DNA Binding and Transactivation

Details of the activation and transport into the nucleus remain unknown. It is assumed that the complex between receptor and hormone and the loosely associated heat shock proteins reaches the nucleus via a protein transport system. After dissociation of the heat shock proteins, the hormone-receptor complex is capable of specific binding on the HRE and of transactivation. 

The chaperones are used as tools in this system for regulation of activity of the steroid hormone receptors. The function of the chaperones is obviously to fix the receptor in a conformation which allows high affinity binding to the hormone and the subsequent steps of specific DNA binding and transactivation. For the steroid hormone receptors this means that they must exist in functionally different conformations. It may be a function of the chaperones to stabilize the particular conformation optimal for hormone binding. 

The HREs are components of the control region of hormone-regulated promotors. Typically, the binding of the activated, hormone-bound receptor to the HRE stimulates the expression of the cognate genes. The mechanisms of transcription activation still remains speculative. Many proteins have been identified which act as coactivators, i.e. they interact with the activated receptor (review Glass et al., 1997). The nature of these proteins suggest that at least two, separate but not mutually exclusive pathways of transcription activation must be considered  

Due to the complex structure of the initiation complex it remains imclear which interactions are responsible for the first mechanism. The coupling between the transactivat-ing domain and the initiation complex can be direct or indirect. There is evidence which indicates that proteins with co-activator function mediate the interaction between HRE-bound receptors and the transcription initiation apparatus. One such protein is RIP-140, which mediates the transcription activation of the estrogen receptor. The AF2 domain can also directly contact the transcriptional apparatus. One component of the RNA polymerase II holoenzyme, the SUGl protein, has been identified as a binding partner for the AF2 domain. The SUGl protein has the function of a co-acti-vator in transcription initiation and is considered a mediator (see 1.4.3.2). 

A second mechanism by which transcriptional activation occurs via remodeling of the chromatin. Steroid receptors, in cooperation with other proteins, can induce a 

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Kim MY, Woo EM, Chong YT, Homenko DR, Kraus WL (2006) Acetylation of estrogen receptor alpha by p300 at lysines 266 and 268 enhances the DNA binding and transactivation activities of the receptor. Mol Endocrinol... 

The steroid hormone receptors are phophoproteins which are usually phosphorylated on several positions. The phosphorylation sites are mainly foimd in the N-terminal region of the receptors. Serine phosphorylation prevails. One rare example of tyrosine phosphorylation is described for the case of estrogen receptors. The consequences of phosphorylation for the receptor proteins are varied. It is conceivable, and in some cases experimentally proven, that it has influence on hormone binding, nuclear transport, DNA binding and transactivation. 

However, the use of recombinant transcriptional activators (RTAs) appears to be the most generally applicable system at present. The construction of RTAs is based on the modular structure of transcription factors, which allows for the combination of DNA binding and transactivation domains derived from different proteins. For example, RTAs have been used to establish a positive feedback loop initiated by transcription from a weak cell type-specific promoter [94], Such... 

PXR 5 C2904T Exon 3 R98S Loss of DNA binding and transactivation in HepG2 cells J = 0.0024 [19]... 

Jiang BH, Rue E, Wang GL, Roe R, Semenza GL. Dimerization, DNA binding, and transactivation properties of hypoxia-inducible factor 1. J Biol Chem 1996 271 17771-17778. 

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