Characterization of two glycosylated boar spermadhesins

CUB domains are not required for OVCH2 function in sperm maturation in the mouse epididymis

BACKGROUND

Ovochymase 2 (Ovch2) is an epididymis-specific gene that is required for male fertility. While a multitude of reproductive tract-specific genes required for male fertility have been identified, OVCH2 is thus far the first protein required for male fertility that contains Complement C1r/C1s, Uegf, Bmp1 (CUB) domains located in tandem in the C-terminus of the protein. Identifying the functional significance of this unique domain has implications in better understanding fertility and infertility and as a potential contraceptive target.

OBJECTIVE

The goals of these studies were to understand the influence and requirement of OVCH2 CUB domains in the localization and functional requirement of OVCH2 in sperm maturation and function.

MATERIALS AND METHODS

To this end, we performed in vivo localization analysis of OVCH2 and reproductive phenotype analysis of mice containing C-terminal FLAG tag on OVCH2, with either the entire protein intact, or CUB2 or both CUB1 and CUB2 genetically ablated. All mice were generated through the CRISPR/Cas9 gene editing approach.

RESULTS

We found that OVCH2 is specifically expressed in the proximal caput epididymidis, and the absence of CUB2 did not affect this localization pattern. Although the absence of both CUB domains significantly reduced sperm motility and progressive motility, this effect was not manifested in a reduction in fertility over a 6-month period mating trial, which showed no significant differences between control and CUB deletant mice. Further, the absence of one or both CUB domains did not affect reproductive organ structure or sperm morphology.

CONCLUSIONS

Our studies demonstrate that the CUB domains are not required for fertility in male mice, at least under the normal animal housing conditions our mice were tested in, and suggest that the enzymatic activity of the OVCH2 protease, in the absence of its CUB domains, is sufficient for normal sperm processing in the epididymis. Although our findings do not preclude the possibility that OVCH2 CUB domains are required under a yet-identified stress condition, our findings demonstrate that the most likely region for deleterious mutations in men with idiopathic infertility and the most vulnerable site for inhibition of OVCH2 protein function is in its protease domain, and not its CUB domains. Our findings have implications in the genetic screening of infertile men and the development of a novel non-hormonal male contraceptive by honing in on the more critical region of a functionally required protein.

Porcine spermadhesin AQN-3 binds to negatively charged phospholipids

The spermadhesin AQN-3 is a major component of porcine seminal plasma. While various studies suggest that this protein binds to boar sperm cells, its attachment to the cells is poorly understood. Therefore, the capacity of AQN-3 to interact with lipids was investigated. For that purpose, AQN-3 was recombinantly expressed in E. coli and purified via the included His-tag. Characterizing the quaternary structure by size exclusion chromatography revealed that recombinant AQN-3 (recAQN-3) is largely present as multimer and/or aggregate. To determine the lipid specificity of recAQN-3, a lipid stripe method and a multilamellar vesicle (MLV)-based binding assay were used. Both assays show that recAQN-3 selectively interacts with negatively charged lipids, like phosphatidic acid, phosphatidylinositol phosphates, and cardiolipin. No interaction was observed with phosphatidylcholine, sphingomyelin, phosphatidylethanolamine, or cholesterol. The affinity to negatively charged lipids can be explained by electrostatic interactions because binding is partly reversed under high-salt condition. However, more factors have to be assumed like hydrogen bonds and/or hydrophobic forces because the majority of bound molecules was not released by high salt. To confirm the observed binding behavior for the native protein, porcine seminal plasma was incubated with MLVs comprising phosphatidic acid or phosphatidyl-4,5-bisphosphate. Attached proteins were isolated, digested, and analyzed by mass spectrometry. Native AQN-3 was detected in all samples analyzed and was - besides AWN - the most abundant protein. It remains to be investigated whether AQN-3, together with other sperm associated seminal plasma proteins, acts as decapacitation factor by targeting negative lipids with signaling or other functional roles in fertilization.

Discovery of predictive biomarkers for litter size in boar spermatozoa

Conventional semen analysis has been used for prognosis and diagnosis of male fertility. Although this tool is essential for providing initial quantitative information about semen, it remains a subject of debate. Therefore, development of new methods for the prognosis and diagnosis of male fertility should be seriously considered for animal species of economic importance as well as for humans. In the present study, we applied a comprehensive proteomic approach to identify global protein biomarkers in boar spermatozoa in order to increase the precision of male fertility prognoses and diagnoses. We determined that l-amino acid oxidase, mitochondrial malate dehydrogenase 2, NAD (MDH2), cytosolic 5'-nucleotidase 1B, lysozyme-like protein 4, and calmodulin (CALM) were significantly and abundantly expressed in high-litter size spermatozoa. We also found that equatorin, spermadhesin AWN, triosephosphate isomerase (TPI), Ras-related protein Rab-2A (RAB2A), spermadhesin AQN-3, and NADH dehydrogenase [ubiquinone] iron-sulfur protein 2 (NDUFS2) were significantly and abundantly expressed in low-litter size spermatozoa (>3-fold). Moreover, RAB2A, TPI, and NDUFS2 were negatively correlated with litter size, whereas CALM and MDH2 were positively correlated. This study provides novel biomarkers for the prediction of male fertility. To the best of our knowledge, this is the first work that shows significantly increased litter size using male fertility biomarkers in a field trial. Moreover, these protein markers may provide new developmental tools for the selection of superior sires as well as for the prognosis and diagnosis of male fertility.

Discovery, primary, and crystal structures and capacitation-related properties of a prostate-derived heparin-binding protein WGA16 from boar sperm

Mammalian sperm acquire fertility through a functional maturation process called capacitation, where sperm membrane molecules are drastically remodeled. In this study, we found that a wheat germ agglutinin (WGA)-reactive protein on lipid rafts, named WGA16, is removed from the sperm surface on capacitation. WGA16 is a prostate-derived seminal plasma protein that has never been reported and is deposited on the sperm surface in the male reproductive tract. Based on protein and cDNA sequences for purified WGA16, it is a homologue of human zymogen granule protein 16 (ZG16) belonging to the Jacalin-related lectin (JRL) family in crystal and primary structures. A glycan array shows that WGA16 binds heparin through a basic patch containing Lys-53/Lys-73 residues but not the conventional lectin domain of the JRL family. WGA16 is glycosylated, contrary to other ZG16 members, and comparative mass spectrometry clearly shows its unique N-glycosylation profile among seminal plasma proteins. It has exposed GlcNAc and GalNAc residues without additional Gal residues. The GlcNAc/GalNAc residues can work as binding ligands for a sperm surface galactosyltransferase, which actually galactosylates WGA16 in situ in the presence of UDP-Gal. Interestingly, surface removal of WGA16 is experimentally induced by either UDP-Gal or heparin. In the crystal structure, N-glycosylated sites and a potential heparin-binding site face opposite sides. This geography of two functional sites suggest that WGA16 is deposited on the sperm surface through interaction between its N-glycans and the surface galactosyltransferase, whereas its heparin-binding domain may be involved in binding to sulfated glycosaminoglycans in the female tract, enabling removal of WGA16 from the sperm surface.

