Srg3 (SWI3-related gene product) is a mouse homolog of yeast SWI3, Drosophila melanogaster MOIRA (also named MOR/BAP155), and human BAF155 and is known as a core subunit of SWI/SNF complex. This complex is involved in the chromatin remodeling required for the regulation of transcriptional processes associated with development, cellular differentiation, and proliferation. We generated mice with a null mutation in the Srg3 locus to examine its function in vivo. Homozygous mutants develop in the early implantation stage but undergo rapid degeneration thereafter. An in vitro outgrowth study revealed that mutant blastocysts hatch, adhere, and form a layer of trophoblast giant cells, but the inner cell mass degenerates after prolonged culture. Interestingly, about 20% of heterozygous mutant embryos display defects in brain development with abnormal organization of the brain, a condition known as exencephaly. Histological examination suggests that exencephaly is caused by the failure in neural fold elevation, resulting in severe brain malformation. Our findings demonstrate that Srg3 is essential for early embryogenesis and plays an important role in the brain development of mice.

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 functional activity of integrins is dynamically regulated by T cell receptor stimulation and by protein kinase C (PKC). We report a novel function for the PKC effector protein kinase D1 (PKD1) in integrin activation. Constitutively active and kinase-inactive PKD1 mutants lacking the PKD1 pleckstrin homology (PH) domain block phorbol ester- and TCR-mediated activation and clustering of beta1 integrins. The PH domain of PKD1 mediates the association of PKD1 with the GTPase Rap1 and is central to Rap1 activation and membrane translocation in T cells. Furthermore, PKD1 and Rap1 associate with beta1 integrins in a manner that is dependent on the carboxy-terminal end of the beta1 integrin subunit cytoplasmic domain. beta1 integrin expression is required for Rap1 activation and membrane localization of the PKD1-Rap1 complex. Therefore, PKD1 promotes integrin activation in T cells by regulating Rap1 activation and membrane translocation via interactions with the beta1 integrin subunit cytoplasmic domain.

The cellular location and substrate specificity of the catalytic subunit (C) of protein phosphatase 2A (PP2A) depend on its interaction with A and B subunits. The distribution of epitope-tagged wild-type or mutated C subunits was studied by transient expression in COS-7 cells. Wild-type tagged C expressed at low levels formed ABC trimer and AC dimer like the endogenous C. Single mutations of C at the site of phosphorylation (Y307F) or carboxymethylation (L309Q) resulted in recovery of only AC dimer. Double mutation of both residues resulted in association of C with alpha 4 protein (alpha 4), a novel subunit of PP2A, instead of with A and B subunits. Thus, the distribution of C between ABC trimer, AC dimer, and alpha 4C complexes can be affected by modifications of the C-terminal residues. The alpha 4 protein is a homologue of the yeast Tap42 protein that functions downstream of the TOR protein to regulate protein synthesis. Transient overexpression of FLAG-alpha 4 resulted in increased dephosphorylation of elongation factor 2, but had no effect on phosphorylation of either p70S6 kinase or PHAS-I (eIF4E-BP). Signals that affect phosphorylation or methylation of the C subunit of PP2A may promote subunit exchange and direct phosphatase activity to specific intracellular substrates.