Ammo Terminal Transactivation Domain

The large NTD of the AR is unique among NRs in that it is responsible for almost the entire transactivation potential of the protein. It contains at least three overlapping constitutive transactivation domains, which, in the context of the full-length receptor, are activated or inhibited by conformational changes resulting from hormone binding at the LBD or by other mechanisms. In particular, two activation functions, AF-1 (aa 51-210) and AF-5 (aa 369-492), which contain LxxLL-like motifs, appear to cooperate to facilitate ligand-induced N and C interactions, the latter with an interaction surface that overlaps with AF-2 in helix 12 of the LBD (Fig. 16.1).

Figure 16.1 Androgen receptor (AR) gene and protein structure. A: Schematic representation of the AR gene on chromosome Xq11-12 showing important binding sites for sex determining region Y (SRY) and SP1 transcription factors. Individual exons are separated by up to 16 kb of intronic sequence. B: AR mRNA transcript showing alternative splice and polyadenylation sites. Translation is primarily directed from the first of two initiating methionine residues. C: Structure of the predominant (AR-B) form of the AR. Indicated are the N-terminal transactivation do-

Figure 16.1 Androgen receptor (AR) gene and protein structure. A: Schematic representation of the AR gene on chromosome Xq11-12 showing important binding sites for sex determining region Y (SRY) and SP1 transcription factors. Individual exons are separated by up to 16 kb of intronic sequence. B: AR mRNA transcript showing alternative splice and polyadenylation sites. Translation is primarily directed from the first of two initiating methionine residues. C: Structure of the predominant (AR-B) form of the AR. Indicated are the N-terminal transactivation do-

main (NTD), DNA-binding domain (DBD), hinge region (H), ligand-binding domain (LBD), and ligand-dependent activation function (AF-2). D: The structure of the NTD showing the position of the two polymeric amino acid stretches (polyglutamine, Qn; polyglycine, Gn), ligand-independent activation function (LIAF), constitutive activation functions (AF1 and AF5), and the position of the two LxxLL-like motifs implicated in the interaction between the NTD and LBD. Structures are not to scale.

Access to the hydrophobic binding surface of LxxLL-like peptides of coactivators also is blocked by helix 12 of the LBD. An N-C interaction can occur intra- or intermolecularly and is associated with increased affinity of receptor binding to androgen response elements of target gene promoters and stabilization of the receptor complex. It is not clear if there is functional redundancy between AF-1 and AF-5 in the AR NTD and AF-2 in the LBD or whether both NTD activation functions are absolutely required for recruitment of coregulatory factors and transcription initiation. We and others have shown that p160 cofactors can form a bridge to tether the AR N and C termini via interaction between a glutamine-rich region of the cofactor and the LxxLL-like motifs in the AR NTD and between LxxLL motifs of the cofactor and AF-2 in the AR.22,23 These interactions may be promoted by the cofactor to stabilize the receptor for maximal activity.

The transactivation potential of subdomains in the AR NTD is modulated by two polymorphic trinucleotide microsatellite repeats, CAG and GGN, in the coding sequence. The normal size distribution of these microsatellites in the population is 9-39 CAG and 14-27 GGN repeats, with a modal repeat size of 21 and 23, respectively.24,25 These repeats encode variable-length polyglutamine (polyQ) and polyglycine (polyG) tracts, respectively, in the receptor (Fig. 16.1). The NTD also contains a strong ligand-inde-pendent activation function (LIAF, aa 502-535; see "Multiple Pathways of Androgen Receptor Activation" below) located downstream of AF-5 (Fig. 16.1), which appears to be silenced by interaction with an inhibitory subdomain (ISD) in the NTD containing the polyQ region.22

Expansion of the CAG microsatellite to 40 or more repeats causes a rare, X-linked, adult-onset, neurodegenerative disorder called spinal and bulbar muscular atrophy (SBMA), or Kennedy's disease.26,27 In addition to progressive muscle weakness and atrophy due to loss of brain stem and spinal cord motor neurons, men with this disorder frequently present with symptoms of partial androgen insensitivity (i.e., gy-necomastia and testicular atrophy), indicative of aberrant AR function.28,29 Receptor proteins encoded by SBMA AR alleles have normal andro-

gen-binding affinities but reduced transactiva-tion capacity compared to wild-type AR.30,31 Indeed, an inverse relationship between AR transactivation activity and CAG repeat length has been established over a CAG size range encompassing normal AR alleles.22,32-34

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