Multiple proteins present in purified porcine sperm apical plasma membranes interact with the zona pellucida of the oocyte

An important step in fertilization is the recognition and primary binding of the sperm cell to the zona pellucida (ZP). Primary ZP binding proteins are located at the apical plasma membrane of the sperm head. In order to exclusively study primary zona binding proteins, plasma membranes of sperm heads were isolated, highly purified and subsequently solubilized with a mild or a strong solubilization procedure. Native, highly purified ZP ghosts were used as the binding substrate for solubilized sperm plasma membrane proteins, and a proteomic approach was employed to identify ZP binding proteins. Two-dimensional gel electrophoresis of ZP fragments with bound sperm proteins showed very reproducibly 24 sperm protein spots to be associated to the zona ghosts after mild plasma membrane solubilization whereas only three protein spots were detected after strong plasma membrane solubilization. This indicates the involvement of multiple sperm proteins in ZP binding. The three persistently bound proteins were identified by a tandem mass spectrometry as isoforms of AQN-3 and probably represent the main sperm protein involved in ZP binding. P47, fertilin beta and peroxiredoxin 5 were also conclusively identified. None of the identified proteins has a known acrosomal origin, which further indicated that there was no sample contamination with secondary ZP binding proteins from the acrosomal matrix. In this study, we showed and identified multiple zona binding proteins involved in primary sperm-zona binding. Although we were not able to identify all of the proteins involved, this is a first step in understanding the event of primary sperm-zona interactions and the relevance of this for fertilization is discussed.

Separation, characterization and identification of boar seminal plasma proteins

Methods used for the isolation, separation and characterization of boar seminal plasma proteins are discussed, as well as techniques applied to study their binding properties. Attention is paid to interactions of these proteins with different types of saccharides and glycoconjugates, with membrane phospholipids, and to interactions between proteins. Boar seminal plasma contains different types of proteins: spermadhesins of the AQN and AWN families; DQH and PSP proteins belong to the most abundant. Some of these proteins are bound to the sperm surface during ejaculation and thus protein-coating layers of sperm are formed. Sperms coated with proteins participate in different types of interactions occurring in the course of the reproduction process, e.g. formation of the oviductal sperm reservoir, sperm capacitation, oocyte recognition and sperm binding to the oocyte.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update covering the period 1999-2000

This review describes the use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the analysis of carbohydrates and glycoconjugates and continues coverage of the field from the previous review published in 1999 (D. J. Harvey, Matrix-assisted laser desorption/ionization mass spectrometry of carbohydrates, 1999, Mass Spectrom Rev, 18:349-451) for the period 1999-2000. As MALDI mass spectrometry is acquiring the status of a mature technique in this field, there has been a greater emphasis on applications rather than to method development as opposed to the previous review. The present review covers applications to plant-derived carbohydrates, N- and O-linked glycans from glycoproteins, glycated proteins, mucins, glycosaminoglycans, bacterial glycolipids, glycosphingolipids, glycoglycerolipids and related compounds, and glycosides. Applications of MALDI mass spectrometry to the study of enzymes acting on carbohydrates (glycosyltransferases and glycosidases) and to the synthesis of carbohydrates, are also covered.

Zinc Ions Induce the Unfolding and Self-Association of Boar Spermadhesin PSP-I, a Protein with a Single CUB Domain Architecture, and Promote Its Binding to Heparin

Spermadhesins are a family of seminal plasma proteins composed of a single CUB domain, which appear to be involved in various aspects of the fertilization process in pigs. PSP-I and PSP-II, the most abundant porcine spermadhesins, occur in seminal plasma as noncovalent heterodimers devoid of heparin-binding capability. Of note is the stability of this dimer, which is significantly affected by physiologically relevant conditions such as Zn2+ ions. Here, we show that PSP-I and PSP-II when separated appear to conserve the overall fold of the CUB domain observed in the crystal structure of the PSP-I/PSP-II heterodimer, as concluded from gel filtration, analytical ultracentrifugation, differential scanning calorimetry, and circular dichroism analyses. However, Zn2+ concentrations in the range of those found in boar seminal plasma induce the unfolding and self-association of PSP-I, apparently as a consequence of the exposure of hydrophobic core residues, whereas they have no effect on PSP-II. Remarkably, Zn2+-denatured and self-associated (but not structured monomeric) PSP-I is retained on a heparin column, resembling the behavior of free PSP-I and homologous spermadhesins of the heparin-binding fraction of boar seminal plasma, which also exhibit different aggregation states. Thus, the modulation of the structural organization and heparin-binding ability of PSP-I by Zn2+ might be a physiological phenomenon in seminal plasma.

Capacitation-dependent concentration of lipid rafts in the apical ridge head area of porcine sperm cells

Lipid architecture of the plasma membrane plays an important role in the capacitation process of the sperm cell. During this process, an increase in membrane fluidity takes place, which coincides with a redistribution of cholesterol to the apical region of the head plasma membrane and subsequently an efflux of cholesterol. Cholesterol is also a major player in the formation of lipid rafts or microdomains in the membrane. Lipid rafts favour specific protein-protein interactions by concentrating certain proteins in these microdomains while excluding others. In this study, we investigated the organization of lipid rafts during in vitro capacitation of boar sperm cells. We report on the presence of the lipid raft-specific proteins caveolin-1 and flotillin-1 in sperm cells. Capacitation induced a change in membrane distribution of these proteins. Lipid analysis on detergent-resistant membranes (DRMs) of sperm cells indicated that capacitation induces a lipid raft concentration rather than a disintegration of lipid rafts, because the total amount of lipid in the DRM fraction remained unaltered. Using a proteomic approach, we identified several major DRM proteins, including proteins involved in capacitation-dependent processes and zona pellucida binding. Our data indicate that sperm raft reorganization may facilitate capacitation-specific signalling events and binding to the zona pellucida.

