spacing activity. The differences exhibited in these complexes are attributed to the associated subunits of each complex. Ioc2 has an atypical PHD motif that is often found in chromatin associated proteins and Ioc4 has a PWWP motif, a putative DNA binding domain. Ioc3 has no conserved domains that have been previously characterized, neither does it bear any similarity to proteins which their function has been defined. No work has been done to identify the domain organization or the contribution of these subunits to the activity of the ISW1 complexes. The goals of this project was to characterize the domain architecture of the Ioc2 and Ioc3 subunits of the ISW1 chromatin remodeling complexes, and also identify the key defining features of these two complexes. Using mutational analysis, this work explicitly identifies regions within the subunits that are required for the maintenance of complex integrity, modulation of nucleosome interaction, directional remodeling, and spacing activity. Further characterization of these domains revealed that their deletion results in remodeling and spacing activity reminiscent of its sister complex, suggesting that these domains serve as the distinguishing features that define the ISW1a and ISW1b complexes. This work was supported by Public Health Service grant GM 70864 from the National Institute of Health.