Structure/function analyses of human sex hormone-binding globulin by site-directed mutagenesis

Characterization and comparison of recombinant full-length ursine and human sex hormone-binding globulin

Sex hormone‐binding globulin (SHBG) regulates the bioavailability of sex steroid hormones in the blood. Levels of SHBG increase markedly in brown bears (Ursus arctos) during hibernation, suggesting that a key regulatory role of this protein is to quench sex steroid bioavailability in hibernation physiology. To enable characterization of ursine SHBG and a cross species comparison, we established an insect cell‐based expression system for recombinant full‐length ursine and human SHBG. Compared with human SHBG, we observed markedly lower secretion levels of ursine SHBG, resulting in a 10‐fold difference in purified protein yield. Both human and ursine recombinant SHBG appeared as dimeric proteins in solution, with a single unfolding temperature of ~ 58 °C. The thermal stability of ursine and human SHBG increased 5.4 and 9.5 °C, respectively, in the presence of dihydrotestosterone (DHT), suggesting a difference in affinity. The dissociation constants for [³H]DHT were determined to 0.21 ± 0.04 nm for human and 1.32 ± 0.10 nm for ursine SHBG, confirming a lower affinity of ursine SHBG. A similarly reduced affinity, determined from competitive steroid binding, was observed for most steroids. Overall, we found that ursine SHBG had similar characteristics to human SHBG, specifically, being a homodimeric glycoprotein capable of binding steroids with high affinity. Therefore, ursine SHBG likely has similar biological functions to those known for human SHBG. The determined properties of ursine SHBG will contribute to elucidating its potential regulatory role in hibernation physiology.

Studies of metabolite-protein interactions: a review

The study of metabolomics can provide valuable information about biochemical pathways and processes at the molecular level. There have been many reports that have examined the structure, identity and concentrations of metabolites in biological systems. However, the binding of metabolites with proteins is also of growing interest. This review examines past reports that have looked at the binding of various types of metabolites with proteins. An overview of the techniques that have been used to characterize and study metabolite-protein binding is first provided. This is followed by examples of studies that have investigated the binding of hormones, fatty acids, drugs or other xenobiotics, and their metabolites with transport proteins and receptors. These examples include reports that have considered the structure of the resulting solute-protein complexes, the nature of the binding sites, the strength of these interactions, the variations in these interactions with solute structure, and the kinetics of these reactions. The possible effects of metabolic diseases on these processes, including the impact of alterations in the structure and function of proteins, are also considered.

Sex hormone-binding globulins characterization and gonadal gene expression during sex differentiation in the rainbow trout, Oncorhynchus mykiss

Sex hormone-binding globulin (SHBG) binds androgens and estrogens in the blood of many vertebrates, including teleost fish. In mammals, SHBG is synthetized in the liver and secreted into the blood. In fish, shbga also exhibits a hepatic expression. In salmonids, in which the gene has been duplicated, the recently discovered shbgb gene exhibits a predominantly ovarian expression. The present work aimed at gaining new insight into shbgb gene structure and expression during gonadal sex differentiation, a steroid-sensitive process, and Shbgb protein structure and binding characteristics; specifically, rainbow trout (Oncorhynchus mykiss) shbgb was analyzed. shbgb structure was analyzed in silico while expression was characterized during gonadal sex differentiation using all-male and all-female populations. We observed that shbgb gene and cognate-protein structures are similar to homologs previously described in zebrafish and mammals. The shbgb gene is predominantly expressed in differentiating female gonads, with increased expression around the end of ovarian differentiation. In the ovary, shbgb mRNA was detected in a subset of somatic cells surrounding the ovarian lamellae. Furthermore, Shbgb binds steroids with a higher selectivity than Shbga, exhibiting a higher affinity for estradiol compared to Shbga. In conclusion, Shbgb binding characteristics are clearly different from those of Shbga. Shbgb is expressed in the differentiating ovary during a period when the synthesis and action of testosterone and estradiol must be tightly regulated. This strongly suggests that Shbgb participates in the regulation of steroid metabolism and/or mediation, that is, needed during early gonadal development in rainbow trout.

