Isotype-restricted corepressor recruitment: a constitutively closed helix 12 conformation in retinoic acid receptors beta and gamma interferes with corepressor recruitment and prevents transcriptional repression.
Retinoic acid receptors (RARs) are ligand-regulated transcription factors that play multiple roles in vertebrate development and differentiation. RARs as a class are capable of both repressing and activating target gene expression. Transcriptional repression is mediated through the recruitment of corepressor proteins such as SMRT. Notably, vertebrates encode three major forms of RARs, alpha, beta, and gamma, and these distinct RAR isotypes differ in the ability to recruit a corepressor. RAR alpha strongly interacts with SMRT and can repress target gene transcription, whereas RAR beta and -gamma interact with SMRT only weakly and fail to repress. We report here the use of a genetic suppressor approach, based on a yeast two-hybrid interaction assay using Saccharomyces cerevisiae, for the isolation of RAR beta mutants that have gained the RAR alpha-like corepressor phenotype, i.e., a strong interaction with SMRT and the ability to repress gene expression in vertebrate cells. Analysis of these gain-of-function mutants indicates that the different corepressor interaction properties of RAR alpha, -beta and -gamma are determined by a gating mechanism through which amino acid differences in the helix 3 region of these receptors influence the position of the receptor C-terminal helix 12 domain. As a consequence, the RAR beta and RAR gamma receptors appear to adopt a constitutively closed helix 12 conformation in the absence of hormone that may approximate the conformation of RAR alpha when bound to hormone agonist. This closed helix 12 conformation in RAR beta and RAR gamma blocks corepressor binding, prevents repression, and permits significant levels of target gene activation even in the absence of hormone. We refer to this phenomenon as a "gate-latch" model of corepressor regulation.
Farboud
Behnom
B
Section of Microbiology, Division of Biological Sciences, University of California at Davis, Davis, California 95616, USA.
Hauksdottir
Herborg
H
Wu
Yun
Y
Privalsky
Martin L
ML
D000595:Amino Acid Sequence; D001665:Binding Sites / Q000235:genetics; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D008954:Models, Biological; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D056985:Nuclear Receptor Co-Repressor 2; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D018168:Receptors, Retinoic Acid / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism; D000072482:Retinoic Acid Receptor alpha; D017386:Sequence Homology, Amino Acid; D014158:Transcription, Genetic; D020798:Two-Hybrid System Techniques; D000097768:Retinoic Acid Receptor gamma
D004268:DNA-Binding Proteins; D056985:Nuclear Receptor Co-Repressor 2; D018168:Receptors, Retinoic Acid; D011994:Recombinant Proteins; D012097:Repressor Proteins; D000072482:Retinoic Acid Receptor alpha; D000097768:Retinoic Acid Receptor gamma; C539130:NCOR2 protein, human; C000607071:RARA protein, human; C082925:retinoic acid receptor beta