Secondary structure and shape of plasma sex steroid-binding protein--comparison with domain G of laminin results in a structural model of plasma sex steroid-binding protein

Cell surface proteoglycans syndecan-1 and -4 bind overlapping but distinct sites in laminin α3 LG45 protein domain

Keratinocyte migration during epidermal repair depends on interactions between cellular heparan sulfate proteoglycan receptors, syndecan-1 and -4, and the C-terminal globular domains (LG45) of the extracellular matrix protein laminin 332. This study investigates the molecular basis of the binding specificity of the syndecan-1 and -4 receptors expressed by human keratinocytes. We used site-directed mutagenesis to alter a recombinant LG45 protein by substituting the most critical basic residues with glutamine. All proteins were expressed in mammalian cells, purified, and characterized biochemically. We used in vitro binding assays, including surface plasmon resonance, to examine interactions between mutated LG45 and heparan sulfates, syndecan-1 and -4. We identify a major heparin binding domain on the outer edge of a β-strand of LG45 surrounded by a track of converging low affinity residues. This domain harbors distinctive syndecan-1 and -4 binding-specific sequences. This is the first study to demonstrate a binding specificity of two proteoglycans produced by a single cell type. In addition, we found that although syndecan-1 interacts exclusively through its glycosaminoglycan chains, syndecan-4 binding relies on both its core protein and its heparan sulfate chains. These results suggest that LG45 may trigger different signals toward keratinocytes depending on its interaction with syndecan-1 or -4.

Definition, expression, and characterization of a protein domain in the N-terminus of pregnancy-associated plasma protein-A distantly related to the family of laminin G-like modules

Although pregnancy-associated plasma protein-A (PAPP-A), a modulator of insulin-like growth factor (IGF) activity through its cleavage of IGF-binding protein (IGFBP)-4 and -5, has been known for more than two decades, knowledge about its domain architecture is still incomplete. Using position-specific iterative BLAST, we have identified distant relatives of the PAPP-A N-terminal sequence stretch of 250 residues. We present evidence that a protein domain with weak similarity to known laminin G-like (LG) modules is contained within this region, and we propose that PAPP-A and PAPP-A2 are new and unique members in the group of LG proteins as the pappalysins represent the first examples where LG modules are associated with proteinases. Fourteen beta-strands characteristic for the LG structure were tentatively located within the PAPP-A LG (PA-LG) module using secondary structure prediction and sequence alignment. Upon mammalian expression of PAPP-A truncation mutants, we defined domain boundaries showing that PA-LG is an autonomously folding unit, which spans the first 243 residues. We were unable to express PAPP-A variants which lack the PA-LG module, suggesting a possible role in stabilization of the proteolytic domain. To obtain larger amounts of protein for functional and structural analysis, the defined PA-LG domain was expressed in bacteria and folded in vitro. In addition, the availability of recombinant PA-LG module may potentially improve diagnostic assays based on the measurement of PAPP-A antigen, and also facilitate the study of PAPP-A in animal model systems.

Structure/function analyses of human sex hormone-binding globulin: effects of zinc on steroid-binding specificity

In humans, sex hormone-binding globulin (SHBG) binds and transports the biologically most important androgens and estrogens in the blood, and regulates the access of these steroids to their targets tissues. In addition to binding sex steroids, SHBG has specific binding sites for divalent cations including calcium and zinc. Zinc binding to a site at the entrance of the steroid-binding pocket in human SHBG has been shown to reduce its affinity for estrogens, while having no impact on the binding of C19 steroids. Crystallographic studies indicate that C18 and C19 steroids are bound in opposite orientations within the SHBG steroid-binding site, and we have obtained new information that supports a molecular model explaining the mechanism by which zinc alters the affinity of human SHBG for estrogens, by studying directly the estradiol-binding properties SHBG variants created by site-directed mutagenesis. In this model, the coordination of a zinc ion by the side chains of residues Asp65 and His136 eliminates a critical hydrogen bond between Asp65 and the hydroxyl at C3 of estrogens, such as estradiol and 2-methoxyestradiol, and causes disorder in a polypeptide loop segment that covers the steroid-binding site. The combination of these structural changes explains the specific decrease in the affinity of human SHBG for C18 steroids in the presence of a zinc ion.

