Molecular characterization by mass spectrometry of the human estrogen receptor ligand-binding domain expressed in Escherichia coli.

The ligand binding domain of the human estrogen receptor (hER-LBD), encompassing the sequence MDPS282AG...V595, has been expressed at high levels in Escherichia coli from a pET-23d vector, and a purified preparation has been characterized both by mass spectrometry and biochemical methods. Inclusion bodies from the bacterial expression were solubilized by sonication and the hER-LBD was purified to near homogeneity by affinity chromatography over an estradiol-Sepharose column in urea-containing buffer. This material ran as a single peak on reversed-phase HPLC, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed a band with apparent molecular mass of 31-32 kilodaltons (kDa), somewhat smaller than that expected from the construct (35.6 kDa). Edman degradation revealed a single sequence of MDPSAGDMRA, consistent with an intact N terminus. Further characterization of this material using low resolution matrix-assisted laser desorption ionization mass spectrometry indicated an apparent single protein species of average mass 33,143 daltons (Da). Detailed molecular analysis by electrospray ionization mass spectrometry, however, revealed that this purified hER-LBD preparation was actually composed of a number of both modified and unmodified LBD protein fragments between 32,900-33,400 Da. The bulk of this 33-kDa protein mixture consisted of three LBD protein fragments with C termini at Ala571 (70%), Ala569 (23%), and Ser575 (4%), with only a trace amount (3%) of the full length expressed LBD (MDPS282...V595; 35, 602 Da). These four protein species also appear to be partially chemically modified by carbamylation. The functional integrity of this hER-LBD preparation was investigated by a ligand-exchange assay, which showed that the hER-LBD retained full estradiol-binding capacity; specific, covalent labeling was also observed using either the electrophilic affinity-labeling ligand tamoxifen aziridine (TAZ) or the photoaffinity-labeling ligand hexestrol diazirine. Thus, this expressed hER-LBD preparation, while appearing nominally pure by conventional biochemical techniques and having the expected ligand-binding capacity, was shown by sensitive high resolution electrospray ionization mass spectrometry techniques to be largely truncated 20-26 amino acids from the expected C terminus and to have a substantial level of covalent modification arising from the urea.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of the structural core of the human estrogen receptor ligand binding domain by selective proteolysis/mass spectrometric analysis.

The structure of the ca. 250 amino acid hormone binding domain of the human estrogen receptor (hER-LBD), expressed in E. coli and purified as a complex with estradiol, has been probed by selective proteolysis, with analysis of the protein fragments both by classical methods (SDS-PAGE and Edman N-terminal sequencing) and by mass spectrometry (HPLC-coupled electrospray ionization mass spectrometry (LC/ESI-MS)). Rapid cleavage by several proteases (trypsin, chymotrypsin, thermolysin, and Asp-N endoproteinase) is observed within a localized region (residues 297-303) at the N-terminus. In contrast, proteolytic scission at the C-terminus is less localized and more progressive; initial cuts by trypsin, chymotrypsin, thermolysin, V8, and Asp-N proteinases are observed to occur in the region 553-571, followed by further cleavage with thermolysin (548) and trypsin (548, 531, and 529). Thus, N304 and K529 define the protease-resistant N- and C-termini of a core structure for this domain that appears to contain the elements sufficient for ligand binding. The remaining segment of this domain (530-553), which is known to embody elements essential for ligand-modulated transcription activation (AF-2), is likely a surface-exposed region that, through these studies, is shown to be accessible to proteases. Only a single region within the 26 kDa ligand-binding core (N304-K529) has been identified as being readily accessible to proteases; rapid proteolysis using the proteases trypsin, chymotrypsin, and thermolysin, is localized to residues 465-468, with cleavage occurring at residues K467, L466, and both T465 and S468, respectively. The flexibility implied by the cuts in this internal 465-468 region suggest that the hER-LBD may actually consist of two subdomains. These proteolysis studies provide a substantially refined view of the conformational nature of the human estrogen receptor ligand binding domain.

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.