Transcriptional Coactivators

Cytokines such as TNFa and IL-1 3, acting via NF-kB, can induce histone acetylation in both a time- and concentration-dependent manner. Upon DNA binding, NF-kB recruits transcriptional coactivators such as CREB binding protein (CBP) and p300/CBP-associated factor (PCAF). 

In the case of liganded NRs, ligand binding is the first and ciucial molecular event that switches the function of these transcription factors from inactive to active state by inducing a conformational change in the LBD of the receptor (Fig. 1). This specific conformation allows the second step of NR activation that corresponds to the recruitment of coregulatoiy complexes, which contain chromatin-modifying enzymes required for transcription. The transcriptional coactivators are very diverse and have expanded to more than hundred in number. These include the pi 60 family of proteins,... 

Coactivators enhancing the transcriptional activity of steroid hormone receptors activators include SRC-1 (steroid-receptor co-activator 1) or TEF2 (transcriptional intermediary factor 2), which are recruited by the DNA/ steroid hormone receptor complex. Their main role is to attract other transcriptional coactivators with histone acetyltransferase activity in order to decondense chromatin and allow for the binding of components of the general transcription apparatus. 

The core unit of the chromatin, the nucleosome, consists of histones arranged as an octamer consisting of a (H3/ H4)2-tetramer complexed with two histone H2A/H2B dimers. Accessibility to DNA-binding proteins (for replication, repair, or transcription) is achieved by posttranslational modifications of the amino-termini of the histones, the histone tails phosphorylation, acetylation, methylation, ubiquitination, and sumoyla-tion. Especially acetylation of histone tails has been linked to transcriptional activation, leading to weakened interaction of the core complexes with DNA and subsequently to decondensation of chromatin. In contrast, deacetylation leads to transcriptional repression. As mentioned above, transcriptional coactivators either possess HAT activity or recruit HATs. HDACs in turn act as corepressors. 

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Fig. 10.2. FSPIM analysis of the interaction between maize transcriptional coactivators—GCN5 and ADA2—fused to CFP and YFP. GCN5 is a histone acetyltransferase that, in conjunction with adaptor protein ADA2, modulates transcription in diverse eukaryotes by affecting the acetylation status of the core histones in nucleosomes [63]. CFP- and YFP-tagged proteins, expressed in protoplasts, were excited by the 458 nm and the 514 nm laser lines sequentially. CFP fluorescence was selectively detected by an FIFT 458 dichroic mirror and BP 470-500 band pass emission filter while YFP fluorescence was selectively detected by using an HFT 514 dichroic mirror and...

The spatial conformation that the ligand-receptor acquires, particularly the spatial disposition that helix 12 of the LBD attains when it binds to estradiol, is key for the subsequent recruitment of the transcription cofactors (Fig. 1.9). Indeed, the arrival of estradiol restructures the entire domain, making helix 12 rotate and close the hole where the leucine zipper sequence of the corepressor had been lodged before (Fig. 1.10). Consequently, both molecules, corepressor and receptor, lose their affinity and their bond is undone. Another structure capable of interacting with gene transcription coactivators is formed at the same place on the receptor (MacGregor et al. 1998 McDonnell et al. 2002). 

Bianghi, F., Denti, S., Granata, A., Bossi, G., Geginat, J., Villa, A., Rogge, L., Pardi, R. Integrin LFA-1 interacts with the transcriptional coactivator JABl to modulate AP-1 activity. Nature 2000, 404, 617-621. 

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