Nuclear receptor Transactivation domain

Chromatin remodeling, transcription factor modification by various enzyme activities, and the communication between the nuclear receptors and the basal transcription apparatus are accomplished by protein-protein interactions with one or more of a class of coregulator molecules. The number of these coregulator molecules now exceeds 100, not counting species variations and splice variants. The first of these to be described was the CREB-binding protein, CBP. CBP, through an amino terminal domain, binds to phosphorylated serine 137 of CREB and mediates transactivation in response to cAMP. It thus is described as a coactivator. CBP and... 

The participation of the nuclear receptors in the machinery of gene transcription takes place by means of specific domains of the molecule known as transactivators (abbreviation for transcription activators). These are made up of sequences of amino acids that interact by means of protein-protein contacts with other transcription factors. The artificial alteration of these sequences has as a consequence the inability of the hormone to induce gene expression (Beato et al. 1996 Klug et al. 1987 Lones et al. 1995). 

The other transactivator domain, TAF2, is found immersed in the LBD and acts only when the hormone-receptor complex is formed. A sequence of 15 well-conserved amino acids from the different members of the family of nuclear receptors, and situated very close to the carboxyl end of the receptors, participates in it (Gruber et al. 2002 Nilsson et al. 2001). 

A typical nuclear receptor contains a domain responsible for the DNA binding (domain C), the ligand binding and dimerization (E), and for the transactivation and other protein-protein interactions (A,B,E,F). Furthermore, there are also nuclear localization signals (D). 

The Ugand binding domain (section E in fig. 4.5) of the nuclear receptors harbors several functions. Apart from the specific binding site for the hormone, one finds further structural elements in this domain which mediates dimerization of the receptors as well as structural elements important for the ligand-mediated transactivation. 

Dahlman-Wright K, et al. Dehneation of a small region within the major transactivation domain of the human glucocorticoid receptor that mediates transactivation of gene expression. Proc. Natl. Acad. Sci. U.S.A. 1994 91 1619-1623. Hadzopoulou-Cladaras M, et al. Functional domains of the nuclear receptor hepatoc)4e nuclear factor 4. J. Biol. Chem. 1997 272 539-550. 

Jain, S., K.M. Dolwick, J.V. Schmidt and C.A. Bradheld. Potent transactivation domains of the Ah receptor and Ah receptor nuclear translocator map to their carboxyl termini. J. Biol. Chem. 269 31518—31524,1994. 

Fig. 16.13. Steroid hormone receptors. A. Domains of the steroid hormone receptor. The transactivation domain (TAD) binds coactivators DNA-binding domain (DBD) binds to hormone response element in DNA ligand-binding domain (LBD) binds hormone NLS is the nuclear localization signal the dimerization sites are the portions of the protein involved in forming a dimer. The inhibitor binding site binds heat shock proteins and masks the nuclear localization signal. B. Transcriptional regulation by steroid hormone receptors. Additional abbreviations HSP, heat shock proteins GRE, glucocorticoid response element GIZ, glucocorticoid receptor.

It is important in the initial experiments to rule out the possibility that the receptor-bait construct activates reporter-gene expression on its own The nuclear receptors contain activation domains and some function as transactivators in yeast. If the bait construct is a weak activator by itself, it can lead to a number of false positive interactions and should not be used as a two-hybrid bait. 

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