Characterization of human seminal plasma proteins homologous to boar AQN spermadhesins

Spermadhesins, proteins secreted by the boar sexual accessory glands, are believed to play an important role in sperm capacitation and primary contact of sperm and egg. We have previously found human seminal plasma proteins immunobiochemically related to boar AQN and AWN spermadhesins. In this study, we characterized further the AQN spermadhesin-related proteins, here designated as hSA (human spermadhesin-like) proteins. On Western blot, we immunodetected 14, 16 and 18 kDa forms of hSA proteins (hSA-14, hSA-16 and hSA-18, respectively) cross-reacting with rabbit antibody against AQN spermadhesins. Each relative molecular-mass form of hSA comprised three isoelectric isoforms (6.0, 6.8 and 8.4) as shown by 2D-PAGE. Glycoprotein analysis revealed that all hSA-16 and hSA-18 isoforms were N-glycosylated, and those of hSA-14 were non-glycosylated. Two isoforms of hSA-14 (pI 6.0 and 8.4) had affinity to heparin. Size-exclusion chromatography of human seminal plasma indicated that hSA proteins formed high molecular-mass complexes either with other hSA proteins or with seminal plasma lactoferrin and/or its fragments. Similarity of biochemical properties (relative molecular masses, isoelectric points and existence of non- and N-glycosylated forms) of hSA proteins and those of boar AQN spermadhesins, together with a previously described N-terminal amino acid sequence of one hSA protein identical to AQN spermadhesins, imply that hSA proteins are structurally related to boar AQN spermadhesins. However, localization of hSA proteins on the sperm tail and neck suggests that their biological role differs from that of boar AQN spermadhesins located on the sperm head.

Mutual Interactions of Boar Seminal Plasma Proteins Studied by Immunological and Chromatographic Methods

PROBLEM

Boar seminal plasma contains various types of proteins. Most of them belong to spermadhesins and some of them contain fibronectin type II domain. Almost all of these proteins are present under physiological conditions in aggregated forms differing in their relative molecular mass, composition, and binding properties. The study of mutual specific interactions between proteins of boar seminal plasma that could be involved in the formation of aggregated forms and most probably of sperm coating layers is the subject of our present communication.

METHODS OF STUDY

Aggregated forms of boar seminal plasma proteins separated by size exclusion chromatography were analysed by Reversed-phase High-performance Liquid Chromatography (RP HPLC), SDS-PAGE and immunochemical methods (ELISA and immunoblotting). Mutual interactions between proteins were investigated by size exclusion chromatography of a mixture of separated monomer proteins and affinity chromatography of boar seminal plasma on immobilized spermadhesins.

RESULTS

Composition of proteins that are adsorbed from boar seminal plasma to immobilized spermadhesins corresponds to that of aggregated forms found in seminal plasma. Mutual interactions between monomer forms observed by size exclusion chromatography are in good agreement with results of affinity chromatography. An existence of interactions between spermadhesins of the AQN and AWN families and the DQH sperm surface protein, as well as between proteins involved in the formation of heterodimer porcine seminal plasma I (PSP I)/PSP II was proved.

CONCLUSION

Mutual specific interactions between protein components of boar seminal plasma were shown. These interactions participate in the formation of aggregated forms of proteins in seminal plasma and probably also in the arrangement and remodelling of protein coating layers of sperm. Aggregation of seminal plasma proteins is probably an important phenomenon in the fertilization process.

Influence of porcine spermadhesins on the susceptibility of boar spermatozoa to high dilution

The effect of heparin-binding and non-heparin-binding spermadhesins on the viability, motility, and mitochondrial activity of boar spermatozoa at the high dilution (300,000 sperm/ml) to which sperm are exposed during the process of sex sorting by flow cytometry was investigated. Incubation of spermatozoa with heparin-binding spermadhesins caused a time- and dose-dependent decrease in the percentage of functional spermatozoa. The percentage of viable spermatozoa incubated at 38 degrees C with heparin-binding spermadhesins diluted in PBS (1 mg/ml) dropped from 75% (0.5 h) to 4% (5 h), whereas the percentage of viable spermatozoa incubated in PBS without proteins (control) decreased from 85% (0.5 h) to 19% (5 h). Addition of non-heparin-binding PSP-I/PSP-II spermadhesin to the PBS resulted in a concentration-dependent increment of the percentage of viable cells (65% after 5-h incubation), with maximum effect at 1.5 mg/ml. The heparin-binding spermadhesins totally suppressed sperm motility and mitochondrial activity after 5 h of incubation. The same parameters of sperm incubated in the presence of 1.5 mg/ml of PSP-I/PSP-II were 50% and 58%, respectively, and the percentages of control sperm displaying motility and mitochondrial activity were 21% and 26%, respectively. Moreover, the viability, motility, and mitochondrial activity all decreased on incubation of spermatozoa with mixtures of PSP-I/PSP-II and heparin-binding spermadhesins as the concentration of the latter increased. We conclude that PSP-I/PSP-II and the heparin-binding spermadhesins exert antagonistic effects on the functionality of highly diluted boar spermatozoa. The finding that PSP-I/PSP-II contributes to maintaining sperm with high viability, motility, and mitochondrial activity for at least 5 h at physiological temperature points to its potential use as an additive for sperm preservation, specifically of highly diluted, flow-sorted spermatozoa for sex preselection.

Affinity chromatography of bull seminal proteins on mannan-Sepharose

The interaction of bull seminal plasma proteins and sperm with mannan was investigated using an enzyme-linked binding assay (ELBA). A high mannan-binding activity was found in the protein fraction interacting with heparin. Mannan binding to seminal plasma proteins was inhibited by D-mannose and D-fructose, but not by D-mannose-6-phosphate, D-glucose-6-phosphate, ovalbumin and ovomucoid. Mannan inhibited the binding of bovine zona pellucida glycoproteins both to bull sperm and seminal plasma proteins. Yeast mannan immobilized to divinyl sulfone-activated Sepharose was used for the isolation of mannan-binding proteins. The protein components of this fraction were identified on the basis of relative molecular mass determination and N-terminal amino acid sequencing: RNAase dimer, PDC-109 and a protein homologous to BSP-30K (relative molecular mass 14,500). The isolated proteins were characterized by a high zona pellucida binding activity.

Expression of spermadhesin genes in porcine male and female reproductive tracts

Spermadhesins, secretory proteins of the male genital tract, constitute the bulk of seminal plasma proteins in pig. The nucleotide sequences of spermadhesins AWN, AQN-1, and AQN-3 from the cDNAs, derived from seminal vesicles, were determined. The coding sequences of spermadhesins revealed an overall sequence similarity of 40% at nucleotide level. Expression of spermadhesins (AWN, AQN-1, AQN-3, PSP-I, and PSP-II) in porcine male and female reproductive tracts were studied by means of RT-PCR and immunological approaches. All spermadhesins are transcribed and translated in seminal vesicles and prostate. In caudal epididymis mRNA transcripts of all spermadhesins have been detected by RT-PCR. PSP-I showed additional signals in caput epididymis and rete testis. Translation can be detected only for AWN in tissue extracts by SDS-PAGE and Western blotting. Remarkably, AWN is the only spermadhesin that is also expressed in the uterus, the uterotubal junction, and the oviduct of the female genital tract as shown by RT-PCR, cDNA-sequencing, and immunological analysis. In sows at estrus and interestrus, in gilts and gilts 12 hr after insemination no obvious differences were noticed in the pattern of AWN-immunoreactivity in epithelial cells either of the uterotubal junction, isthmus, or ampulla. While strong staining was observed in the superficial uterine glands and in the glands of the uterotubal junction during estrus, in diestrus this declined distinctly. The role of spermadhesins in relation to their expression is discussed.

