The mammalian SWI/SNF complex is an evolutionarily conserved ATP-dependent chromatin remodeling complex that consists of nine or more components. SRG3, a murine homologue of yeast SWI3, Drosophila MOIRA, and human BAF155, is a core component of the murine SWI/SNF complex required for the regulation of transcriptional processes associated with development, cellular differentiation, and proliferation. Here we report that SRG3 interacts directly with other components of the mammalian SWI/SNF complex such as SNF5, BRG1, and BAF60a. The SWIRM domain and the SANT domain were required for SRG3-SNF5 and SRG3-BRG1 interactions, respectively. In addition, SRG3 stabilized SNF5, BRG1, and BAF60a by attenuating their proteasomal degradation, suggesting its general role in the stabilization of the SWI/SNF complex. Such a stabilization effect of SRG3 was not only observed in the in vitro cell system, but also in cells isolated from SRG3 transgenic mice or knock-out mice haploinsufficient for the Srg3 gene. Taken together, these results suggest the critical role of SRG3 in the post-transcriptional stabilization of the major components of the SWI/SNF complex.The mammalian SWI/SNF complexes are evolutionarily conserved ATP-dependent chromatin remodeling complexes, which use the energy of ATP hydrolysis to mobilize nucleosomes and remodel chromatin structure (1, 2). These complexes play important roles in transcriptional regulation, thereby controlling diverse cellular processes including proliferation, differentiation, cell death, and tumorigenesis (3-6). The mammalian SWI/SNF complexes are multisubunit complexes that consist of invariant core components and variable components (7). The subunit diversity of mammalian SWI/ SNF complexes suggests that different complexes might have tissue-specific roles during development (8). The core components of the mammalian SWI/SNF complexes are BRG1 or hBRM, SNF5/INI1/BAF47, BAF155/SRG3, and BAF170 (9). BRG1 and BRM are DNA-dependent ATPase homologous to yeast SWI2/SNF2. Biochemical experiments have shown that although BRG1 or BRM alone can remodel nucleosomal arrays, the addition of other core components (BAF155, BAF170, and SNF5) to BRG1 stimulates the remodeling activity of BRG1 at a rate that is comparable with the entire complex in vitro (10).Human SNF5 was initially identified by the yeast two-hybrid system through its interaction with human immunodeficiency virus type 1 integrase (11). It was shown that human SNF5 interacts with c-Myc, thereby enhancing c-Myc-mediated transactivation by recruiting the SWI/SNF complex to the E-box (12). Furthermore, human SNF5 is known as a tumor suppressor in atypical teratoid and malignant rhabdoid tumors and the majority of these tumors have deletion or point mutations in SNF5 leading to disruption of normal function of SNF5 (13,14).Srg3 (Swi3-related gene), a murine homologue of yeast Swi3, Drosophila Moira, and human Baf155, was initially isolated as a gene highly expressed in the thymus but at a low level in the peripheral lymphoid organ (15). It...

The process of thymocyte development requires an exquisite regulation of many genes via transcription factors and chromatin remodeling activities. Even though the SWI/SNF chromatin remodeling complex has been thought to play important roles during thymocyte development, its known function is very limited. In this study, we show that the SWI/SNF chromatin remodeling activity is finely regulated during thymocyte maturation process, especially during thymocyte selections. We found that TCR signaling directly down-regulates mBRG1 and SWI3-related gene, the core components of murine SWI/SNF complex, during thymocyte maturation. Constitutive expression of SWI3-related gene in developing thymocytes attenuated the down-regulation of the SWI/SNF complex and resulted in a change in the expression of genes such as linker for activation of T cells and casitas B lineage lymphoma, which affected the TCR-mediated intracellular signaling pathway. The defects in TCR signaling resulted in the disruption of both positive and negative selections in specific TCR transgenic mice systems. Our results state, for the first time, that the chromatin remodeling activity needs to be finely controlled for proper thymocyte selection and maturation processes.

Nitric oxide (NO) plays many roles in the immune system. It has been known that NO rescues thymocytes from glucocorticoid (GC)-induced apoptosis. However, the downstream target of NO in the protection from GC-induced thymocyte apoptosis has yet to be identified. We previously reported that GC sensitivity of developing thymocytes is dependent on the expression level of SRG3. In the present report, we found that NO repressed the SRG3 expression in both primary thymocytes and 16610D9 thymoma cells. Specifically, NO down-regulated the transcription of SRG3 via the inactivation of the transcription factor Sp1 DNA-binding activity to the SRG3 promoter. In addition, overexpression of SRG3 by a heterologous promoter reduced NO-mediated rescue of thymocytes from GC-induced apoptosis. These observations strongly suggest that NO may be involved in protecting immature thymocytes from GC-induced apoptosis by repressing the SRG3 expression in thymus.