Determination of alpha-subunit contact regions of human follicle-stimulating hormone beta-subunit using synthetic peptides

Molecular architecture and biorecognition processes of the cystine knot protein superfamily: part I. The glycoprotein hormones

In this review article, the reader is introduced to recent advances in our knowledge on a subset of the cystine knot superfamily of homo- and hetero-dimeric proteins, from the perspective of the endocrine glycoprotein hormone family of proteins: follitropin (FSH), Iutropin (LH), thyrotropin. (TSH) and chorionic gonadotropin (CG). Subsequent papers will address the structure-function behaviour of other members of this increasingly significant family of proteins, including various members of the transforming growth factor-beta (TGF-beta) family of proteins, the activins, inhibins, bone morphogenic growth factor, platelet derived growth factor-beta, nerve growth factor and more than 35 other proteins with similar topological features. In the present review article, specific emphasis has been placed on advances with the glycoprotein hormones (GPHs) that have facilitated greater insight into their physiological functions, molecular structures and most importantly the basis of the molecular recognition events that lead to the formation of hetero-dimeric structures as well as their specific and selective recognition by their corresponding receptors and antibodies. Thus, this review article focuses on the structural motifs involved in receptor recognition and the current techniques available to identify these regions, including the role of immunological methodology, peptide fragment design and synthesis and mutagenesis to delineate their structure-function relationships and molecular recognition behaviour.

Human follitropin heterodimerization and receptor binding structural motifs: identification and analysis by a combination of synthetic peptide and mutagenesis approaches

The family of human glycoprotein hormones, including follitropin (FSH), are heterodimeric proteins, each composed of single alpha- and beta-subunits that are tightly associated but non-covalently linked. To study structure and function relationships of FSH, synthetic peptides were used to inhibit subunit association, to map epitopes of FSH antibodies and as antigens to generate site specific antipeptide antibodies which could be used for topographic analysis. Interpretation of such results are generally more straightforward than when peptides are used with radioreceptor assays or in cell cultures which are complex systems. The data we collected using the synthetic peptide approach suggested that FSH residues homologous to human chorionic gonadotropin (hCG) loops L3 beta and L2 alpha are involved in subunit contact. FSH residues homologous to hCG loops L2 beta and L3 alpha seemed involved in receptor binding. Loop L2 beta also seemed involved in subunit contact. Those data provided a rationale for extensive mutagenesis of the four regions of hFSH. Mutagenesis data provided additional information and higher resolution of function when combined with the three dimensional structure of hCG. In the aggregate, this information has provided a reasonable model of the receptor binding site of hFSH. Our current model of the FSH receptor site is that of a discontinuous functional epitope including L3 beta, L2 alpha and L3 alpha. The juxtaposition of residues beta D93, alpha K5 1, alpha Y88 and of alpha Y89 in the 'binding-facet' of hFSH suggest the feasibility of designing a synthetic peptide mimetic of FSH. Additional residues of the alpha-subunit are involved, along this facet of the molecule. The data collected studying hFSH therefore demonstrates that the alpha-subunit features prominently in the mechanism of FSH binding to and stabilizing the interaction with its receptor. In contrast, the beta-subunit determinant loop serves as discriminator in addition to stabilizing the binding interaction whereas mutagenesis data indicates that L2 beta does neither. Instead, L2 beta appears to stabilize FSH conformation, possibly, the alpha-subunit, required for competent binding. In this regard, synthetic peptides provided data which were a useful guide to plan mutagenesis studies and which contributed to the process of understanding the structure and function of the gonadotropins.

A synthetic peptide representing residues 7 to 21 of human luteinizing hormone beta-subunit binds calcium, facilitates calcium uptake by liposomes and possesses sequence similarity to calcium-binding domains of calmodulin

We have previously demonstrated that a synthetic peptide corresponding to residues 1 to 15 of hFSH-beta-subunit (hFSH-beta-(1-15)) shares sequence homology with the calcium-binding domains of calmodulin (CaM), binds calcium with an affinity similar to that of CaM-binding loop III (CaM III) and facilitates uptake of external calcium by liposomes and cultured rat Sertoli cells. We noted a sequence homologous to hFSH-beta-(1-15) between residues 7 to 21 of the beta-subunits of both hLH and hCG. A peptide amide representing this region of hLH-beta (hLH-beta-(7-21)) and an N-terminal truncated analog (hLH-beta-(10-21)) that more closely resembles the calcium-binding loops of calmodulin were synthesized and tested for their ability to bind 45Ca2+, and facilitate uptake of 45Ca2+ by liposomes. hLH-beta-(7-21) and hLH-beta-(10-21) bound significant amounts of 45Ca2+ with affinities of 1.2 +/- 0.4 and 0.7 +/- 0.3 mM, respectively, values virtually identical to that reported for a synthetic peptide corresponding to CaM III. The two peptides also facilitated specific, concentration-related and saturable uptake of 45Ca2+ by liposomes, which was sensitive to blockade by voltage-independent calcium-channel antagonists. Our data suggest that residues 7 to 21 of the beta-subunit of hLH may be associated with the previously demonstrated calcium requirement for specific binding of LH to its receptor, and that the calcium-binding property of this region is related to its ability to facilitate uptake of calcium into liposomes via the formation of calcium-conducting transmembrane channels.

