Proteolytic activity of the purified hormone-binding subunit in the estrogen receptor

The HDQVH-motif in domain E of the estradiol receptor alpha is responsible for zinc-binding and zinc-induced hormone release

The estradiol receptor alpha and proteolytic fragments thereof which contain the entire ligand-binding domain E, bind 65Zn with high affinity. Four putative double-histidine zinc-binding sequences can be identified within the hormone-binding domain E: HDQVH [amino acid (aa) 373-377], HIH (aa 474-476), HFRH (aa 513-516) and HRLH (aa 547-550). Only the HDQVH-motif is responsible for the 1:1 zinc-binding to domain E because the proteolytic (endo-Lys-C) 17 kDa fragment (aa 303-467) from porcine estradiol receptor alpha possesses the zinc-binding ability but none of the fragments containing the other motifs. In addition, H373A- and H377A-mutants lack the metal-binding capacity. Moreover, divalent metal ions are able to release estradiol out of the binding-niche. The order for this feature parallels the competition pattern of 65Zn-binding: Mg2+ < Ni2+ << Zn2+ < or = Cu2+. Mutant estradiol receptor alpha fragments (H373A and H377A) lack the zinc-induced hormone release.

Three-dimensional model for the hormone binding domains of steroid receptors

We have used a motif-based structural search method to identify structural homologs of the hormone binding domains of the nuclear receptors from among a set of known protein structures and have found the closest similarity with members of the subtilisin-like serine proteases. These proteins consist of an open twisted sheet of parallel beta-strands flanked on both sides by alpha-helices. The alignment with the protease scaffold was refined by using multiple sequence prealignment of different sets of nuclear receptors, and alternative model structures were screened by considering their consistency with the results of biochemical experiments defining the ligand binding pocket. In the most favored model, nearly all of the residues thought to be involved in ligand binding map to a pocket of appropriate dimensions where the subtilisin-like proteases have their active site. The three-dimensional model that we propose for the hormone binding domains of the nuclear receptors provides a framework for the design of experiments to further investigate nuclear receptor structure and function.

Assignment of the ligand binding site of the porcine estradiol receptor to the N-terminal 17 kDa part of domain E

Ligand-filled porcine estradiol receptor was adsorbed to heparin-Sepharose, from which a 26 kDa fragment was released by papain. Its mass was reduced to a 17 kDa fragment by trypsin. The sedimentation velocities of both estradiol binding fragments increased after reaction with the MAB 13H2. The same antibody identified the denatured fragments on Western blots. The N-terminal 21 amino acid sequence was obtained from the 17 kDa peptide which corresponds to amino acids 303-323 of the human receptor with four amino acids exchanged. This indicates that the ligand binding site resides in the N-terminal 17 kDa portion of domain E.