| Research Abstract |
In this study, we examined the molecular mechanisms underlying phenomena that transcription factor AP-1 selectively transactivates target genes in response to each stimulus. Our current study describes the novel functions of Brm and BRG1, catalytic subunits of the SWI/SNF chromatin remodeling complex, as both transactivators of one of AP-1 target gene, the telomerase reverse transcriptase (TERT), and as accelerators of exon inclusion in the transcripts of TERT gene, in concert with p54^<nrb>. We show that a DBHS (for Drosophila behavior, human splicing) family protein, p54^<nrb>, directly binds both BRG1 and Brm, as well as an additional core subunit of this complex, BAF60a, and provide evidence that p54^<nrb> binds SWI/SNE-like complexes.
Polypyrimidine tract-binding protein (PTB)-associated splicing factor (PSF), another DBHS family protein known to directly bind p54^<nrb>, was also found to be associated with the SWI/SNE-like complex. p54^<nrb> and PSF are multifunctional nuclear pro
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teins involved in transcriptional control, pre-mRNA splicing, the nuclear retention of 'A to I' -edited RNAs, and DNA recombination. Moreover, when a BRG1-deficient human cell line was transfected with retroviral vectors harboring shRNAs that target Brm, the isolated knocked-down clones show reduced expression levels of TERT. This reduction accompanies the enhancement of exclusion of exons 7 and 8 in the TERT transcripts, which leads to the dominant production of a β-site-deleted inactive protein. These Brm-deficient clones also display telomere shortening and growth arrest within two months of the knockdown.
In the TERT gene locus in human tumor cells, Brm, and probably BRG1 also, in concert with p54^<nrb> initiates efficient gene transcription, and these factors are also involved in the subsequent splicing of the TENT transcripts via the acceleration of exon-inclusion. This contributes to the maintenance of active telomerase in human tumor cells.
We discuss the significance of the assembly of p54^<nrb>, PSF and Brm for the promotion of exon inclusion, and speculate about the possible underlying mechanisms that modulating this process. Since p54^<nrb> co-localizes with Brm at the TERT promoter, we believe that our current findings provide significant mechanistic insights into novel functions of the SWI/SNE (or -like) complexes, which appear to dynamically alter their composition throughout transcription and during post-transcriptional processes. Less
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