The ATP-dependent chromatin-remodeling complexes play important roles in gene regulation by opening chromatin structures for transcriptional activators or repressors. The prototype of this type of complexes is the SWI/SNF complex, which was found from diverse organisms, including yeast, Drosophila, mouse and human. It is required for proper expression of homeotic genes and segmentation in Drosophila, and mutation in one subunit of the complex causes pediatric rhabdoid cancer in human. I have purified several human SWI/SNF-related complexes. By microsequencing, I have identified and cloned most of the subunits from the major form of the complex. We have microsequenced and cloned the largest subunit, BAF250. Sequence analysis revealed that BAF250 contains a DNA binding domain similar to yeast SWI1, and several LXXLL motifs which have been previously shown to be able to interact with nuclear hormone receptors. Using transient transfection assays, we found that BAF250 infact facilitates transcriptional activation by glucocorticoid receptor (GR). The region containing LXXLL motifs of BAF250 also interacts with GR in vitro. This work suggests that BAF250 may be a targeting subunit of hSWI/SNF, and may mediate the recruitment of the complex to DNA-bound glucocorticoid receptors. This work was recently published in MCB (Nie et al., Vol 20:8879, 2000). Currently, we have cloned a novel human homolog of BAF250a, termed BAF250b. The two genes share over 60% of identity and possess same type of domain structure. Importantly, while BAF250a is ubiquitously expressed at relatively constant levels in different tissues, BAF250b displays tissue-restrictive expression patterns, suggesting that BAF250b could be part of a tissue-restricted chromatin-remodeling complex. We have now isolated the BAF250b-containing complex. It shares several identical subunits with BAF250a complex but also contains its own unique components. Identification of these unique subunits hints that this complex could be involved in leukemia. We are continuing to investigate the structure and function of this new complex.
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