Intracellular folding pathway of the cystine knot-containing glycoprotein hormone alpha-subunit

Cystine knot growth factors and their functionally versatile proregions

The cystine knot disulfide pattern has been found to be widespread in nature, since it has been detected in proteins from plants, marine snails, spiders and mammals. Cystine knot proteins are secreted proteins. Their functions range from defense mechanisms as toxins, e.g. ion channel or enzyme inhibitors, to hormones, blood factors and growth factors. Cystine knot proteins can be divided into two superordinate groups. (i) The cystine knot peptides, also referred to - with other non-cystine knot proteins - as knottins, with linear and cyclic polypeptide chains. (ii) The cystine knot growth factor family, which is in the focus of this article. The disulfide ring structure of the cystine knot peptides is made up by the half-cystines 1-4 and 2-5, and the threading disulfide bond is formed by the half-cystines, 3-6. In the growth factor group, the disulfides of half-cystines 1 and 4 pass the ring structure formed by the half-cystines 2-5 and 3-6. In this review, special emphasis will be devoted to the growth factor cystine knot proteins and their proregions. The latter have shifted into the focus of scientific interest as their important biological roles are just to be unravelled.

Screening of feature genes in distinguishing different types of breast cancer using support vector machine

Objective

To screen the feature genes in estrogen receptor-positive (ER+) breast cancer in comparison with estrogen receptor-negative (ER−) breast cancer.

Methods

Nine microarray data of ER+ and ER− breast cancer samples were collected from Gene Expression Omnibus database. After preprocessing, data in five training sets were analyzed using significance analysis of microarrays to screen the differentially expressed genes (DEGs). The DEGs were further analyzed via support vector machine (SVM) function in e1071 package of R to construct a SVM classifier, the efficacy of which was verified by four testing sets and its combination with training sets using a leave-one-out cross-validation. Feature genes obtained by SVM classifier were subjected to function- and pathway-enrichment via the Database for Annotation, Visualization and Integrated Discovery and KEGG Orthology Based Annotation System, respectively.

Results

A total of 526 DEGs were screened between ER+ and ER− breast cancer. The SVM classifier demonstrated that these genes could distinguish different subtype samples with high accuracy of larger than 90%, and also showed good sensitivity, specificity, positive/negative predictive value, and area under receiver operating characteristic curve. The inflammatory and hormone biological processes were the common enriched results for two different function analyses, indicating that the inflammatory (ie, IL8) and hormone regulation (ie, CGA) genes may be the involved feature genes to distinguish ER+ and ER− types of breast cancer.

Conclusion

The gene-expression profile data can provide feature genes to distinguish ER+ and ER− samples, and the identified genes can be used for biomarkers for ER+ samples.

Rapid Maturation of Glycoprotein Hormone Free α-Subunit (GPHα) and GPHαα Homodimers

The dynamics of glycoprotein hormone α-subunit (GPHα) maturation and GPHαα homodimer formation were studied in presence (JEG-3 choriocarcinoma cells) and absence (HeLa cells) of hCGβ. In both cases, the major initially occurring GPHα variant in [35S]Met/Cys-labeled cells carried two N-glycans (Mr app = 22 kDa). Moreover, a mono-N-glycosylated in vivo association-incompetent GPHα variant (Mr app = 18 kDa) was observed. In JEG-3 cells the early 22-kDa GPHα either associated with hCGβ, or showed self-association to yield GPHαα homodimers, or was later converted into heavily glycosylated large free GPHα (Mr app = 24 kDa). Micro-preparative isolation of intracellular GPHαα homodimers of JEG-3 cells and their conversion by reduction revealed that they consisted of 22-kDa GPHα monomers and not of large free GPHα. In HeLa cells, the large free GPHα variant was not observed, whereas GPHαα homodimers were present. Intracellularly, early GPHαα homodimers (35 kDa) and late variants (JEG-3: 44 kDa, HeLa: 39 kDa) were found. Both cell types secreted 45 kDa GPHαα homodimers. Large free GPHα and GPHαα homodimers were more rapidly sialylated than hCG αβ-heterodimers indicating a sequestration mechanism in the secretory pathway. In GPHαα homo- as well as hCG αβ-heterodimers the subunit interaction site, located on loop 2 of GPHα (amino acids 33–42), became immunologically inaccessible indicating similar spatial orientation of GPHα in both types of dimers. The studies demonstrate the formation, in vivo dynamics of GPHαα homodimers, and the pathways of the cellular metabolism of variants of GPHα, monoglycosylated GPHα and large free GPHα.

