The conserved residues of the ligand-binding domains of steroid receptors are located in the core of the molecules

The relationship between conserved residues and biochemical functions of steroid receptors was investigated. Pairwise three-dimensional (3D) alignment of the ligand-binding domains of the human estrogen (1A52) and progesterone (1A28) receptors revealed two conserved domains; Asn313-Ser456 and Gln471-Lys531 (numbering reflects the sequence in the human estrogen receptor). Alignment of the protein sequences of 39 steroid receptors revealed 36 highly conserved residues (i.e., the residues commonly found in more than 80% of sequences aligned). They were distributed throughout the sequences but formed a contiguous 3D structure. Most of these highly conserved residues were buried in the ligand-binding domain, but several residues were exposed on the surface. The well-known functions commonly associated with the ligand-binding domain of steroid receptors are ligand binding, HSP90 binding, transcriptional activation and dimerization. The relationship between the residues and these functions were checked. To determine the residues involved in dimerization, the differences between the solvent accessibilities of the monomeric and dimeric forms were calculated. These results revealed 32 residues of 1A52 and 15 residues of 1A28 potentially involved in dimerization. Their distribution areas do not overlap greatly. Comparing these putative dimerization sites with highly conserved residues, many of the exposed conserved residues were observed on the side of the domain opposite are the dimerization sites. Some highly conserved residues are located in a steroid-binding site and in transcriptional activation domain. However, few of them were observed in the HSP90 binding site. These results indicate that the core structure made by most of the highly conserved residues among the ligand-binding domains of steroid receptors is important. These conserved residues may be essential for conformational change in the ligand-binding domain from its inactive to active form.