Purification and primary structure of a new bovine spermadhesin

Z13 is a new seminal plasma protein made up of two disulfide-linked 13-kDa subunits that was identified in our laboratory by 2D PAGE. In this report we present the purification of Z13 from bovine seminal plasma. In solution, the protein is a nonglycosylated dimer that presents one interchain disulfide bond and does not show heparin-binding properties. The complete primary structure and the localization of the S-S bridges are reported. The results suggest that Z13 is a new protein of the spermadhesin family whose members are thought to play a prominent role in different aspects of fertilization.

Structural characterization of the oligosaccharide chains of native and crystallized boar seminal plasma spermadhesin PSP-I and PSP-II glycoforms

The PSP-I/PSP-II heterodimer is the major protein of boar seminal plasma. Both subunits are glycoproteins of the spermadhesin family and each contains a single N-glycosylation site. After enzymatic release of the oligosaccharides from isolated PSP-I and PSP-II, mainly neutral and monosialylated oligosaccharides, and small amounts of disialylated oligosaccharides, were recovered from both proteins. Twenty-two neutral oligosaccharides, 11 monosialylated glycans and three disialylated carbohydrate chains were characterized using mass spectrometric and NMR techniques. PSP-I and PSP-II share the same glycans but differ in their relative molar ratios. Most glycan structures are proximally alpha1-6-fucosylated, diantennary complex-type bearing nonsialylated or alpha2-6-sialylated N-acetyllactosamine or di-N-acetyllactosamine antennae. The majority of nonsialylated N-acetyllactosamine antennae bear terminal alpha1-3-linked Gal residues. In addition, the N-acetylglucosamine residue of nonsialylated N-acetyl and di-N-acetyllactosamine antennae can be modified by an alpha1-3-linked fucose residue. Structures of higher antennarity, as well as structures 3,6-branched at galactose residues, were found in smaller amounts. In one oligosaccharide, N-acetylneuraminic acid is substituted by N-glycolylneuraminic acid. Mass spectrometric analysis of PSP-I and PSP-II glycoforms isolated from crystallized PSP-I/PSP-II heterodimer showed the coexistence of major PSP-I and PSP-II glycoforms in the hexagonal crystals. Oligosaccharides with the NeuNAcalpha2-6GalNAcbeta1-4GlcNAc-R motif block adhesive and activation-related events mediated by CD22, suggesting a possible immunoregulatory activity for PSP-I/PSP-II.

Characterization of boar spermadhesins by monoclonal and polyclonal antibodies and their role in binding to oocytes

PROBLEM

The role of Ala-Trp-Asn (AWN) and Ala-Gln-Asn (AQN) families of spermadhesive sperm proteins in fertilization.

METHOD OF STUDY

The preparation and characterization of polyclonal antibodies against AWN and AQN spermadhesins and one monoclonal antibody (MAb), designated Bo.5, against AWN spermadhesin. The use of biochemical and immunocytochemical methods for characterization of spermadhesins on the sperm membrane of boar spermatozoa and in the cryostat sections of boar reproductive organs.

RESULTS

Polyclonal anti-AWN and anti-AQN antibodies specifically reacted with AWN and AQN proteins, respectively. MAb Bo.5 detected the 17-, 16-, and 14-kDa protein members of AWN subfamily. The monoclonal, as well as the polyclonal, AWN antibodies remarkably decreased the sperm binding to the egg surface in an in vitro sperm zona pellucida binding assay.

CONCLUSIONS

Presented results demonstrate that polyclonal antibodies and MAb Bo.5 against spermadhesins specifically recognize the membrane-associated antigens and inhibit the binding of sperm to oocytes. Reduced binding of sperm to oocytes, due to the antibodies, indicates the role of these spermadhesins in sperm-egg primary binding.

Spermadhesins: a new protein family. Facts, hypotheses and perspectives

Spermadhesins are a novel family of secretory proteins expressed in the male genital tract of pig, horse and bull. They are major products of the seminal plasma and have been found to be peripherally associated to the sperm surface. The structure and function of spermadhesins have been thoroughly investigated in the pig, which exhibits the greatest diversity of members: AWN, AQN-1, AQN-2, PSP-I and PSP-II and its glycosylated isoforms. They are multifunctional proteins showing a range of ligand-binding abilities, e.g. carbohydrates, sulfated glycosaminoglycans, phospholipids and protease inhibitors, suggesting that they may be involved in different steps of fertilization. Isolated porcine spermadhesins bind the zona pellucida glycoproteins in a cation-dependent manner with a Kd in a low micromolar range, and AWN, AQN-1 and AQN-3 display similar binding affinity for glycoproteins containing Gal beta(1-3)-GalNAc and Gal beta(1-4)-GlcNAc sequences in O-linked and N-linked oligosaccharides, respectively. During sperm passage through the epididymis AQN-3 and AWN have been shown to bind tightly to the sperm surface by interaction with the phospholipids of the membrane bilayer. At ejaculation the spermadhesins form a protective coat around the sensitive acrosomal region of the sperm head, thus possibly preventing premature acrosome reaction. During in vitro capacitation most of these aggregated sperm adhesins are lost, with the exception of phospholipid-bound spermadhesins. AWN and AQN-3 may now serve as a primary receptor for the oocyte zona pellucida, thus contributing to initial binding and recognition between sperm and egg. The amino acid sequence of spermadhesins does not show any discernible similarity with known carbohydrate recognition domains (CRD). However, they belong to the superfamily of proteins with a CUB domain with a predicted all-beta structure. The crystal structure of the heterodimeric complex of the spermadhesins PSP-I/PSP-II has been solved, showing that the overall structure of both spermadhesins consists of a beta-sandwich with five (parallel and antiparallel) beta-strands. It is the first three-dimensional structure of a zona pellucida-binding protein and reveals the architecture of the CUB domain. The spermadhesins represent a novel class of lectins that may be involved in sequential steps of fertilization, at least in the pig.

Binding of mannose-6-phosphate and heparin by boar seminal plasma PSP-II, a member of the spermadhesin protein family

PSP-I/PSP-II, a heterodimer of glycosylated spermadhesins, is the major component of boar seminal plasma. Similarly to other spermadhesins, the PSP-II subunit is a lectin which displays heparin- and zona pellucida glycoprotein-binding activities. We have investigated the ligand binding capabilities of the heterodimer and the isolated subunits using several polysaccharides, glycoproteins, and phospholipids. PSP-II binds the sulfated polysaccharides heparin and fucoidan in a dose-dependent and seemingly-specific manner. In addition, PSP-II binds oligosaccharides containing exposed mannose-6-phosphate monoester groups and the binding is selectively inhibited by mannose-6-phosphate and glucose-6-phosphate. Inhibition experiments indicate that binding of PSP-II to sulfated polysaccharides and mannose-6-phosphate-containing oligosaccharides involves distinct but possibly overlapping binding sites. Heterodimer formation with PSP-I abolishes both the heparin and the mannose-6-phosphate binding capabilities, suggesting that the corresponding sites may be located at the dimer interface. Using the crystal structure of PSP-I/PSP-II heterodimer as a template, we have explored possible binding sites which satisfy the observed binding characteristics. In the proposed models, PSP-II Arg43 appears to play a pivotal role in both heparin- and mannose-6-phosphate-complexation as well as in heterodimer formation.

