Radioimmunoassays for bull seminal plasma proteins (BSP-A1/-A2, BSP-A3, and BSP-30-Kilodaltons), and their quantification in seminal plasma and sperm

Three proteins, BSP-A1/-A2, BSP-A3, and BSP-30 kilodaltons (collectively called BSP proteins), represent the major proteins of bovine seminal plasma (BSP). At ejaculation, these proteins bind to the sperm surface and induce molecular changes in the plasma membrane that are deemed to be essential for sperm capacitation. The present study was carried out to develop specific radioimmunoassays (RIAs) for the quantification of each of the BSP proteins in BSP and sperm. RIAs were developed using polyclonal antibodies raised in rabbits against each BSP protein. The purified and iodinated BSP proteins were used as standard and tracer, respectively. The RIAs that were developed were shown to be specific for each protein and the cross-reactivity toward various antigens was negligible (<2%). The average sensitivity limit was 5 ng/ml of sample for BSP-A1/-A2 and BSP-A3, and 40 ng/ml of sample for BSP-30-kDa. The concentration of BSP proteins was determined by analyzing the RIA data with spline function. BSP proteins represented 40% to 57% of seminal plasma total protein (25% to 47% of BSP-A1/-A2, 3% to 5% of BSP-A3, and 3% to 7% of BSP-30 kDa) and 4% to 6% of sperm total protein (2.5% to 4% of BSP-A1/-A2, 0.4% to 0.9% of BSP-A3, and 0.5% to 1% of BSP-30-kDa). We also determined the concentration of BSP proteins that were sperm-bound after semen cryopreservation in Tris-egg yolk-glycerol extender. A significant decrease (70%-80%) in sperm-bound BSP proteins was noted after cryopreservation. The availability of reliable RIA procedures should aid in the further understanding of the role of BSP proteins in sperm function as well as their effect on sperm cryopreservation.     

Complete amino acid sequence of BSP-A3 from bovine seminal plasma. Homology to PDC-109 and to the collagen-binding domain of fibronectin.

Bovine seminal plasma was shown to contain three similar proteins, called BSP-A1, BSP-A2 and BSP-A3. Both BSP-A1 and BSP-A2 were shown to be molecular variants of a recently characterized peptide called PDC-109. They seem to differ only in their degree of glycosylation and otherwise seem to possess an identical amino acid composition. The work in the present paper deals with the complete characterization of the third member of this series, namely BSP-A3. The complete amino acid sequence revealed that it is composed of 115 amino acids and predicts a Mr of 13,403. An analysis of the primary structure of BSP-A3 revealed a high degree of internal homology, with two homologous domains composed of 39 (residues 28-66) and 43 (residues 73-115) amino acids. An exhaustive computer-bank search for the similarity of this sequence to any known protein, or segment thereof, revealed two significant homologies. The first is between PDC-109 and BSP-A3, which is so high that we can confidently predict that both proteins evolved from a single ancestral gene. The collagen-binding domain of bovine fibronectin (type II sequence) was also found to be highly homologous to both BSP-A3 and PDC-109.     

Purification and biochemical characterization of three major acidic proteins (BSP-A1, BSP-A2 and BSP-A3) from bovine seminal plasma.

Three major acidic proteins of bovine seminal plasma, BSP-A1, BSP-A2 and BSP-A3, were purified to homogeneity, by employing fast protein liquid chromatography, gel filtration and h.p.l.c. The proteins were purified on the basis of their stimulatory effect on the basal release of gonadotropins by rat anterior-pituitary cells in culture. All three proteins migrated as distinct single bands in the presence or absence of 2-mercaptoethanol in SDS/polyacrylamide-gel electrophoresis. Their Mr values were estimated to be between 15,000 and 16,500 by SDS/polyacrylamide-gel electrophoresis. Similar Mr estimates were obtained when they were subjected to gel filtration on a calibrated column of Sephadex G-75 equilibrated in 0.05 M-acetic acid, pH 3.0. However, BSP-A1 and BSP-A2 were eluted as aggregated molecules (Mr 60,000-120,000) during gel filtration on Sephadex G-200 equilibrated in 0.05 M-NH4HCO3, pH 8.5, or phosphate buffer, pH 7.0, containing 0.15 M-NaCl. In the presence of 8 M-urea both BSP-A1 and BSP-A2 were eluted at positions corresponding to Mr values of 17,000-20,000. BSP-A1 and BSP-A2 had an identical amino acid composition, which differed largely from that of BSP-A3. All three proteins contained aspartic acid as the N-terminal residue, and cysteine was identified as the C-terminal residue. BSP-A1 and BSP-A2 are glycoproteins containing galactosamine, sialic acid and neutral sugars, but BSP-A3 did not contain any covalently attached sugars. Whereas BSP-A2 and BSP-A3 were eluted unadsorbed, BSP-A1 bound to wheat-germ lectin-Sepharose 6MB and could be eluted by the competing sugar N-acetyl-D-glucosamine. Treatment of BSP-A1 and BSP-A2 with trypsin resulted in complete loss of gonadotropin-release activity, but BSP-A3 retained full activity. Antibody raised against BSP-A1 did not cross-react with BSP-A3, or vice versa. All these properties indicated marked structural differences between BSP-A3 and BSP-A1 (or BSP-A2). On the basis of amino acid composition it was concluded that BSP-A1, BSP-A2 and BSP-A3 are the same as the gonadostatins [Esch, Ling, Bohlen, Ying & Guillemin (1983) Biochem. Biophys. Res. Commun. 113, 861-867].     