Naturally occurring mutants inform SHBG structure and function

SHBG transports and regulates the activities of androgens and estrogens. Several single nucleotide polymorphisms in the human SHBG gene have been linked to sex steroid-dependent diseases, including those associated with the metabolic syndrome. The N-terminal laminin G-like domain of SHBG includes binding sites for calcium, sex steroids, and fibulin family members, as well as a dimerization domain. We have found that 8 of 18 uncharacterized nonsynonymous single nucleotide polymorphisms within this domain alter the production or biochemical properties of SHBG in ways not previously recognized. O-Linked glycosylation at Thr7 is disrupted in SHBG T7N, whereas abnormal glycosylation of SHBG G195E limits its secretion. Three SHBG mutants (R135C, L165M, and E176K) bind estradiol with abnormally high affinity. SHBG R135C also has an increased interaction with fibulin-2. Two different substitutions within the dimer interface at R123 (R123H and R123C) reduce the affinity for 5α-dihydrotestosterone, while increasing the relative binding affinity for estradiol. SHBG T48I is defective in calcium binding, which leads to a defect in dimerization, reduced affinity for sex steroids, and an enhanced interaction with fibulin-2, which can all be restored by calcium supplementation. These naturally occurring mutants provide insight into SHBG structure and function, and defects in SHBG production or function need to be considered in the context of its utility as a biomarker of diseases.

Steroid ligands bind human sex hormone-binding globulin in specific orientations and produce distinct changes in protein conformation

The amino-terminal laminin G-like domain of human sex hormone-binding globulin (SHBG) contains a single high affinity steroid-binding site. Crystal structures of this domain in complex with several different steroid ligands have revealed that estradiol occupies the SHBG steroid-binding site in an opposite orientation when compared with 5 alpha-dihydrotestosterone or C19 androgen metabolites (5 alpha-androstan-3 beta,17 beta-diol and 5 alpha-androstan-3 beta,17 alpha-diol) or the synthetic progestin levonorgestrel. Substitution of specific residues within the SHBG steroid-binding site confirmed that Ser(42) plays a key role in determining high affinity interactions by hydrogen bonding to functional groups at C3 of the androstanediols and levonorgestrel and the hydroxyl at C17 of estradiol. Among residues participating in the hydrogen bond network with hydroxy groups at C17 of C19 steroids or C3 of estradiol, Asp(65) appears to be the most important. The different binding mode of estradiol is associated with a difference in the position/orientation of residues (Leu(131) and Lys(134)) in the loop segment (Leu(131)-His(136)) that covers the steroid-binding site as well as others (Leu(171)-Lys(173) and Trp(84)) on the surface of human SHBG and may provide a basis for ligand-dependent interactions between SHBG and other macromolecules. These new crystal structures have also enabled us to construct a simple space-filling model that can be used to predict the characteristics of novel SHBG ligands.

Photoaffinity labeling of human sex hormone-binding globulin using 17alpha-alkylamine derivatives of 3beta-androstanediol substituted with azidonitrophenylamido, azidonitrophenylamino, or trifluoroazidonitrophenylamino chromophores. Localization of Trp-84 in the vicinity of the steroid-binding site

Purified human SHBG was photoaffinity labeled with 17alpha-aminomethyl (M), 17alpha-aminoethyl (E), and 17alpha-aminopropyl (P) derivatives of [3alpha-(3)H]-5alpha-androstane-3beta,17beta-diol coupled to 5-azido-2-nitrobenzoylamido (ANB), 4-azido-2-nitrophenylamino (ANP), and 5-azido-2-nitro-3,4,6-trifluorophenylamino (ANTFP) chromophores. Successful labeling was achieved in all cases except for the two photoreagents with the shortest side chains, namely, ANP-M and ANTFP-M derivatives. Edman sequencing and mass spectrometry of immunopurified photolabeled tryptic fragments revealed that radioactivity was present either on the sequence of residues 73-94, uniquely at the level of Trp-84 (stable covalent labeling), or on one of the two overlapping sequences of residues 126-134 and 126-135, at the level of Pro-130 (labile labeling) and Lys-134 (either stable or partially labile labeling), respectively. The same Trp-84 was photolabeled with the three ANB derivatives of increasing lengths, and by the ANP-P photoreagent. This residue was the exclusive target for the shortest [(3)H]ANB-M photoreagent but was a minor site for the longest [(3)H]ANB-P photoreagent, essentially recovered at the level of Pro-130. The [(3)H]ANB-E photoreagent of intermediate size also labeled exclusively Trp-84, except in some experiments in which photolabeling was recovered predominantly at the level of Pro-130. The [(3)H]ANP-P photoreagent with an overall length similar to that of the ANB-P photoreagent labeled simultaneously Trp-84 (minor site) and Lys-134. The other [(3)H]ANP-E, [(3)H]ANTFP-E, and [(3)H]ANTFP-P derivatives labeled in all cases Lys-134. These findings indicate that the conserved Trp-84 and the two Pro-130 and Lys-134 residues are all located in the vicinity of the D ring of steroid ligands and remain freely accessible from the C17alpha position, thus providing biochemical data delineating the corresponding region of the steroid-binding site.