Follitropin conformational stability mediated by loop 2 beta effects follitropin-receptor interaction

Follicle-stimulating hormone (FSH) is in the family of pituitary/placental glycoprotein hormones which also includes luteinizing hormone (LH), chorionic gonadotropin (hCG), and thyroid-stimulating hormone. These hormones are heterodimers composed of common alpha- and similar but unique beta-subunits. The 21 amino acid loop between Y33 and F53 of the FSH beta-subunit (L2 beta) can be switched into L2 beta of hCG beta without a loss of receptor binding, yet mutation of hFSH beta 37LVY39 to 37AAA39 was antecendent to a 20-fold reduction in receptor binding (based on ID50). A mutation in the LH beta gene, which causes Q54 to be R, causes hypogonadism. This residue is conserved in the glycoprotein hormones and corresponds to Q48 in hFSH beta. Mutation of hFSH beta 48QKTCT52 to 48AAACA52 resulted in a failure of heterodimer formation. In the current study single mutations were made to pinpoint which of the seven hFSH beta residues in the 37LVY39 to 37AAA39 and the 48QKTCT52 to 48AAACA52 mutants were responsible for the observed phenotypes. A single mutation of T52 to alanine was sufficient to cause a reduction in expression of heterodimeric hormone. Single mutants Q48A, T50A, V38A, Y39A, and, to a lesser extent, T52A formed heterodimer. However, these hFSH mutants were markedly unstable at pH 2.0. Thus, acid dissociation can be used to reveal metastable forms of this protein. Mutant hFSH beta Q48A was also 8-fold less active than wild-type hFSH when assayed for binding to hFSH receptors. hFSH beta V38A and Y39A mutants affected receptor binding; however, neither mutation alone caused greater than a 2-fold decrease in receptor binding activity. In summary, these results identify single important residues in the long loop (between Y33 and F53) of the hFSH beta-subunit which are required for proper subunit interactions that provide conformational stability which in turn is necessary for FSH-receptor interaction.

Scanning-alanine mutagenesis of long loop residues 33-53 in follicle stimulating hormone beta subunit

Follicle stimulating hormone (FSH) is a gonadotropin and member of the pituitary/placental glycoprotein hormone family which bind to G-protein-coupled receptors. These hormones are heterodimers composed of a common alpha and distinct beta -subunits. Previous experimental evidence suggested that the FSH beta -subunit long loop comprised of amino acids Tyr33 to Phe53 is involved in receptor binding and activation and in subunit interaction. According to recently reported crystal structures of human chorionic gonadotropin (hCG), the homologous long loop of the beta -subunit of hCG associates with the alpha -subunit and is partially exposed to solvent. This report describes the results of scanning alanine mutagenesis used to determine if amino acid side chains in this region of the molecule are required for receptor binding and/or subunit contact. Five mutations were made which spanned this loop and the mutant FSH beta-subunits were co-expressed with alpha-subunit in a Baculovirus-infected insect-cell expression system. Mutation of 48QKTCT52 to 48AAACA52 produced a FSH beta-subunit that failed to form heterodimer, consistent with the crystal structure of hCG which shows these amino acids are buried at the subunit interface. The four remaining mutants produced heterodimer and were assayed for binding to and activation of human FSH receptors. Mutation of 37LVY39 to 37AAA39 caused a 20-fold reduction binding (ID50 of 7.0 nM compared with 0.3 nM for wildtype). Mutation of 34TRDL37 to 34AAAA37 or 44RPKI47 to 44APAA47 caused lesser but measurable effects with ID50 values of 1.1 nM and 1.9 nM, respectively. The (40)KDPA(43) to 40KDPA43 to 40AAPA43 mutation had little effect on receptor binding (ID50 = 0.5 nM).

