Isolation and recombination of ovine lutropin subunits with complete recovery of biological activity

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.

Structural features of mammalian gonadotropins

There are two species for which both pituitary and placental gonadotropins are readily available, humans and horses. The human gonadotropins are better characterized than equine gonadotropins. Nevertheless, the latter are very interesting because they provide exceptions to some of the general structure-function principles derived from studies on human and other mammalian gonadotropins. For example, separate genes encode the hLH beta and hCG beta subunits while a single gene encodes eLH beta and eCG beta. Thus, eCG and eLH differ only in their oligosaccharide moieties and eLH is the only LH that possesses the O-glycosylated C-terminal extension previously believed to be restricted to chorionic gonadotropins. Truncation experiments involving eLH beta and hCG beta have suggested the C-terminal extension has no effect on receptor binding. However, the largest of three eCG forms which differ only in the extent of O-glycosylation possessed reduced affinity for LH and FSH receptors. This result suggested that effects of O-glycosylation need to be considered when examining the glycosylation differences between eLH and eCG responsible for the 10-fold lower eCG receptor binding affinity compared with that of eLH. Contribution of alpha Asn56 N-linked oligosaccharides to the different biological activities of eLH and eCG has been evaluated following selective removal using peptide-N-glycanase digestion of native equine alpha-subunit preparations. Hormones-specific patterns of glycosylation were observed on alpha Asn56 of eLH, eFSH, and eCG. Removal of alpha Asn56 oligosaccharides increased the rate of subunit association, the extent of association, and receptor binding activity. Some unassociated alpha-subunit oligosaccharides were identified which may interfere with subunit association because they were more abundant in unassociated subunit oligosaccharide maps than in a total oligosaccharide map. This was most striking in the case of eCG alpha in which two minor peaks became the major oligosaccharide peaks detectable in the unassociated eCG alpha fraction following association with eLH beta and eFSH beta. The biological activities exhibited by hybrid hormones, eLH alpha reassociated with oLH beta and pLH beta, found to be greater than those of oLH and pLH provided an interesting exception to the general rule that the beta-subunit determines the potency of the heterodimer. LH receptor binding activities of eLH beta-chimeric ovine/equine alpha-subunits suggested that the equine alpha-subunit N-terminal domain may be responsible for this effect. Equine FSH has higher FSH receptor binding activity than human, ovine, and porcine FSH preparations. This probably results from two factors. First, the presence of the equine alpha-subunit promotes receptor binding as noted above. Second, the overall -2 charge of the eFSH beta determinant loop, which is less negative that the -3 observed in other species, results from the presence of an Asn residue at position 88 instead of Asp. This apparently facilitates binding to the FSH receptor.

Human platelet adenylate cyclase: persistent binding of guanine nucleotide is central to the regulation of both hormone-stimulated and basal activities

Prostaglandin E1 stimulation of human platelet adenylate cyclase, in purified plasma membranes, occurs without the addition of exogenous GTP. Possible contamination of the adenylate cyclase assay mixture by GTP either from nonspecifically bound nucleotide in the plasma membrane or from the substrate ATP was ruled out as follows: (a) variation of the membrane concentration, repeated washing, inclusion of EDTA, GDP beta S, or GMP in the wash step, or UDP in the assay, are all without effect, and (b) analysis of the substrate by high-performance liquid chromatography revealed no contaminating GTP. Other prostaglandins (I2, E2, D2) also activate cyclase without the addition of GTP. In sharp contrast, stimulation of adenylate cyclase in the human neutrophil plasma membrane by prostaglandin E1 shows an obligatory requirement for GTP, under identical assay conditions. GDP beta S pretreatment amplifies the fold cyclase stimulation by GTP in the presence and absence of prostaglandin E1, by lowering the basal activity. This alteration occurs without lowering the GTP-independent prostaglandin E1 activation, and is specific for inhibitory guanine nucleotides (GDP beta S, GMP, GDP) in the pretreatment. Extensive washing with buffer or incubation with other nucleotides, epinephrine, or prostaglandin E1 prior to the assay, is without effect. GTP gamma S treatment of the membrane induces a high-activity state and abolishes the GDP beta S effect on basal activity as well as prostaglandin E1 activation of cyclase. The results suggest distinct patterns of prostaglandin stimulation in platelet and neutrophil cyclase systems, and further imply that guanine nucleotide, prebound to specific sites within the GTP-regulatory proteins, may modify the kinetic characteristics of platelet adenylate cyclase.