Thermodynamics of phosphorylcholine and lysophosphatidylcholine binding to the major protein of bovine seminal plasma, PDC-109

PDC-109 binds to sperm plasma membranes by specific interaction with choline phospholipids and induces cholesterol efflux, a necessary event before capacitation - and subsequent fertilization - can occur. The binding of phosphorylcholine (PrC) and lysophosphatidylcholine (Lyso-PC) with PDC-109 was investigated by monitoring the ligand-induced changes in the absorption spectrum of PDC-109. At 20 degrees C, the association constants (K(a)), for PrC and Lyso-PC were obtained as 81.4M(-1) and 2.02 x 10(4) M(-1), respectively, indicating that the binding of Lyso-PC to PDC-109 is 250-fold stronger than that of PrC. From the temperature dependence of the K(a) values, enthalpy of binding (DeltaH(0)) and entropy of binding (DeltaS(0)), were obtained as -79.7 and -237.1 J mol(-1)K(-1) for PrC and -73.0 kJ mol(-1) and -167.3 J mol(-1)K(-1) for Lyso-PC, respectively. These results demonstrate that although the binding of these two ligands is driven by enthalpic forces, smaller negative entropy of binding associated with Lyso-PC results in its significantly stronger binding.     

Influence of the bovine seminal plasma protein PDC-109 on cholesterol in the presence of phospholipids

The interaction of the major bovine seminal plasma protein PDC-109 with cholesterol was studied by employing spin-labelled analogues. It could be shown that PDC-109 does not interact directly with cholesterol molecules. However, in the presence of phospholipids we found a strong reduction of cholesterol motion by PDC-109. The fraction of immobilized cholesterol was largest for phosphorylcholine-containing lipids. This is consistent with the preferential interaction between PDC-109 and phosphatidylcholine. It is concluded that a stronger association and interaction of PDC-109 with phosphatidylcholine leads to an enhanced fraction of immobilized cholesterol analogues, but not to a phospholipid-dependent specific interaction between the protein and cholesterol. Moreover, the interaction of PDC-109 with various spin-labelled analogues of phosphatidylcholine (lysoPC, diacylPC) was investigated. In membranes of lipid vesicles the protein caused an immobilization of the phosphatidylcholine analogues mainly in the outer membrane leaflet, with no differences between diacylPC and lysoPC. The results are of relevance for understanding the physiological role of PDC-109 in the genesis of sperm cells.     

Inhibition of acrosine-like protease activity by a lectin affinity chromatographic bovine seminal plasma fraction containing the PDC-109 and aSFP proteins

These studies showed that the fractionation of bovine seminal plasma based on lectin agarose affinity chromatography, employing lectins specific to asparagine linked oligosaccharides, and a lectin specific for fucosylated glycans, lead to products with an inhibitory effect on the acrosine-like protease activity. This effect decreases when glycocompounds containing fucosylated Lewis^x structures are removed, suggesting that these compounds might have some role in the modulation of this activity in the bull. In the fraction devoid of high mannose, hybrid and non-bisecting lactosaminic oligosaccharide-containing glycocompounds, PDC-109 and aSFP proteins were detected and characterized at microscale.     