A novel four-amino acid determinant defines conformational freedom within chorionic gonadotropin beta-subunits

On the basis of apparent molecular mass heterogeneity following reducing versus nonreducing SDS-PAGE, we determined that the beta-subunit of macaque (Macaca fascicularis) chorionic gonadotropin (mCG-beta) is more conformationally constrained than the beta-subunit of human chorionic gonadotropin (hCG-beta). The amino acid sequences of these two subunits are 81% identical. To determine the conformational variance source, which was not due to glycosylation differences, we generated a series of hCG-beta-mCG-beta chimeras and identified domains that contributed to CG-beta conformational freedom. We discovered that the CG-beta 54-101 domain contained a small subdomain, residues 74-77, that regulated the conformational freedom of the beta-subunit; i.e., when residues 74-77 were of macaque origin (PGVD), the mutated hCG-beta subunit displayed macaque-like conformational rigidity, and when residues 74-77 were of human origin (RGVN), the mutated mCG-beta subunit displayed human-like conformational freedom and microheterogeneity. Additionally, CG-beta N-terminal domain residues (8, 18, 42, and 46-48) were also found to influence CG-beta conformational freedom when residues 74-77 were of human but not macaque origin. The biological significance of the CG-beta conformational variance was tested using a biological assay that showed that the hCG-alpha-hCG-beta heterodimer facilitated human CG receptor-mediated cAMP-driven luciferase reporter gene activity in HEK cells nearly 1 order of magnitude more effectively than the hCG-alpha-mCG-beta chimera. Together, these data demonstrate that two essential amino acid residues within a four-amino acid subdomain regulated CG-beta conformational freedom and that a conformational difference between hCG-beta and mCG-beta was recapitulated in the context of receptor-mediated CG heterodimer signal transduction activation.

Time-dependent folding of immunological epitopes of the human chorionic gonadotropin beta-subunit

We have explored the possibility to use 14 different monoclonal antibodies in order to follow the formation of the respective epitopes during the biosynthesis of hCG subunits and their association in JEG-3 choriocarcinoma cells using pulse (30s to 5 min)-chase (0-180 min) experiments. We found central cystine knot epitope structures (epitope beta1) to be formed immediately and simultaneously with epitopes on the protruding hCG-beta loops 1 and 3. We found also differences in the time-dependent folding of beta2 and beta4 epitopes, which are highly overlapping structures on the loops 1+3. These differences were reinforced by decreasing the temperature during the pulse-chase experiments to 25 degrees C. Moreover, we describe for the first time an intracellular intact hCG beta-subunit form that showed the transient expression of the hCG-beta-core fragment epitope beta11 in the course of the maturation of this subunit which casts new light on the presence of hCG-beta-core fragment in Down's syndrome, tumors and pregnancy.

The effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on chorionic gonadotrophin activity in pregnant macaques

As many as 62% of all human conceptions are lost prior to 12 weeks of pregnancy and it is unknown how many of these losses result from environmental hazards. Previous studies have shown that single doses of 1, 2, and 4 microg/kg 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) administrated orally to cynomolgus macaques during the peri-implantation period leads to early fetal loss (EFL) within 10-20 days. TCDD induced EFL is associated with a reduction in the biological activity of monkey chorionic gonadotrophin (mCG) but no change in the immunoreactive mCG profile. These studies are consistent with either a direct effect of TCDD on differentiation of the trophoblast and an indirect effect on mCG synthesis, or a direct effect on mCG synthesis and secretion independent of trophoblast development. The present study was designed to test the hypothesis that the action of TCDD is directly on mCG synthesis rather than on the differentiation of the trophoblast. Female macaques (Macaca fascicularis) were treated with a single dose of TCDD (4 microg/kg b.wt.) on Gestational Day 20, a stage of pregnancy following initial trophoblast differentiation and invasion. Circulating mCG concentrations were monitored for the next 6 days. Compared to the controls, the peak level of serum bioactive mCG was lower in the treated group (P<0.05), with a decrease observed on the day following exposure. The bioactive/immunoreactive mCG ratio was also lower in the treated group compared to the controls (P<0.05). There was no difference in serum immunoreactive mCG levels between the groups. Histological evaluation of the embryo-placental unit showed increased apoptosis and vascular congestion after treatment but was otherwise grossly normal. Because exposure of the conceptus to TCDD following differentiation of the trophoblast decreased the bioactivity of circulating mCG, we conclude that the action of TCDD in the placenta is directly on mCG synthesis.

Disulfide folding pathways of cystine knot proteins. Tying the knot within the circular backbone of the cyclotides

The plant cyclotides are a fascinating family of circular proteins that contain a cyclic cystine knot motif. The knotted topology and cyclic nature of the cyclotides pose interesting questions about folding mechanisms and how the knotted arrangement of disulfide bonds is formed. In the current study we have examined the oxidative refolding and reductive unfolding of the prototypic cyclotide, kalata B1. A stable two-disulfide intermediate accumulated during oxidative refolding but not in reductive unfolding. Mass spectrometry and NMR spectroscopy were used to show that the intermediate contained a native-like structure with two native disulfide bonds topologically similar to the intermediate isolated for the related cystine knot protein EETI-II (Le-Nguyen, D., Heitz, A., Chiche, L., El Hajji, M., and Castro B. (1993) Protein Sci. 2, 165-174). However, the folding intermediate observed for kalata B1 is not the immediate precursor of the three-disulfide native peptide and does not accumulate in the reductive unfolding process, in contrast to the intermediate observed for EETI-II. These alternative pathways of linear and cyclic cystine knot proteins appear to be related to the constraints imposed by the cyclic backbone of kalata B1 and the different ring size of the cystine knot. The three-dimensional structure of a synthetic version of the two-disulfide intermediate of kalata B1 in which Ala residues replace the reduced Cys residues provides a structural insight into why the two-disulfide intermediate is a kinetic trap on the folding pathway.

The cystine knot promotes folding and not thermodynamic stability in vascular endothelial growth factor

Cystine knots consist of three intertwined disulfide bridges and are considered major determinants of protein stability in proteins in which they occur. We questioned this function and observed that removal of individual disulfide bridges in human vascular endothelial growth factor (VEGF) does not reduce its thermodynamic stability but reduces its unexpected high thermal stability of 108 degrees C by up to 40 degrees C. In wild-type VEGF (deltaG(u,25)(0) = 5.1 kcal.mol(-1)), the knot is responsible for a large entropic stabilization of TdeltaS(u,25)(0) = -39.3 kcal mol(-1), which is compensated for by a deltaH(u,25)(0) of -34.2 kcal mol(-1). In the disulfide-deficient mutants, this entropic stabilization disappears, but instead of a decrease, we observe an increase in the thermodynamic stability by about 2 kcal.mol(-1). A detailed crystallographic analysis of the mutant structures suggests a role of the cystine knot motif in protein folding rather than in the stabilization of the folded state. When assuming that the sequential order of the disulfide bridge formation is conserved between VEGF and glycoprotein alpha-subunit, the crystal structure of the mutant C61A-C104A, which deviates by a root mean square deviation of more than 2.2 A from wild-type VEGF, identifies a true folding intermediate of VEGF.

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.