Identification of regions of the follitropin (FSH) beta-subunit that interact with the N-terminus region (residues 9-30) of the FSH receptor

We have recently identified a region, N-terminus residues 9-30, in the extracellular domain of the follicle-stimulating hormone (FSH) receptor capable of binding FSH, but not luteinizing hormone (LH) or thyroid-stimulating hormone (FSH) (Dattatreyamurty and Reichert (1992) Mol. Cell. Endocrinol. 87, 9-17). The objectives of the present study were to examine the interaction between a synthetic peptide corresponding to this receptor sequence and the beta-subunit of FSH, and to identify which FSH-beta regions are involved in the interaction. FSH-beta subunit and synthetic FSH-beta peptides 1-15, 71-85 and 101-111 effectively bound 125I-labeled FSH rec-(9-30) peptide, and binding was inhibited by excess unlabeled FSH receptors. Scatchard analysis indicated that the synthetic FSH-beta peptides had affinities for FSH rec-(9-30) peptide in the order of 10(6) M-1 (Ka), with the sum of individual peptide affinities (Ka = 1.21 x 10(7) M-1) closely approximating that of the intact beta-subunit (1.02 x 10(7) M-1). Polyclonal antibodies raised against FSH rec-(9-30) peptide completely inhibited the binding of 125I-labeled receptor peptide to hFSH, hFSH-beta, and hFSH-beta peptides 1-15, 71-85 and 101-111. Our results indicate that recognition of FSH-beta by N-terminus region (9-30) of the FSH receptor involves contact with residues in three discontinuous binding regions on FSH-beta.(ABSTRACT TRUNCATED AT 250 WORDS)

Solution structure of a synthetic peptide corresponding to a receptor binding region of FSH (hFSH-beta 33-53)

The receptor binding surface of human follicle-stimulating hormone (hFSH) is mimicked by synthetic peptides corresponding to the hFSH-beta chain amino acid sequences 33-53 [Santa-Coloma, T. A., Dattatreyamurty, D., and Reichert, L. E., Jr. (1990), Biochemistry 29, 1194-1200], 81-95 [Santa-Coloma, T. A., Reichert, L. E., Jr. (1990), J. Biol. Chem. 265, 5037-5042], and the combined sequence (33-53)-(81-95) [Santa-Coloma, T. A., Crabb, J. W., and Reichert, L. E., Jr. (1991), Mol. Cell. Endocrinol. 78, 197-204]. These peptides have been shown to inhibit binding of hFSH to its receptor. Circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy were used to determine the structure of the first peptide in this series, the 21 amino acid peptide hFSH-beta-(33-53), H2N-YTRDLVYKDPARPKIQKTCTF-COOH. Analysis of CD data indicated the presence of approximately equal amounts of antiparallel beta-pleated sheet, turns including a beta-turn, "other" structures, and a small amount of alpha-helix. The major characteristics of the structure were found to be relatively stable at acidic pH and the predominant effect of increased solvent polarity was a small increase in alpha-helical content. One- and two-dimensional NMR techniques were used to obtain full proton and carbon signal assignments in aqueous solution at pH 3.1. Analysis of NMR results confirmed the presence of the structural features revealed by CD analysis and provided a detailed picture of the secondary structural elements and global folding pattern in hFSH-beta-(33-53). These features included an antiparallel beta-sheet (residues 38-51 and 46-48), turns within residues 41-46, and 50-52 (a beta-turn) and a small N-terminal helical region comprised of amino acids 34-36. One of the turns is facilitated by prolines 42 and 45. Proline-45 was constrained to the trans conformation, whereas proline-42 favored the trans conformer (approximately 70%) over the cis (approximately 30%). Two resonances were observed for the single alanine residue (A-43) sequentially proximal to P-42, but the rest of the structure was minimally affected by the isomerization at proline-42. The major population of molecules, containing trans-42 and trans-45 prolines, presented 120 NOEs. Distance geometry calculations with 140 distance constraints and energy minimization refinements were used to derive a moderately well-defined model of the peptide's structure.(ABSTRACT TRUNCATED AT 400 WORDS)

A synthetic peptide encompassing two discontinuous regions of hFSH-beta subunit mimics the receptor binding surface of the hormone

Synthetic peptides corresponding to discontinuous segments of the hFSH-beta subunit, amino acids 33-53 and 81-95, have been shown to interact with the follicle-stimulating hormone (FSH) receptor. In this study, we demonstrate that hFSH-beta-(33-53)-(81-95)-peptide amide, a synthetic peptide encompassing these binding regions, possesses higher affinity for the FSH receptor than either synthetic hFSH-beta-(33-53) or hFSH-beta-(81-95). This increased affinity suggests that each binding component is effectively interacting with the receptor, providing evidence that these two separate receptor binding regions of hFSH-beta form a continuous binding surface on the native molecule. These results also suggest that binding surfaces of very complex proteins, such as the heterodimeric glycoprotein hormone FSH, may be mimicked by a linear arrangement of its binding domains. A model based on energetics of the peptide-receptor interaction is also described. The results indicate that the affinity (Ka) of a peptide containing different binding domains can be approximated utilizing the product of the affinity constant of each binding domain (Ka = k1.k2...kn).