PDC-109 (BSP-A1/A2) promotes bull sperm binding to oviductal epithelium in vitro and may be involved in forming the oviductal sperm reservoir

Sperm reservoirs have been found in the oviducts of several species of mammals. In cattle, the reservoir is formed by the binding of sperm to fucose-containing glycoconjugates on the surface of oviductal epithelial cells. A fucose-binding molecule was purified from sperm extracts and identified as PDC-109 (BSP-A1/A2), a protein that is secreted by the seminal vesicles and associates with the plasma membrane of sperm upon ejaculation. The objective of this study was to demonstrate that PDC-109 promotes bull sperm binding to oviductal epithelium. PDC-109 was purified from bovine seminal plasma, and polyclonal antibodies were produced in rabbits. The antibodies detected PDC-109 on ejaculated sperm by indirect immunofluorescence and Western blots of extracts, but PDC-109 was not detected on epididymal sperm. When added to epididymal sperm, purified PDC-109 was absorbed onto the plasma membrane overlying the acrosome, as demonstrated by indirect immunofluorescence and by labeling sperm directly with fluorescein-conjugated PDC-109. When added to explants of oviductal epithelium, significantly fewer epididymal sperm than ejaculated sperm became bound. Addition of PDC-109 to epididymal sperm increased epithelial binding to the level observed for ejaculated sperm. In addition, binding of ejaculated sperm to oviductal epithelium was inhibited by addition of excess soluble PDC-109. Ejaculated sperm lost the ability to bind to oviductal epithelium after heparin-induced capacitation, but treatment with PDC-109 restored binding. These results demonstrate that PDC-109 enables sperm to bind to oviductal epithelium and plays a major role in formation of the bovine oviductal sperm reservoir.     

Characterisation of the conformational and quaternary structure-dependent heparin-binding region of bovine seminal plasma protein PDC-109

PDC-109, the major heparin-binding protein of bull seminal plasma, binds to sperm choline lipids at ejaculation and modulates capacitation mediated by heparin. Affinity chromatography on heparin-Sepharose showed that polydisperse, but not monomeric, PDC-109 displayed heparin-binding capability. We sought to characterise the surface topology of the quaternary structure-dependent heparin-binding region of PDC-109 by comparing the arginine- and lysine-selective chemical modification patterns of the free and the heparin-bound protein. A combination of reversed-phase peptide mapping of endoproteinase Lys-C-digested PDC-109 derivatives and mass spectrometry was employed to identify modified and heparin-protected residues. PDC-109 contains two tandemly arranged fibronectin type II domains (a, Cys24-Cys61; b, Cys69-Cys109). The results show that six basic residues (Lys34, Arg57, Lys59, Arg64, Lys68, and Arg104) were shielded from reaction with acetic anhydride and 1,2-cyclohexanedione in heparin-bound PDC-109 oligomers. In the 1H-NMR solution structures of single fibronectin type II domains, residues topologically equivalent to PDC-109 Arg57 (Arg104) and Lys59 lay around beta-strand D on the same face of the domain. In full-length PDC-109, Arg64 and Lys68 are both located in the intervening polypeptide between domains a and b. Our data suggest possible quaternary structure arrangements of PDC-109 molecules to form a heparin-binding oligomer.     

Mechanism of membrane binding by the bovine seminal plasma protein,PDC-109: a surface plasmon resonance study

PDC-109, the major protein of bovine seminal plasma, binds to sperm plasma membranes upon ejaculation and plays a crucial role in the subsequent events leading to fertilization. The binding process is mediated primarily by the specific interaction of PDC-109 with choline-containing phospholipids. In the present study the kinetics and mechanism of the interaction of PDC-109 with phospholipid membranes were investigated by the surface plasmon resonance technique. Binding of PDC-109 to different phospholipid membranes containing 20% cholesterol (wt/wt) indicated that binding occurs by a single-step mechanism. The association rate constant (k(1)) for the binding of PDC-109 to dimyristoylphosphatidylcholine (DMPC) membranes containing cholesterol was estimated to be 5.7 x 10(5) M(-1) s(-1) at 20 degrees C, while the values of k(1) estimated at the same temperature for the binding to membranes of negatively charged phospholipids such as dimyristoylphosphatidylglycerol (DMPG) and dimyristoylphosphatidic acid (DMPA) containing 20% cholesterol (wt/wt) were at least three orders of magnitude lower. The dissociation rate constant (k(-1)) for the DMPC/PDC-109 system was found to be 2.7 x 10(-2) s(-1) whereas the k(-1) values obtained with DMPG and DMPA was about three to four times higher. From the kinetic data, the association constant for the binding of PDC-109 to DMPC was estimated as 2.1 x 10(7) M(-1). The association constants for different phospholipids investigated decrease in the order: DMPC > DMPG > DMPA > DMPE. Thus the higher affinity of PDC-109 for choline phospholipids is reflected in a faster association rate constant and a slower dissociation rate constant for DMPC as compared to the other phospholipids. Binding of PDC-109 to dimyristoylphosphatidylethanolamine and dipalmitoylphosphatidylethanolamine, which are also zwitterionic, was found to be very weak, clearly indicating that the charge on the lipid headgroup is not the determining factor for the binding. Analysis of the activation parameters indicates that the interaction of PDC-109 with DMPC membranes is favored by a strong entropic contribution, whereas negative entropic contribution is primarily responsible for the rather weak interaction of this protein with DMPA and DMPG.     

