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Distinct chromatin remodeling complexes can share a common ATPase subunit. The functional characteristics of each remodeling complex are determined by the respective ATPase-associated subunits. The Mi-2 nucleosome remodeling ATPase has so far only been shown to reside within Nucleosome Remodeling and Deacetylase (NuRD) complexes. Here we will review the recent discovery of two Mi-2 related remodelers that function independently of NuRD and that act as SUMO (small ubiquitin-related modifier)-dependent corepressors: First, Mi-2 exists in a novel chromatin remodeling complex, dMec, that does not rely on histone deacetylation to effect transcriptional repression of proneural genes. Second, the Mi-2 related factor dCHD3 acts as a monomer and does not associate with additional subunits in vivo. These recent results have uncovered an unanticipated complexity in the composition and function of CHD (Chromodomain-Helicase-DNA-binding) complexes.

The ATP-dependent chromatin remodeller Mi-2 functions as a transcriptional repressor and contributes to the suppression of cell fates during development in several model organisms. Mi-2 is the ATPase subunit of the conserved Nucleosome Remodeling and Deacetylation (NuRD) complex, and transcriptional repression by Mi-2 is thought to be dependent on its associated histone deacetylase. Here, we have purified a novel dMi-2 complex from Drosophila that is distinct from dNuRD. dMec (dMEP-1 complex) is composed of dMi-2 and dMEP-1. dMec is a nucleosome-stimulated ATPase that is expressed in embryos, larval tissues and adult flies. Surprisingly, dMec is far more abundant than dNuRD and constitutes the major dMi-2-containing complex. Both dNuRD and dMec associate with proneural genes of the achaete-scute complex. However, despite lacking a histone deacetylase subunit, only dMec contributes to the repression of proneural genes. These results reveal an unexpected complexity in the composition and function of Mi-2 complexes.