Ligand Binding, Activation and Corepression of the RXR-Heterodimers

The receptors for the Ts-hormone (T3R) and for the all-trans-vitaminA acid (RAR) exist in mammalian cells in two states. In the absence of hormone they exercise a 

The imderstanding of the biochemical principles of repressive function is still in its initial stages, and thus only models for their mode of action can be currently offered  

In the case of the T3R and RAR specific repressor proteins have been identified which bind to the receptor and mediate an inhibition of transcription (Hoerlein et al, 1995). The repressor proteins are also termed co-repressors Among the repressors are found proteins with histone deacetylase activity (see 1.4.6). The RXR heterodimers possibly stabilize the repressed state of chromatin by recruiting a histone deacetylase to the chromatin. 

The binding of the ligand to the DNA-bound receptor reverses the repression and leads to an active transcriptional enhancement. One model assumes that the hormone induces dissociation of the repressing proteins so that interactions with the co-activa-tors can occur. This leads to stimulation of transcription initiation. 

Activation is a complex process in which various types of co-activators participate. Among these are foimd histone acetylases (see 1.4.6). 

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