A cDNA cloned from pregnant mouse uterus exhibits temporo-spatial expression and predicts a novel protein

A cDNA was cloned from a pregnant mouse uterus cDNA library. On conceptual translation, the cDNA has one long open reading frame that predicts a novel protein of 606 amino acids. This protein is principally composed of two CUB domains and a ZP domain; motifs found in proteins implicated in egg-sperm recognition. Probes derived from the cDNA were used to conduct Northern hybridizations. The expression of this mRNA is temporal; message first appears in the uterus 6 days prior to birth, it increases each subsequent day to attain maximal levels at 3 days prior to birth and then abruptly decreases during the last 3 days of pregnancy. The expression of this mRNA is restricted; message is abundant in the uterus during late pregnancy, but it is not found in non-pregnant uterus or in a variety of adult or fetal tissues. The temporo-spatial expression of this pregnant uterus specific mRNA and the consolidation in the predicted protein of two motifs implicated in early pregnancy events suggests that the product of the gene represented by this mRNA may play an important role in events that transpire during late pregnancy.

The 2.4 A resolution crystal structure of boar seminal plasma PSP-I/PSP-II: a zona pellucida-binding glycoprotein heterodimer of the spermadhesin family built by a CUB domain architecture

The crystal structure of porcine seminal plasma spermadhesin PSP-I/PSP-II heterodimer has been determined in two crystal forms by multiple isomorphous replacement in an hexagonal crystal (space group P6(1)22) and molecular replacement in a trigonal crystal of space group P3(2)21. The crystal structure has been refined at 2.4 A resolution to an R-factor of 20.0% (Rfree = 25.9%) for 14,809 independent reflections with intensities greater than 2 sigma (I), with root-mean-square deviations of 0.009 A and 1.657 degrees from ideal bond lengths and bond angles, respectively. The final model includes 1688 non-hydrogen protein atoms of 221 amino acids and 79 water molecules. PSP-I/PSP-II represents the first crystal structure of a mammalian zona pellucida-binding protein. PSP-II displays a putative carbohydrate-recognition site located around its Asn50. This region shares structural features with sugar binding sites of known lectin structures of the leguminous and galectin families. PSP-I and PSP-II are N-glycosylated at asparagine residues 50 and 98, respectively, and show site heterogeneity. Only the innermost N-acetylglucosamine of PSP-I is defined in the crystal structure. Both subunits of the PSP-I/PSP-II heterodimer are built by a single CUB domain architecture. The CUB domain displays a novel fold, which consists of a compact ellipsoidal beta-sandwich structure (42 A x 27 A x 23 A) organized into two 5-stranded beta-sheets. Each sheet contains parallel and antiparallel beta-strands. Two disulphide bridges, which are conserved in all spermadhesin molecules and many CUB domains, crosslink loop LA and strand beta 4 and loops LE and LG, respectively, at opposite edges of the same face of the domain. The four highly conserved aromatic residues and 15 out of 17 invariant hydrophobic residues, which define the CUB domain signature, display an interior location, suggesting that this hydrophobic core may be essential for maintaining the overall folding of the domain. Most of the hydrophobic core residue characteristics are conserved in the jellyroll topology of certain icosahedral virus capsid proteins, indicating that the CUB domain and the viral proteins share a minimal structural core.

Crystal structure of acidic seminal fluid protein (aSFP) at 1.9 A resolution: a bovine polypeptide of the spermadhesin family

We report the three-dimensional crystal structure of acidic seminal fluid protein (aSFP), a 12.9 kDa polypeptide of the spermadhesin family isolated from bovine seminal plasma, solved by the multiple isomorphous replacement method and refined with data to 1.9 A resolution with a final R-factor of 17.3%. aSFP is built by a single CUB domain architecture, a 100 to 110 amino-acid-residue extracellular module found in 16 functionally diverse proteins. The structure of aSFP reveals that the CUB domain displays a beta-sandwich topology organised into two 5-stranded beta-sheets, each of which contain two parallel and four antiparallel strands. The structure of aSFP is almost identical to that of porcine spermadhesins PSP-I and PSP-II, which in turn show limited structural similarity with jellyroll topologies of certain virus capsid proteins. Essentially, topologically conserved residues in these proteins are those internal amino acids forming the hydrophobic core of the CUB and the jellyroll domains, suggesting their importance in maintaining the integrity of these protein folds. On the other hand, the structure of aSFP shows structural features that are unique to this protein and which may provide a structural ground for understanding the distinct biological properties of different members of the spermadhesin protein family.

The role of apocrine released proteins in the post-testicular regulation of human sperm function

A unifying hypothesis is presented postulating an apocrine release of several seminal proteins which mix and reaggregate in seminal fluid, thereby eventually forming particles designated either as "prostasomes", "vesiculosomes" or "seminosomes". The term "aposomes" should be restricted to the blebs released from secretory cells in the rat dorsal prostate and coagulating gland. Three different proteins present in human seminosomes along with the respective antibodies have been used to identify the localization, function and hypothetical interaction with spermatozoa. The proteins were (1) seminal vesicle-derived fibronectin, (2) prostate-derived 5'-nucleotidase and (3) a hitherto unidentified 100 kD membrane protein from epididymis, seminal vesicle and prostate. I. Fibronectin is an extracellular matrix protein which is also secreted from the seminal vesicles participating in the formation of the seminal clot. Immunofluorescence and immunoelectron microscopy revealed a relatively broad distribution pattern of fibronectin immunoreactivity on spermatozoa from different donors. Adding a fibronectin antiserum at a moderate dilution to vital spermatozoa in vitro resulted in a significant increase in sperm motility. Purified plasma fibronectin added at various concentrations to a vital sperm preparation was found to inhibit sperm motility in a dose-dependent manner. Measurement of calcium fluxes in individual sperm in the presence of fibronectin showed a significant increase. These findings point to a possible post-testicular regulatory function of seminal fibronectin. 2.5'-Nucleotidase (5'-NT) is an enzyme that hydrolyzes nucleotides such as AMP or IMP into inorganic phosphate and the respective nucleoside. The highest amount and activity of 5'-nucleotidase was present in glandular cells of the prostate; much less was detected in seminal vesicles and epididymis. On spermatozoa, the enzyme was localized on the outer leaflet of the plasma membrane covering the acrosomal region. Addition of purified enzyme to an in vitro incubation system of spermatozoa had no effect on sperm motility. A slight reduction of overall motility, however, was observed after addition of 5'-NT antibody to the spermatozoa. When 5'-nucleotidase inhibitors and adenosine channel antagonists were added to the sperm incubation system, a clear-cut inhibition of sperm motility occurred in a dose-dependent manner. This result is interpreted as indicating a significant role of ecto-5'-nucleotidase in the regulation of sperm motility. 3. A polyvalent antiserum against native human prostasomes recognized antigens in the range of 10-14 kD and of approximately 100 kD, respectively, in seminal fluid and prostate homogenates. Immunohistochemical studies revealed the presence of respective antigens in the epididymis, seminal vesicles and the prostate. Immunoelectron microscopy of ultracryo-sections showed labeling both of the apical plasma membrane in the prostate, as well as intraluminal secretory particles indicating the apocrine i.e. plasma-membrane bounded release of these particles. The secretory elements are termed "seminosomes". An affinity-purified fraction within the antiserum recognizes a 100 kD protein which is present both in the apical plasma membrane of the male genital glands, but also in the sperm head and principal piece of human spermatozoa. Incubation of spermatozoa with seminosomes and the respective purified antiserum had no effect on sperm motility. This is in contradistinction to former reports on motility increase induced by the so-called prostasomes.