Resolution of the human sex hormone-binding globulin dimer interface and evidence for two steroid-binding sites per homodimer

Human sex hormone-binding globulin (SHBG) transports sex steroids in the blood. It functions as a homodimer, but there is little information about the topography of its dimerization domain, and its steroid binding stoichiometry is controversial. The prevailing assumption is that each homodimeric SHBG molecule contains a single steroid-binding site at the dimer interface. However, crystallographic analysis of the amino-terminal laminin G-like domain of human SHBG has shown that the dimerization and steroid-binding sites are distinct and that both monomers within a homodimeric complex are capable of binding steroid. To validate our crystallographic model of the SHBG homodimer, we have used site-directed mutagenesis to create SHBG variants in which single amino acid substitutions (V89E and L122E) were introduced to produce steric clashes at critical positions within the proposed dimerization domain. The resulting dimerization-deficient SHBG variants contain a steroid-binding site with an affinity and specificity indistinguishable from wild-type SHBG. Moreover, when equalized in terms of their monomeric subunit content, dimerization-deficient and wild-type SHBGs have essentially identical steroid binding capacities. These data indicate that both subunits of the SHBG homodimer bind steroid and that measurements of the molar concentration of SHBG homodimer in serum samples have been overestimated by 2-fold.

Arginine-140 and isoleucine-141 determine the 17beta-estradiol-binding specificity of the sex-steroid-binding protein (SBP, or SHBG) of human plasma

Arginine-140 and isoleucine-141 were identified as key determinants of 17beta-estradiol (E(2)) binding affinity of the sex-steroid-binding protein (SBP, or SHBG) of human plasma. Amino acid residues that differ between human and rabbit SBP sequences were replaced in the human protein and the products tested for lowered E(2)binding activity as are seen in the rabbit protein. Only mutants containing either R140K or I141L replacements display an E(2) equilibrium dissociation constant (Kd) higher than the wild type, reaching a value of 30 nM when both were present. The 5alpha-dihydrotestosterone (DHT) equilibrium dissociation constant of these mutants was unaffected. The quadruple mutant M107I/I138V/R140K/I141L yielded an E(2) Kd of 65 nM, significantly closer to the 80 nM rabbit SBP E(2) Kd value. Although mutants containing the M107I and I138V replacements in the absence of R140K and I141L had normal E(2) Kds, the presence of the M107I replacement in the quadruple mutant was necessary to obtain an accurate E(2) Kd value by competitive Scatchard analysis. Molecular modeling using coordinates for the recently determined N-terminal domain of human SBP revealed a significant shift of the F56 phenyl ring away from ring A of E(2) in mutant models containing the R140K and I141L replacements. We conclude that R140 and I141 are required for sustaining the right proximity of the phenyl ring of F56 to ring A of 17beta-estradiol, thus optimizing the E(2)-binding affinity of human SBP.

Rabbit sex hormone-binding globulin: expression in the liver and testis during postnatal development and structural characterization by truncated proteins

Although sex hormone binding globulin (SHBG) is found in the blood plasma of adult humans and rabbits and the gene is expressed in their livers, it is not detected in the plasma of adult rodents nor is it expressed in adult rodent livers. Thus the rabbit represents a good model to study the metabolism and function of SHBG in the blood. We have used a cloned rabbit SHBG cDNA to detect mRNA expression in rabbits during the postnatal period, and to construct truncated SHBG proteins for structure/function analysis. The SHBG mRNA appeared in the testis as early as 3 days after birth. The level increased gradually in abundance throughout postnatal development, and attained a maximum at 12 weeks of age when the gonads were fully matured. In contrast, SHBG mRNA in the livers of male and female animals increased to a maximum by 4 weeks of age, and were maintained at this level until 12 weeks before subsiding to the initial levels. The increase and decrease in SHBG mRNA levels in the liver were accompanied by similar changes in serum SHBG. This suggests that SHBG in the blood circulation comes from the liver and this might also provide a source of SHBG for the male reproductive tract before formation of the blood-testis barrier. To elucidate the minimal sequence of rabbit SHBG responsible for steroid-binding, a panel of 13 truncated SHBG proteins was constructed, expressed in Escherichia coli, and biochemically purified for study. It was shown that the complete protein sequence of rabbit SHBG was important for maintaining a stable steroid-protein complex. Unlike human SHBG for which a truncated protein of the first 206 residues of the 373 amino acid protein can still bind steroid, removal of 43 or more residues from the C-terminus of rabbit SHBG completely abolished steroid-binding.

