Loss of biological activity of human chorionic gonadotropin (hCG) by the amino acid substitution on the "CMGCC" region of the alpha-subunit

Single chain human chorionic gonadotropin, hCGalphabeta: effects of mutations in the alpha subunit on structure and bioactivity

The strategy of translationally fusing the subunits of heterodimeric proteins into single chain molecules is often used to overcome the mutagenesis-induced defects in subunit interactions. The approach of fusing the alpha and beta subunits of human Chorionic Gonadotropin (hCG) to produce a single chain hormone (phCGalphabeta) was used to investigate roles of critical residues of the alpha subunit in hormone receptor interaction and biological activity. The alpha subunit was mutated using PCR-based site-directed mutagenesis, fused to the wild type beta subunit and the fusion protein was expressed using Pichia pastoris expression system. Following partial purification, the mutant proteins were extensively characterized using immunological probes, receptor assays, and in vitro bioassays. The mutation hCGalpha P38A, which disrupts subunit interaction in the heterodimeric molecule, produced a fusion molecule exhibiting altered subunit interactions as judged by the immunological criteria, but could bind to the receptor with lower affinity and elicit biological response. Mutation of hCGalpha T54A disrupting the glycosylation at Asparagine 52, believed to be important for bioactivity, also yielded a biologically active molecule suggesting that the glycosylation at this site is not as critical for bioactivity as it is in the case of the heterodimer. The fusion protein approach was also used to generate a superagonist of hormone action. Introduction of four lysine residues in the Loop 1 of the alpha subunit led to the generation of a mutant having higher affinity for the receptor and enhanced bioactivity. Immunological characterization of single chain molecules revealed that the interactions between the subunits were not identical to those seen in the heterodimeric hormone, and the subunits appeared to retain their isolated conformations, and also retained the ability to bind to the receptors and elicit response. These data suggest the plasticity of the hormone-receptor interactions.

Analysis of the three dimensional structure of the CXGXC motif in the CMGCC and CAGYC regions of alpha-and beta-subunits of human chorionic gonadotropin: importance of glycine residue (G) in the motif

The three dimensional structures of the C(1)X(2)G (3)(X(3))X(4)C(5) motif of hCG, which is considered to be important for noncovalent assembly of the alpha- and beta-subunits of glycoprotein hormones were analyzed to assess the importance of glycine (Gly) (G) at site X(3) in the motif by the conformational energy calculation using computational procedures. In the C(1)M(2)G (3)(X(3))C(4)C(5) motif of the alpha-subunit, Ramachandran plot analysis showing the allowed area of the dihedral angles demonstrated that only a Gly residue was allowed at site X(3). In calculating collision with surrounding atoms as a monomer the possible main chain models of the C(1)A(2)G(3)(X(3))Y(4)C(5) motif in the beta-subunit showed that only alanine (Ala) (A) or Gly at site X(3) is allowed to alleviate the collision with the cysteine (Cys) (C) residues which form a disulfide bridge. A mutant of the beta-subunit with the C(1)A(2)A(3)(X(3))Y(4)C(5) motif (Ala at site X(3)) may not compose a heterodimer with the alpha-subunit because of interference of intermolecular hydrogen bond formation. These findings indicate that the Gly residue at site X(3) (G(3)) in the motif is essential for heterodimer formation of glycoprotein hormones. The significance of similar motifs found in various human proteins other than glycoprotein hormones was suggested.

Pharmacogenetics of human androgens and prostate cancer – an update

Prostate cancer is the most common non-skin cancer in the US; it is the second leading cause of death from cancer among US men, and the seventh leading cause of death in the US. This review examines the recent biochemical and pharmacogenetic literature related to prostate cancer, specifically that which focused on constitutional (‘germline’) single nucleotide polymorphisms at ‘functional candidate’ genes for prostate cancer. The investigations summarized in this review demonstrate the need to study the molecular genetics at these loci to rationally develop personalized medicine. In addition, the identification of somatic pharmacogenetic alterations in one of these loci suggests that this may also be a fruitful field of investigations with important clinical applications. Pharmacogenomic investigations of constitutional and tumor DNA may lead to significant advances in chemoprevention, presymptomatic diagnosis and improved treatment of prostate cancer.