Molecules involved in mammalian sperm-egg interaction

To achieve fertilization, sperm and egg are equipped with specific molecules which mediate the steps of gamete interaction. In mammals, the first interaction between sperm and egg occurs at an egg-specific extracellular matrix, the zona pellucida (zp). The three glycoproteins, ZP1, ZP2, and ZP3, that comprise the zp have been characterized from many species and assigned different roles in gamete interaction. A large number of candidate-binding partners for the zp proteins have been described; a subset of these have been characterized structurally and functionally. Galactosyltransferase, sp56, zona receptor kinase, and spermadhesins are thought to participate in the primary binding between sperm and zp and may initiate the exocytotic release of hydrolytic enzymes in the sperm head, the acrosome reaction. Digestion of the zp by these enzymes enables sperm to traverse the zp, at which time the proteins PH20, proacrosin, sp38, and Sp17 are thought to participate in secondary binding between the acrosome-reacted sperm and zp. Once through the zp, sperm and egg plasma membranes meet and fuse in a process reported to involve the egg integrin alpha 6 beta 1 and the sperm proteins DE and fertilin. These molecules and the processes involved in gamete interaction are reviewed in this chapter within a physiological context.

The crystal structures of two spermadhesins reveal the CUB domain fold

Spermadhesins, 12,000-14,000 M(r) mammalian proteins, include lectins involved in sperm-egg binding and display a single CUB domain architecture. We report the crystal structures of porcine seminal plasma PSP-I/PSP-II, a heterodimer of two glycosylated spermadhesins, and bovine aSFP at 2.4 A and 1.9 A resolution respectively.

Localization of adhesion molecules on human spermatozoa by fluorescence microscopy

The expression of adhesion molecules on human spermatozoa of healthy probands was analysed. The localization patterns of adhesion molecules (AM) on the spermatozoal surface were documented by fluorescence microscopy. Spermatozoa were incubated with antibodies against alpha 1 (CD49a), alpha 2 (CD49b), alpha 3 (CD49c), alpha 4 (CD49d), alpha 5 (CD49e), alpha 6 (CD49f) chains of beta 1 integrins, beta 1 (CD29), beta 2 (CD18), alpha V (CD51), beta 3 (CD61) and beta 4 integrin chains, the LFA-3 (Lymphocyte function antigen, CD58) from the immunoglobulin superfamily and the extra-cellular matrix proteins laminin, fibronectin and collagen IV. For collagen IV, alpha 1 and alpha 2 chains no expression could be noticed. Laminin was detected at the acrosomal membrane, fibronectin and beta 4 chain mainly at the equatorial membrane. The fibronectin receptors alpha 3, alpha 4 and alpha 5 chains of the beta 1 integrins were mainly located on acrosomal and equatorial membrane areas. Laminin receptor alpha 6 chain was located postacrosomal and less frequently acrosomal. beta 2 chain and vitronectin receptors alpha V and beta 3 chains had a mainly postacrosomal localization pattern. LFA-3 was found constantly on postacrosomal membrane areas. Double staining technique was used to prove the simultaneous occurrence of fibronectin and its integrin receptors alpha 3, alpha 4 and alpha 5 chains and of alpha V and beta 2 chains on spermatozoa. The localization patterns of integrins on double stained spermatozoa were similar to the patterns described for single stained spermatozoa. The localization of fibronectin appeared to be influenced by the presence of integrins: the typical equatorial fibronectin band disappeared in case of an equatorial localization of integrins.

Binding of boar spermatozoa to porcine oocytes: effect of low molecular weight 17-kDa protein

The effect of seminal plasma and low molecular weight ACR.3 (17 kDa) protein on boar spermatozoa-porcine oocyte binding was examined. Boar seminal plasma that contains the sperm-adhesive ACR.3 protein was added to spermatozoa prior to their coincubation with oocytes, and the binding capacity of the spermatozoa so treated was compared to that of untreated cells. Similarly, purified ACR.3 protein, that binds to the egg zona pellucida, was added to noncapacitated spermatozoa, and the binding capacity of treated and untreated cells was again evaluated. In the two cases, the treatment of spermatozoa reduced their capacity to bind to the zona pellucida. We propose that the reduction in binding is due to competition for the ACR.3 binding sites on the zona pellucida between the soluble ACR.3 protein and the ACR.3 protein attached to the sperm surface. Furthermore, sperm-ZP binding was examined in the presence of ACR.3 monoclonal antibody, which specifically reacts with ACR.3 protein. Preliminary results show that addition of ACR.3 monoclonal antibody to a suspension of boar spermatozoa prior to their coincubation with oocytes did not markedly change sperm-zona binding in comparison with the control untreated spermatozoa. Our results suggest that ACR.3 protein may mediate the primary sperm-egg zona pellucida binding, and that it is one of the likely candidates for the primary sperm-ZP binding protein.