The sex steroid binding protein (SBP or SHBG) of human plasma: identification of Tyr-57 and Met-107 in the steroid binding site

Tyrosine-57 (Y57) and methionine-107 (M107) have been identified in the binding site of the sex steroid binding protein (SBP) (or sex hormone binding globulin) of human plasma by replacing the two amino acids with a number of residues of varying structure. Replacement of Y57 with phenylalanine resulted in a fourfold increase in the K(d) of 5 alpha-dihydrotestosterone but left the K(d) of 17 beta-estradiol unchanged. Except in two cases, no further loss in binding took place when replacing Y57 with other residues, suggesting that the phenolic group of Y57 may form a hydrogen bond with the ligand. Replacement of M107 with isoleucine increased the 5 alpha-dihydrotestosterone K(d) fourfold to a value equal to that of rabbit SBP, which contains isoleucine at the corresponding position; however, the K(d) of 17 beta-estradiol remained unchanged. Replacement of M107 with threonine resulted in a tenfold decrease in 5 alpha-dihydrotestosterone binding affinity, whereas replacement with leucine left the K(d) unchanged. These data indicate that substitutions on the beta-carbon of the amino acid side-chain at position 107 causes significant loss of binding affinity but, as in the case of Y57, the activity was not totally eliminated. We conclude that Y57 and M107 form part of a structural motif within the steroid binding site and specifically contribute binding energy to ring A of 5 alpha-dihydrotestosterone but not to ring A of 17 beta-estradiol. We also propose that the integrated contribution of several side chains may be required to optimize the ligand affinity of the steroid binding site. This proposal may fit a 'lock and key' model where little movement of the side chains occurs during binding as might be expected for a rigid structure like the steroid nucleus.

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.

Heterologous expression of wild type and deglycosylated human sex steroid-binding protein (SBP or SHBG) in the yeast, Pichia pastoris. Characterization of the recombinant proteins

Wild type, partially and fully-deglycosylated human sex steroid-binding protein (SBP or SHBG) cDNAs lacking the native cucaryotic signal sequence were cloned into a yeast expression vector containing the Saccharomyces cerevisiae alpha-factor for extracellular secretion. Following transformation into Pichia pastoris, the wild type and all constructed mutants were successfully expressed. The levels were lower for the deglycosylated mutants indicating that oligosaccharide side chains may play a role in SBP secretion. Under fermentation conditions, the wild type protein was expressed at a level of 4 mg/l while the fully-deglycosylated mutant T7A/N351Q/N367Q was expressed at about 1.5 mg/l. The latter was purified from several fermentation runs and was found to be completely deglycosylated, electrophoretically homogeneous and fully active. The aminoterminus was found to have the sequence NH2QSAHDPPAV- indicating that cleavage of the alpha-factor occurred at the A(+7)-Q(+8) peptide bond. The molecular mass of the subunit was determined to be 39,717.8 Da, which is in complete agreement with the amino acid sequence of the T7A/N351Q/N367/Q mutant. The equilibrium constants for the dissociation of 5alpha-dihydrotestosterone and steroid binding specificity were found to be identical to that of the human plasma protein indicating that the missing N-terminal segment NH2-LRPVLPT and the removal of oligosaccharide side chains do not affect the stability and active conformation of the protein. In conclusion, the data presented reveal that the SBP mutant T7A/N351Q/N367/Q is the protein of choice for solving the three-dimensional structure.

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.

A specific binding protein for cardiac glycosides exists in bovine serum

Searching for a binding protein in blood, which may be involved in the specific transport of cardiac glycosides to their receptor sites on the sodium pump, we isolated a cardiac glycoside-binding protein (CGBG) of 26 kDa from the globulin fraction of bovine serum by affinity chromatography and on a ouabain-Sepharose 4B column by a purification factor of 5000. The cardiac glycoside-binding globulin was labeled specifically and covalently by the protein-reactive digoxigenin derivative HDMA (N-hydroxysuccimidyldigoxigenin-3-O-methylcarbonyl-epsilon-+ ++aminocapro ate). Even very high concentrations of other steroids, such as estrogen, testosterone, progesterone, and cortisone, did not prevent HDMA-labeling (at 5 and 100 nM) of CGBG, but the cardenolides ouabain and digoxin or the bufadienolide proscillaridin A did so. CGBG is a homodimer of two 26-kDa subunits forming disulfide bonds, since HDMA labeling of a protein of 53 kDa was observed in SDS-polyacrylamide gel electrophoresis when beta-mercaptoethanol was absent during SDS denaturation. The N-terminal amino acid sequence K-D-V-Y-R-A-P-D-G-T-Q-S-A showed no sequence similarity with proteins recorded in gene and protein sequence data banks. A 90-kDa cytosolic CGBG exists in bovine kidneys and reacts with antibodies against CGBG. Binding of ouabain to the cardiac glycoside-binding globulin was monitored by quenching of intrinsic tryptophan fluorescence. Such studies reveal two negatively cooperative ouabain binding sites with Kd' of 1.52 nM and Kd' = 75 nM and with an interaction factor of 50 using a Koshland-Némethy-Filmer model. The demonstration of a cardiac glycoside-binding globulin in plasma is consistent with the recent finding of endogenous cardiac glycosides in mammals.