Crystal structure of human sex hormone-binding globulin: steroid transport by a laminin G-like domain

Human sex hormone-binding globulin (SHBG) transports sex steroids in blood and regulates their access to target tissues. In biological fluids, SHBG exists as a homodimer and each monomer comprises two laminin G-like domains (G domains). The crystal structure of the N-terminal G domain of SHBG in complex with 5alpha-dihydrotestosterone at 1.55 A resolution reveals both the architecture of the steroid-binding site and the quaternary structure of the dimer. We also show that G domains have jellyroll topology and are structurally related to pentraxin. In each SHBG monomer, the steroid intercalates into a hydrophobic pocket within the beta-sheet sandwich. The steroid and a 20 A distant calcium ion are not located at the dimer interface. Instead, two separate steroid-binding pockets and calcium-binding sites exist per dimer. The structure displays intriguing disorder for loop segment Pro130-Arg135. In all other jellyroll proteins, this loop is well ordered. If modelled accordingly, it covers the steroid-binding site and could thereby regulate access of ligands to the binding pocket.

Photoaffinity labeling of homologous Met-133 and Met-139 amino acids of rabbit and sheep sex hormone-binding globulins with the unsubstituted Delta 6-testosterone photoreagent

Purified rabbit and sheep sex hormone-binding globulins (SHBGs) were photolabeled by Delta 6-testosterone. The maximal levels of specific incorporation were respectively 0.33 and 0.30 mol of label/mol of homodimer. Tryptic cleavage of photolabeled SHBGs gave a single radioactive peptide for rabbit SHBG and two major radioactive peptides S1 and S2 for sheep SHBG. Edman sequencing of the photolabeled peptide of rabbit SHBG revealed a single sequence corresponding to peptidic fragment Leu-118-Lys-134. Subcleavage of this peptide with elastase led to a single radioactive peptidic fragment corresponding to dipeptide Met-133-Lys-134, identified by mass spectrometry, while deletion of the C-terminal residue with carboxypeptidase B showed that all the radioactivity remained on peptide Leu-118-Met-133, thus demonstrating that photolabeling occurred exclusively on Met-133, the only residue common to the two radioactive subcleaved peptides. Edman sequencing of peptides S1 and S2 of sheep SHBG showed a same single sequence corresponding to residues Gln-126-Arg-140 which contained no identifiable phenylthiohydantoin derivative at cycle 14, thus indicating that in both cases the corresponding Met-139 residue is the main site of photolabeling, as confirmed for peptide S1 by the presence at this cycle of a major peak of radioactivity while in peptide S2 the photoattachment of Delta 6-testosterone was found labile in the conditions of sequencing. The photolabeled peptide S1 was characterized by mass spectrometry which showed the covalent fixation of one mole of Delta 6-testosterone and the presence of a biantennary oligosaccharide attached at Asn-133, which suggests that the steroid-binding site is probably not deeply buried in the SHBG homodimer.

Sequence and functional relationships between androgen-binding protein/sex hormone-binding globulin and its homologs protein S, Gas6, laminin, and agrin

Androgen-binding protein/sex hormone-binding globulin (ABP/SHBG) is an extracellular binding protein that regulates the bioavailability of sex steroids. ABP/SHBG is closely related to the globular (G) domain of vitamin K-dependent protein S family of proteins and more distantly related to the G domains of several extracellular matrix proteins. ABP/SHBG appears to have evolved from the fusion of two ancestral G domains. Expanding evidence suggests that ABP/SHBG has other functions that are mediated through membrane binding, including signal transduction; however, the types of binding proteins (receptors) have not been identified. Sequence comparisons of ABP/SHBG with G domains of its homologs protein S, Gas6, laminin, and agrin have identified regions of ABP/SHBG that may bind receptors related to homolog receptors. These membrane receptors include beta-integrins, alpha-dystroglycan, and receptor tyrosine kinases. The G domains of laminin and related proteins have clearly evolved from a common ancestor to interact with specific receptors and binding proteins. It remains to be determined if ABP/SHBG followed this evolutionary pathway.

