Role of the intra-A-chain disulfide bond of insulin-like peptide 3 in binding and activation of its receptor, RXFP2

doi:10.1016/j.peptides.2010.05.021

Peptides

Volume 31, Issue 9, September 2010, Pages 1730-1736

https://doi.org/10.1016/j.peptides.2010.05.021 Get rights and content

Abstract

INSL3 is a member of the insulin-IGF-relaxin superfamily and plays a key role in male fetal development and in adult germ cell maturation. It is the cognate ligand for RXFP2, a leucine-rich repeat containing G-protein coupled receptor. To date, and in contrast to our current knowledge of the key structural features that are required for the binding of INSL3 to RXFP2, comparatively little is known about the key residues that are required to elicit receptor activation and downstream cell signaling. Early evidence suggests that these are contained principally within the A-chain. To further explore this hypothesis, we have undertaken an examination of the functional role of the intra-A-chain disulfide bond. Using solid-phase peptide synthesis together with regioselective disulfide bond formation, two analogs of human INSL3 were prepared in which the intra-chain disulfide bond was replaced, one in which the corresponding Cys residues were substituted with the isosteric Ser and the other in which the Cys were removed altogether. Both of these peptides retained nearly full RXFP2 receptor binding but were devoid of cAMP activity (receptor activation), indicating that the intra-A-chain disulfide bond makes a significant contribution to the ability of INSL3 to act as an RXFP2 agonist. Replacement of the disulfide bond with a metabolically stable dicarba bond yielded two isomers of INSL3 that each exhibited bioactivity similar to native INSL3. This study highlights the critical structural role played by the intra-A-chain disulfide bond of INSL3 in mediating agonist actions through the RXFP2 receptor.

Introduction

Insulin-like peptide 3 (INSL3; also known as relaxin-like factor and Leydig cell insulin-like peptide) is primarily produced by prenatal and postnatal Leydig cells of the testis and the thecal cells of the ovary [1], [18]. It is involved in mediation of fetal gonad translocation to the inguinal canal during development [18]. More recent studies have uncovered prominent new roles for INSL3 including the modulation of both male and female germ cell function [18], [10]. Thus, INSL3 agonists and antagonists may have considerable potential as specific drugs for novel contraceptive approaches or infertility treatments in both sexes.

INSL3 is a member of the insulin-IGF-relaxin superfamily, which includes insulin, IGF-1, IGF-2, relaxin-1, relaxin-2, relaxin-3, INSL3, INSL4, INSL5 and INSL6. All peptides within this family (except IGF-1) have a common two-chain structure, with two inter-chain and one intra-A-chain disulfide bond. Each member (with the exception of INSL4 which is seemingly unstructured) retains a characteristic tertiary structure characterized by two short α-helices within the termini of the A-chain separated by a turn, and a long central α-helix within the B-chain [18], [19].

RXFP2, the receptor for INSL3, is a member of the leucine-rich repeat containing G-protein coupled receptor family [11]. It is closely related to the relaxin receptor, RXFP1. Our previous studies have determined that the primary binding site of INSL3 to RXFP2 is mediated via key residues within the B-chain, namely, Arg(B16) and Val(B19), with His(B12) and Arg(B20) playing a secondary role. Together, these amino acids combine with the previously identified critical residue, Trp(B27), to form the receptor binding surface [15].

In contrast, little is known about those residues within INSL3 that are required to elicit RXFP2 receptor activation and downstream cell signalling, i.e. to confer agonist properties. The preliminary evidence suggests that these are contained principally within the A-chain [3], [4]. As the intra-chain disulfide bond of the A-chain is a common motif for all peptide members of the relaxin family, we undertook to determine the role it plays in RXFP2 receptor binding and activation. Synthetic human INSL3 analogs were prepared in which the intra-A-chain disulfide bond was either removed or replaced by a dicarba homolog.

Section snippets

Materials

9-Fluorenylmethoxycarbonyl (Fmoc)-protected amino acids and peptide synthesis reagents (HBTU, HOBt) were obtained from GL Biochem (Shanghai, China), and Novabiochem (Sydney, Australia) respectively. Fmoc-amino acid-preloaded PEG polystyrene resins were obtained from Applied Biosystems (Melbourne, Australia). All solvents and reagents used for solid-phase synthesis were of analytical quality and used without further purification.

Reversed-phase (RP) HPLC

The mobile phase in all RP-HPLC systems consisted of 0.1%

Results

To determine the importance of the intra-A-chain disulfide bond of human INSL3 on the structure and activity, two synthetic analogs were prepared in which the disulfide bond was removed. In one analog, [A(Cys^10,15 → Ser)]INSL3, the two Cys residues which form this bond were replaced with the isosteric Ser residue. The second analog, [A(ΔCys^10,15)]INSL3, had the two Cys residues deleted altogether. The A-chains of the two analogs were each assembled using Fmoc-solid-phase synthesis [6], [7] and,

Discussion

Disulfide bonds are common structural motifs in naturally occurring cyclic peptides. In some cases, they constitute part of a peptide binding domain or active site where they can undergo disulfide exchange or reduction to release metal-chelating thiol groups. These events can be important for the activity of peptides and proteins. In most peptides, however, the cystine bridge serves only a skeletal, structural role to maintain the secondary and tertiary fold of the native peptide. In our

Acknowledgements

This work was funded by National Health and Medical Research Council of Australia Project grants #350245 and 509048 to JDW, RADB and RAH. We thank Linda Chan for the amino acid analyses and Tania Ferraro and Sharon Layfield for performing binding and cAMP assays.

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Role of the intra-A-chain disulfide bond of insulin-like peptide 3 in binding and activation of its receptor, RXFP2

Abstract

Introduction

Section snippets

Materials

Reversed-phase (RP) HPLC

Results

Discussion

Acknowledgements

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

Dicarba analogues of the cyclic enkephalin peptides H-Tyr-c[d-Cys-Gly-Phe-d(or l)-Cys]NH(2) retain high opioid activity

J Med Chem

Structure of the transmembrane signal initiation site of the relaxin-like factor (RLF/INSL3)

Biochemistry