Hybrids from equine LH: alpha enhances, beta diminishes activity

LH hybrids were prepared by combining eLH alpha and eLH beta with the corresponding subunits of oLH, pLH and hCG. Recombinants were isolated by gel filtration and assessed by SDS-polyacrylamide gel electrophoresis under both dissociating and non-dissociating conditions. All combinations of subunits produced hybrid LH molecules. Hybrids prepared by combining eLH beta with oLH alpha, pLH alpha or hCG alpha were very inactive in rat radioligand and Leydig cell in vitro bioassays. Hybrids prepared with eLH alpha were very active in both assays. The greatest potentiating activity was observed when eLH alpha was combined with pLH beta. The resulting hybrid was 49 times as active as pLH in stimulating steroidogenesis by Leydig cells.

Deglycosylated ovine lutropin: preparation and characterization by in vitro binding and steroidogenesis

Ovine lutropin (oLH) subunits were deglycosylated in anhydrous hydrofluoric acid. All possible combinations of native and deglycosylated alpha- and beta-subunits were prepared and characterized. The two native plus deglycosylated counterpart preparations showed properties intermediate between the native only and the deglycosylated subunit hybrid. For brevity, only the properties of the latter hybrid were described in detail. Deglycosylation did not alter subunit-subunit recombination, protein conformation (circular dichroism) or the peptide chains (N-termini). Deglycosylation enhanced receptor-binding activity, but the carbohydrate moieties are important for maximal steroidogenesis (Leydig cell testosterone production). The deglycosylated hybrid molecule acts as an antagonist or agonist to native hormone action depending on the test system and conditions. The deglycosylated hormone binds to the receptor much more rapidly than the native hormone and binding plateaued at a higher level in the case of the deglycosylated hormone, although cross-labeling of tracers (either native or deglycosylated) indicated that competition was for the same type of receptors. To eliminate internalization of the hormone as a factor in the binding studies we used broken cell (membrane) preparations of Leydig cells. Differences in the preparations are thus attributable to a greater 'on-rate' for the deglycosylated hormone. The 'off-rate' from the receptor was essentially equal for both native and deglycosylated hormones. For this reason the deglycosylated hormone has only limited antagonistic action for native hormone activity.

Purification and receptor binding properties of complexes between lutropin and monovalent antibodies against its alpha subunit

A complex between bovine lutropin (LH) and monovalent antibodies (Fab fragments) directed against its alpha subunit, which is common to the glycoprotein hormones, has been purified by gel filtration and chromatography on concanavalin A-Sepharose. The complex is heterogenous with respect to molecular size; 70--80% of the hormone is complexed with either two or three Fab fragments. The LH-Fab alpha complexes retain only about 13% receptor binding activity as compared to LH when measured in a radioligand receptor assay in which the radiolabeled ligand is human choriogonadotropin. (Use of the human hormone as labeled ligand permits direct measurement of competition between receptor and the bovine complex because the alpha portion of the human hormone does not cross react significantly with antibodies directed against bovine alpha subunits.) Complex formation does not lead to dissociation of the lutropin into its subunits, as shown with a homologous LH-beta immunoassay which distinguishes free beta subunit from intact LH. Complexing of LH with Fab-alpha fragments also causes little or no change in the affinity of the hormone's beta subunit for anti-LH-beta antibodies indicating that significant changes in beta subunit conformation did not occur. The data show that at least two well-separated antigenic regions on the alpha subunit are exposed to the surface in the intact hormone. They are also in agreement with the proposal that the loss of binding activity to receptor is due to steric effects rather than to changes in conformation or dissociation, and that there may be sites on the alpha subunit which interact directly with the receptor.