Sex hormone-binding globulin/androgen-binding protein: steroid-binding and dimerization domains

Plasma sex hormone-binding globulin (SHBG) and testicular androgen-binding protein (ABP) are homodimeric glycoproteins that share the same primary structure, and differ only with respect to the types of oligosaccharides associated with them. The biological significance of these differences is not understood, but enzymatically deglycosylated SHBG and a non-glycosylated SHBG mutant both bind steroids normally. Various affinity-labelling experiments, and studies of recombinant SHBG mutants have indicated that a region encompassing and including Met-139 in human SHBG represents an important component of its steroid-binding site. Analyses of chimeric proteins comprising various portions of human SHBG and rat ABP have also indicated that residues important for the much higher affinity of human SHBG for steroid ligands are probably located within the N-terminal portion of these molecules. Recent studies of SHBG mutants have confirmed this, and a deletion mutant containing only the first 205 N-terminal residues of human SHBG has been produced which dimerizes and binds steroids appropriately. The introduction of amino-acid substitutions between Lys-134 and Phe-148 of SHBG has also indicated that residues including and immediately N-terminal of Met-139 may influence steroid-binding specificity, while those immediately C-terminal of Met-139 represent at least a part of the dimerization domain. These studies have also demonstrated that dimerization is induced by the presence of steroid ligand in the binding site, and that divalent cations play an important role in this process. Together, these data have led us to conclude that SHBG is a modular protein, which comprises an N-terminal steroid-binding and dimerization domain, and a C-terminal domain containing a highly-conserved consensus sequence for glycosylation that may be required for other biological activities, such as cell-surface recognition.

Resolution of the steroid-binding and dimerization domains of human sex hormone-binding globulin by expression in Escherichia coli

To determine the minimal sequence requirements for steroid binding and dimerization of human sex hormone-binding globulin (SHBG), the SHBG polypeptide and various SHBG deletion mutants were expressed as glutathione S-transferase (GST) fusion proteins in Escherichia coli. Fusion proteins containing the complete SHBG sequence, or the first 177 N-terminal residues of SHBG, bound steroids with high affinity and specificity. Further deletions from the C-terminus severely compromised steroid-binding activity, as did N-terminal deletions beyond residue 18 in the SHBG sequence. Thus, residues 18-177 in SHBG encompass a region required for its steroid-binding activity, and a disulfide bridge normally present between Cys-164 and Cys-188 in SHBG is not obviously essential for steroid binding. Most of the GST/SHBG fusion proteins undergo cleavage at 4 degrees C, releasing immunoreactive polypeptides that correspond approximately in size to their respective SHBG sequences. The 23-kDa immunoreactive cleavage product released from the fusion protein containing residues 1-205 in the SHBG sequence (SHBG 1-205) has a 50-fold greater steroid-binding capacity but a 7.5-fold lower affinity than its parent fusion protein. In addition, the 22-kDa immunoreactive polypeptide released from SHBG(1-194) binds steroid, and its dimerization is promoted by steroid ligands that bind SHBG with high affinity. These data suggest that the N-terminal region of SHBG dimerizes readily in the absence of GST and in doing so acquires steroid-binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Over-expression of human sex steroid-binding protein (hSBP/hABP or hSHBG) in insect cells infected with a recombinant baculovirus. Characterization of the recombinant protein and comparison to the plasma protein

Human sex steroid-binding protein (hSBP/hABP or hSHBG) was over-expressed in High Five and Sf9 cells adhered to plates and in suspension. The adherent cells expressed to levels of 2.3 mg/l and 1.4 mg/l after 4 and 6 days, respectively, while Sf9 cells grown in suspension yielded 4.67 mg/l after 6 days. Recombinant hSBP/hABP, purified to homogeneity by immunoadsorption, was found to fold similarly to native plasma hSBP/hABP and to display similar sequence epitopes after heat denaturation. The recombinant protein binds dihydrotestosterone, testosterone, and 17 beta-estradiol with KdS of 0.6, 2.4, and 14.2 nM, respectively, which are similar to plasma hSBP/hABP. The recombinant protein contains N-linked and O-linked oligosaccharide side-chains but the monomer exhibits a slightly lower molecular weight than plasma hSBP/hABP (40 kDa vs 44 kDa) which may be due to the absence of one N-linked side-chain or to shorter oligosaccharide side-chains. The partial N-terminal sequence LRPVLP(T)Q of recombinant hSBP/hABP is identical to plasma hSBP/hABP but appears to be less heterogeneous. These results indicate that recombinant baculovirus SBP represents a good model for investigating the structure of plasma hSBP/hABP. The expression system will allow the isolation of preparative amounts of SBP mutants generated by combinatorial site-directed mutagenesis to advance investigations on structure-function relationships and undertake crystallization trials for X-ray diffraction analyses.