Studies on the relevance of the glycan at Asn-52 of the alpha-subunit of human chorionic gonadotropin in the alphabeta dimer

Glycosylation of Asn-52 of the alpha-subunit (alphaAsn-52) is required for bioactivity of the alphabeta-dimeric human chorionic gonadotropin (hCG), although at a molecular level the effect of the glycan at alphaAsn-52 is not yet understood. To study the role of this glycan for heterodimer stability, the beta-subunit was recombined in solution with either the alpha-subunit or the alpha-subunit enzymically deglycosylated at alphaAsn-52. Enzymic deglycosylation avoids modification of the glycans at alphaAsn-78 and disturbing the protein folding. The efficiency of recombination after 16 h is 80%, independent of whether alphaAsn-52 is glycosylated or not. The dissociation constant of the hCG complex, with or without the glycan at alphaAsn-52, is less than 1 x 10(-5) s(-1), indicating that the glycan at alphaAsn-52 does not contribute significantly to the stability of the dimer. CD and NMR spectra indicate a local conformational difference between both alphabeta-dimeric hCG variants, most probably involving amino acids of the hCG beta-subunit close to the glycan at alphaAsn-52. These data explain the native-like receptor-binding abilities of hCG lacking the glycan at alphaAsn-52. It is proposed that for bioactivity the glycan at alphaAsn-52 is necessary for inducing and stabilizing a conformational change in hCG upon binding to the receptor, resulting in activation of the signal-transduction pathway.

Molecular, structural, and cellular biology of follitropin and follitropin receptor

Follitropin and the follitropin receptor are essential for normal gamete development in males and females. This review discusses the molecular genetics and structural and cellular biology of the follitropin/follitropin receptor system. Emphasis is placed on the human molecules when possible. The structure and regulation of the genes for the follitropin beta subunit and the follitropin receptor is discussed. Control of systemic and cellular protein levels is explained. The structural biology of each protein is described, including protein structure, motifs, and activity relationships. Finally, the follitropin/follitropin receptor signal transduction system is discussed.

Structural biology of human follitropin and its receptor

In this review, the current understanding of structure-activity relationships of human follitropin and of the extracellular domain of its receptor is described. Comprehensive mutagenesis of human follitropin combined with the three-dimensional structure of human follitropin has ushered in a new era of understanding of how this complex hormone binds to and activates its receptor. Comparison of human choriogonadotropin and follitropin structures has proved invaluable in understanding how these human glycoprotein hormones have conserved primary sequence that enables high affinity binding while diverging in amino acids that provide specificity. Moreover, by comparison of the structures of deglycosylated and glycosylated human choriogonadotropin and glycosylated human follitropin, there appears to be no influence of oligosaccharides upon backbone conformation of human glycoprotein hormones. Extensive structure-activity relationships of human follitropin receptor have been studied, and new insights gained here as well. These studies indicate that follitropin binds to the central module of the extracellular domain of the follitropin receptor. Biophysical analyses of purified follitropin receptor extracellular domain further revealed conformational changes affected by hormone binding and by the solvent environment. Further, secondary structure analysis of the purified extracellular domain of follitropin receptor favors the leucine-rich repeat motif model of the glycoprotein hormone receptors. Together, the studies indicate that there are only a few residues that contribute to the overall energy of binding. Formation of a weak collisional complex between follitropin and its receptor likely involves complementation of compatible surfaces and steric hindrance by oligosaccharides, followed by conformational change and formation of active site residue salt bridges. In this regard and in light of these new data, current models of the glycoprotein hormone receptors may need to be re-evaluated.

Functional contributions of noncysteine residues within the cystine knots of human chorionic gonadotropin subunits

Human chorionic gonadotropin (hCG) is a heterodimeric member of a family of cystine knot-containing proteins that contain the consensus sequences Cys-X(1)-Gly-X(2)-Cys and Cys-X(3)-Cys. Previously, we characterized the contributions that cystine residues of the hCG subunit cystine knots make in folding, assembly, and bioactivity. Here, we determined the contributions that noncysteine residues make in hCG folding, secretion, and assembly. When the X(1), X(2), and X(3) residues of hCG-alpha and -beta were substituted by swapping their respective cystine knot motifs, the resulting chimeras appeared to fold correctly and were efficiently secreted. However, assembly of the chimeras with their wild type partner was almost completely abrogated. No single amino acid substitution completely accounted for the assembly inhibition, although the X(2) residue made the greatest individual contribution. Analysis by tryptic mapping, high performance liquid chromatography, and SDS-polyacrylamide gel electrophoresis revealed that substitution of the central Gly in the Cys-X(1)-Gly-X(2)-Cys sequence of either the alpha- or beta-subunit cystine knot resulted in non-native disulfide bond formation and subunit misfolding. This occurred even when the most conservative change possible (Gly --> Ala) was made. From these studies we conclude that all three "X" residues within the hCG cystine knots are collectively, but not individually, required for the formation of assembly-competent hCG subunits and that the invariant Gly residue is required for efficient cystine knot formation and subunit folding.