Boar spermadhesins AQN-1 and AQN-3: oligosaccharide and zona pellucida binding characteristics

AQN-1 and AQN-3 form part of the complement of surface-associated lectins which coat the plasma membrane overlying the acrosomal cap of in vitro capacitated boar spermatozoa. They belong to the spermadhesin protein family and have binding affinity for glycoconjugates of the zona pellucida, the extracellular investment surrounding mammalian eggs. The oligosaccharide and zona pellucida binding characteristics of spermadhesins AQN-1 and AQN-3 were investigated using a solid-phase assay and biotinylated glycoprotein ligands. Both sperm proteins bind glycoproteins containing Gal beta (1-4)-GlcNAc and Gal beta-(1-3)-GalNAc oligosaccharide sequences with dissociation constants (Kd) of 0.08 to 0.8 microM, and to zona pellucida glycoproteins with Kd = 0.15-0.25 microM. However, 5-N-acetyl neuraminic acid alpha (2-3/6)-linked to the galactose residue decreases the affinity of glycosylated ligands to AQN-1 three-fold, although it did not affect oligosaccharide binding to AQN-3. In addition, AQN-3 binds preferentially to glycoproteins with either a linear or tri- and tetraantennary carbohydrates than to those containing diantennary N-acetyllactosamine structures. The similar but distinct oligosaccharide recognition capabilities of spermadhesins AQN-1 and AQN-3 (this work) and AWN-1 (Dostálová, Z, Calvete, J.J., Sanz, L., and Töpfer-Petersen, E. (1995) Eur. J. Biochem. 230, 329-336) suggest that, in the pig, sperm-zona pellucida binding might be mediated by lectins displaying similar although distinct carbohydrate-recognition abilities.

Crystallization and preliminary X-ray diffraction analysis of boar seminal plasma spermadhesin PSP-I/PSP-II, a heterodimer of two CUB domains

Boar spermadhesin PSP-I/PSP-II (M(r) 29 000-30 000), a non-covalent heterodimer of two CUB domains, was crystallized in two crystal forms. Complete diffraction data sets for hexagonal (space group P6(1,5)22) and trigonal (space group P3(1,2)21) crystals have been collected up to 2.9 and 2.5 angstrom resolution, respectively. Cell constants of the hexagonal and trigonal crystal forms are a=b=87.2 angstrom, c=152.4 angstrom, and a=b=96.2 angstrom, c=70.8 angstrom, respectively. The calculated packing parameters (Vm) are 2.8 and 3.2 angstrom(3)/DA for the hexagonal and trigonal crystal forms, respectively, indicating that, in both cases, the asymmetric unit is constituted by one PSP-I/PSP-II heterodimer. This paper reports the first crystals of a protein built up by a CUB domain architecture.

Mapping the heparin-binding domain of boar spermadhesins

Boar spermadhesins are a group of seminal plasma, heparin-binding proteins which appear to be involved in sperm capacitation and gamete interaction. Using a proteolytic protection assay we have identified regions of AQN-1, AQN-3, PSP-I and AWN which remain attached to a heparin-Sepharose column following in-column digestion of bound spermadhesins with chymotrypsin and elastase. In addition, the complete amino acid sequence of spermadhesin AWN was synthesized as overlapping peptides, and their ability to bind to a heparin-Sepharose column and to inhibit the interaction of soluble heparin with purified ELISA plate-coated AWN was tested. Both approaches gave similar results and as a whole showed that different regions of AWN may converge in its tertiary structure to form a composite heparin-binding site. The conformational heparin-binding surface resides on the GFCC'C'' face of the proposed structural model for AWN and is in an opposite location to the carbohydrate-binding region of the spermadhesin.

Analysis of the structural organization and thermal stability of two spermadhesins. Calorimetric, circular dichroic and Fourier-transform infrared spectroscopic studies

The CUB domain is a widespread 110-amino-acid module found in functionally diverse, often developmentally regulated proteins, for which an antiparallel beta-barrel topology similar to that in immunoglobulin V domains has been predicted. Spermadhesins have been proposed as a subgroup of this protein family built up by a single CUB domain architecture. To test the proposed structural model, we have analyzed the structural organization of two members of the spermadhesin protein family, porcine seminal plasma proteins I/II (PSP-I/PSP-II) heterodimer and bovine acidic seminal fluid protein (aSFP) homodimer, using differential scanning calorimetry, far-ultraviolet circular dichroism and Fourier-transform infrared spectroscopy. Thermal unfolding of PSP-I/PSP-II and aSFP were irreversible and followed a one-step process with transition temperatures (Tm) of 60.5 degrees C and 78.6 degrees C, respectively. The calorimetric enthalpy changes (delta Hcat) of thermal denaturation were 439 kJ/mol for PSP-I/PSP-II and 660 kJ/mol for aSFP dimer. Analysis of the calorimetric curves of PSP-I/PSP-II showed that the entire dimer constituted the cooperative unfolding unit. Fourier-transform infrared spectroscopy and deconvolution of circular dichroic spectra using a convex constraint analysis indicated that beta-structure and turns are the major structural element of both PSP-I/PSP-II (53% of beta-sheet, 21% of turns) and aSFP (44% of beta-sheet, 36% of turns), and that the porcine and the bovine proteins contain little, if any, alpha-helical structure. Taken together, our results indicate that the porcine and the bovine spermadhesin molecules are probably all-beta-structure proteins, and would support a beta-barrel topology like that predicted for the CUB domain. Other beta-structure folds, such as the Greek-key pattern characteristic of many carbohydrate-binding protein domains cannot be eliminated. Finally, the same combination of biophysical techniques was used to characterize the residual secondary structure of thermally denatured forms of PSP-I/PSP-II and aSFP, and to emphasize the aggregation tendency of these forms.

Effect of glycosylation on the heparin-binding capability of boar and stallion seminal plasma proteins

Boar and stallion seminal plasmas were fractionated using affinity chromatography on heparin-Sepharose. In both species, among other proteins, the heparin-binding (H+) and non-heparin-binding (H-) fractions each contained glycoforms of either porcine PSP-I or equine HSP-1 and HSP-2. However, porcine H+/PSP-I eluted as a monomeric protein, whereas H-/PSP-I formed a heterodimer with PSP-II, another major seminal plasma protein. On the other hand, the stallion proteins H+/HSP-1 and H+/HSP-2 eluted together as an aggregate of relative molecular mass (M(r)) 90,000, whereas H-/HSP-1 and H-/HSP-2 eluted as monomers (15,000). Remarkably, when PSP-I and PSP-II from the H- fraction were separated, both proteins bound to heparin. Altogether these data show that glycosylation has an indirect effect on the heparin-binding ability of PSP-I, HSP-1 and HSP-2 through modulation of their aggregation state.

Cloning of boar SPMI gene which is expressed specifically in seminal vesicle and codes for a sperm motility inhibitor protein

Boar semen contains a seminal plasma motility inhibitor (SPMI) that blocks the motility of demembranated-reactivated spermatozoa as well as of intact spermatozoa. In this paper, we describe the primary structure of SPMI, the coding of boar SPMI cDNA gene and its expression in various porcine tissues. Nucleotide sequence analysis of the 645-bp SPMI cDNA predicts a coded polypeptide of 137 amino acid residues which includes a 21-residue signal peptide and a 116-residue secreted protein. The amino acid sequence of SPMI was found to be highly homologous to AQN-3, a member of spermadhesin family proteins of boar that bind to spermatozoa. Expression of the boar SPMI gene detected by Northern blot analysis revealed that its expression is very abundant in seminal vesicles and specific to this tissue.