Structure, function, and regulation of androgen-binding protein/sex hormone-binding globulin

Despite over 20 years of research, the functions of ABP and SHBG remain elusive. The major reason for this lack of knowledge has been the unavailability of natural mutants with clinical defects for study. There is strong evidence that these binding proteins do act to modulate the gene regulatory actions of nuclear sex steroid receptors by controlling the availability of androgens and estrogens. In plasma, SHBG controls the metabolic clearance rate of sex steroids. In addition there is strong evidence that they have a much broader function. The identification of plasma membrane receptors in target tissues and the finding of homologous domains in several developmental proteins support other functions. Moreover, other experiments suggest the proteins may actually be hormones or growth factors. These findings are not compatible with a model that has the proteins only regulating free steroid hormone levels. Obviously, much more experimentation will be necessary to reveal the functions of ABP and SHBG. The recent discoveries have offered several clues to their functions and open new routes for study. These experiments, coupled with newly developed techniques, such as gene knockout by homologous recombination, make one optimistic that the functions of these unique proteins will be deciphered in the near future.

Functions of agrin and agrin-related proteins

Agrin, a molecule produced by motoneurons that induces the aggregation of nicotinic acetylcholine receptors (nAChRs), has recently been structurally characterized. Agrin-related proteins (ARPs) that arise from differential splicing are synthesized by neurons and muscle. The C-terminal region of agrin that instructs muscle to aggregate nAChRs contains three laminin A modules separated by epidermal growth factor-like modules. Alternative splicing in the laminin A modules leads to the formation of at least three ARPs that are devoid of nAChR-aggregating activity. In their N-terminal regions, both agrin and ARPs contain nine follistatin-related modules that, like those in follistatin and in another related protein, osteonectin, may have the capability to bind members of the transforming growth factor beta (TGF-beta) or platelet-derived growth factor (PDGF) families. This review proposes that these follistatin-like regions of agrin and ARPs might bind and localize growth factors, and thus provide a matrix-bound concentration of them. Beyond agrin's role in inducing AChR aggregation, the function of agrin and ARPs to provide a localized reservoir of growth factors could contribute to the formation and maintenance of the long-lasting synaptic architecture by specifying and limiting the area of influence of these molecules.

Localization of the steroid-binding site of the human sex steroid-binding protein of plasma (SBP or SHBG) by site-directed mutagenesis

The amino-terminal region of the human sex steroid-binding protein of plasma (SBP or SHBG) containing K134 and M139 was found to represent part of the steroid-binding site. This was accomplished by constructing and expressing site-directed mutants having the following replacements: M139L, M139K, M139S, K134A, H235S, and Y57F. The results indicated that M139L and H235S were fully-active, K134A and Y57F were 50 and 67% active, M139K was 7% active, and M139S was inactive. These results support affinity-labeling data indicating that both K134 and M139 are located in or near the site, and suggest that Y57 may play a role in steroid binding. The fully active H235S mutant reveals that H235 is not involved in the steroid-binding process.

Characterization of Met-139 as the photolabeled amino acid residue in the steroid binding site of sex hormone binding globulin using delta 6 derivatives of either testosterone or estradiol as unsubstituted photoaffinity labeling reagents

Immunopurified human sex hormone binding globulin (SHBG) was photoinactivated and photolabeled by radioinert and radioactive photoaffinity labeling steroids delta 6-testosterone (delta 6-T) and delta 6-estradiol (delta 6-E2). The maximal levels of specific incorporation of these two reagents were 0.50 and 0.33 mol of label/mol of SHBG, respectively. Covalently labeled SHBG fractions were citraconylated, reduced, carboxymethylated, and cleaved by trypsin. Separation of tryptic digests by reverse-phase liquid chromatography gave single radioactive peaks at the same retention times with both steroid reagents. However, the two labeled peptidic fractions could be distinguished by capillary electrophoresis and immunodetection with anti-steroid antibodies, whereas the covalent attachment of radioactivity was confirmed by thin-layer chromatography on silica gel. Edman degradation of the two labeled peptides showed a single sequence His-Pro-Ile-([3H]X)-Arg corresponding to the pentapeptide His-Pro-Ile-Met-Arg 136-140 of SHBG sequence. The coincidence, in both cases, of the absence of an identifiable amino acid residue and of the elution of the most intense peak of radioactivity at the fourth cycle of Edman degradation suggests that the same Met-139 residue was labeled by delta 6-[1,2-3H2]T or by delta 6-[17 alpha-3H]E2. Liquid secondary ion mass spectrometry of the two peptides showed [M+H]+ ions at m/z 939.8 or 923.8, corresponding respectively to the addition of delta 6-T or delta 6-E2 to the pentapeptide. The presence of the steroid molecule in the delta 6-[3H]T-pentapeptide conjugate was confirmed by the difference of 2 mass units with the [M+H]+ peak of the delta 6-[4-14C]T-pentapeptide conjugate.