Boar spermadhesin PSP-II: location of posttranslational modifications, heterodimer formation with PSP-I glycoforms and effect of dimerization on the ligand-binding capabilities of the subunits

Spermadhesin PSP-II was isolated from the non-heparin-binding fraction of boar seminal plasma; its disulphide bridge pattern, and the location of a single N-glycosylation site were established. PSP-II forms a heterodimer with specific N-glycoforms of PSP-I. Although both subunits possess heparin-binding capability, the PSP-I/PSP-II complex does not. The heterodimer contains binding sites for zona pellucida glycoproteins and soybean trypsin inhibitor located in the PSP-II subunit. However, the PSP-I/PSP-II heterodimer binds only loosely to the sperm surface and is easily removed during in vitro capacitation, suggesting that the zona pellucida binding activity may not be relevant for gamete interaction. Our results show that dimerization of spermadhesins PSP-I and PSP-II markedly affects their binding capabilities.

Boar spermadhesin AWN-1. Oligosaccharide and zona pellucida binding characteristics

We have analyzed the oligosaccharide recognition ability of boar spermadhesin AWN-1 using biotinylated glycoproteins with defined carbohydrate chains as probes. Our results show that AWN-1 bound to proteins containing O-linked NeuAc alpha(2-3/6)-Gal beta(1-3)-GalNAc (PDC-109 and fetuin) with a Kd = 0.7 microM. AWN-1 also bound to NeuAc alpha(2-3/6)-Gal beta(1-4)-GlcNAc sequences in N-linked triantennary structures of fetuin, but not to the same oligosaccharide in the diantennary structures of IgG or fibrinogen. The absence of terminal sialic acid decreased fivefold the binding affinity. By competitive ELISA, peptides containing the N-linked oligosaccharide sequence inhibited the binding of the parent glycoprotein to immobilized AWN-1 5-45 times less effectively than those carrying O-linked NeuAc alpha(2-3/6)-Gal beta(1-3)-GalNAc structures. In addition, AWN-1 bound with a Kd = 0.3 microM to solubilized, biotinylated porcine zona pellucida glycoproteins. PDC-109 competed effectively with zona pellucida glycoproteins for AWN binding, whereas fetuin was a poor competitor. On the other hand, AWN epitopes were demonstrated on in vitro capacitated boar spermatozoa which were able to bind to, and penetrate, zona-encased oocytes. These data indicate that spermadhesin AWN-1 may play a role in pig fertilization as a sperm-associated lectin of broad specificity though preferential affinity for certain O-linked oligosaccharide structures of the oocyte's zona pellucida.

Interaction of non-aggregated boar AWN-1 and AQN-3 with phospholipid matrices. A model for coating of spermadhesins to the sperm surface

Boar spermadhesins are 12-14 kDa lectins which coat the entire acrosomal cap sperm head surface. The large amount of spermadhesins AQN-1, AQN-2, AQN-3, and AWN-1 present on in vitro capacitated spermatozoa (approximately 7 x 10(6) molecules of each spermadhesin per cell) suggested that they may bind to major component(s) of the sperm surface. We have investigated both the aggregation state of spermadhesins in seminal plasma using gel filtration chromatography, and their ability to bind to the major phospholipids of the boar sperm plasma membrane, i.e. phosphorylcholine and phosphorylethanolamine. The bulk (90%) of spermadhesins AQN-3 and AWN-1 were eluted as aggregated proteins (Mr > 50,000) with the void volume of a Sephadex G-50 column; the remaining 10% of the total amount of seminal plasma AWN-1 and AQN-3 were recovered, together with the whole amount of AQN-1 and AQN-2, in a fraction containing low-molecular-mass proteins (Mr 16,000-30,000). None spermadhesin of either gel-filtration fraction bound to a phosphorylcholine affinity matrix. On the other hand, low-molecular-mass (monomeric or dimeric) AQN-3 and AWN-1 were the only spermadhesins retained in a phosphorylethanolamine affinity column. Both AQN-3 and AWN-1 purified from seminal plasma by reverse-phase HPLC retained their lipid-binding capability. In addition, immobilization of AQN-3 and AWN-1 onto a phosphorylethanolamine matrix did not interfere with the ability of the proteins to bind bovine glycoprotein PDC-109, indicating that the structural determinants for the binding lipid and carbohydrates lay on different structural domains.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunolocalization and quantitation of acidic seminal fluid protein (aSFP) in ejaculated, swim-up, and capacitated bull spermatozoa

Acidic seminal fluid protein (aSFP, 12.9 kDa), a major protein of bull seminal plasma, belongs to the spermadhesin protein family. Boar spermadhesins become bound to the sperm head's surface at ejaculation and are thought to play a role as capacitation factors and/or in gamete recognition and binding. Here, we have investigated the topographical distribution and fate of bovine spermadhesin aSFP during sperm capacitation in order to assess whether aSFP could be involved in similar aspects of the fertilization process as its boar homologous proteins. 5.7 +/- 2.1 x 10(6) molecules/spermatozoa were quantitated on the surface of fresh ejaculated and washed sperm. The binding site of aSFP was restricted to a thin coat at the apical part of the acrosomal cap. The amount of aSFP in swim-up sperm was 1.8 +/- 1.0 x 10(6) molecules/spermatozoa, but decreased dramatically to 22 +/- 10 x 10(3) and to undetectable levels after incubation of sperm for 1.5 h and 18 h, respectively, in capacitation medium. This indicates that the bull spermatozoa surface may be completely depleted of spermadhesin aSFP before spermatozoa reach the surroundings of the investing egg. Therefore, our results suggest that aSFP may act as a decapacitation factor on bull spermatozoa rather than as a zona pellucida binding molecule.

Bovine seminal plasma aSFP: localization of disulfide bridges and detection of three different isoelectric forms

Acidic seminal fluid protein (aSFP) is a major 13 kDa protein isolated from bull seminal plasma and characterized as a new growth factor which stimulates in vitro cell division and progesterone secretion by ovarian cells. Here, we establish that the four cysteines of oxidized aSFP form two disulfide bridges between nearest-neighbour residues. This pattern is conserved in boar spermadhesins, with which aSFP shares up to 50% amino acid sequence identity, and other proteins of the recently identified CUB domain family. Using isoelectric focusing in combination with sulfhydryl group-specific blotting, the three forms of aSFP were identified as completely oxidized (pI 4.7), partly reduced (pI 4.8) and fully reduced at pI 5.1. These results indicate that native aSFP possesses two pairs of cysteine residues of different reactivity. The observation that aSFP can protect sperm from oxidative damage might be explained by its reduction/oxidation behaviour.