Correlation of membrane binding and hydrophobicity to the chaperone-like activity of PDC-109, the major protein of bovine seminal plasma

The major protein of bovine seminal plasma, PDC-109 binds to choline phospholipids present on the sperm plasma membrane upon ejaculation and plays a crucial role in the subsequent events leading to fertilization. PDC-109 also shares significant similarities with small heat shock proteins and exhibits chaperone-like activity (CLA). Although the polydisperse nature of this protein has been shown to be important for its CLA, knowledge of other factors responsible for such an activity is scarce. Since surface exposure of hydrophobic residues is known to be an important factor which modulates the CLA of chaperone proteins, in the present study we have probed the surface hydrophobicity of PDC-109 using bisANS and ANS. Further, effect of phospholipids on the structure and chaperone-like activity of PDC-109 was studied. Presence of DMPC was found to increase the CLA of PDC-109 significantly, which could be due to the considerable exposure of hydrophobic regions on the lipid-protein recombinants, which can interact productively with the nonnative structures of target proteins, resulting in their protection. However, inclusion of DMPG instead of DMPC did not significantly alter the CLA of PDC-109, which could be due to the lower specificity of PDC-109 for DMPG as compared to DMPC. Cholesterol incorporation into DMPC membranes led to a decrease in the CLA of PDC-109-lipid recombinants, which could be attributed to reduced accessibility of hydrophobic surfaces to the substrate protein(s). These results underscore the relevance of phospholipid binding and hydrophobicity to the chaperone-like activity of PDC-109.     

Isothermal titration calorimetric studies on the interaction of the major bovine seminal plasma protein, PDC-109 with phospholipid membranes

The interaction of the major bovine seminal plasma protein, PDC-109 with lipid membranes was investigated by isothermal titration calorimetry. Binding of the protein to model membranes made up of diacyl phospholipids was found to be endothermic, with positive values of binding enthalpy and entropy, and could be analyzed in terms of a single type of binding sites on the protein. Enthalpies and entropies for binding to diacylphosphatidylcholine membranes increased with increase in temperature, although a clear-cut linear dependence was not observed. The entropically driven binding process indicates that hydrophobic interactions play a major role in the overall binding process. Binding of PDC-109 with dimyristoylphosphatidylcholine membranes containing 25 mol% cholesterol showed an initial increase in the association constant as well as enthalpy and entropy of binding with increase in temperature, whereas the values decreased with further increase in temperature. The affinity of PDC-109 for phosphatidylcholine increased at higher pH, which is physiologically relevant in view of the basic nature of the seminal plasma. Binding of PDC-109 to Lyso-PC could be best analysed in terms of two types of binding interactions, a high affinity interaction with Lyso-PC micelles and a low-affinity interaction with the monomeric lipid. Enthalpy-entropy compensation was observed for the interaction of PDC-109 with phospholipid membranes, suggesting that water structure plays an important role in the binding process.     

31P NMR and AFM studies on the destabilization of cell and model membranes by the major bovine seminal plasma protein, PDC-109

The effect of PDC-109 binding to dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylglycerol (DPPG) multilamellar vesicles (MLVs) and supported membranes was investigated by (31)P NMR spectroscopy and atomic force microscopy. Additionally, the effect of cholesterol on the binding of PDC-109 to phosphatidylcholine (PC) membranes was studied. Binding of PDC-109 to MLVs of DMPC and DPPG induced the formation of an isotropic signal in their (31)P NMR spectra, which increased with increasing protein/lipid ratio and temperature, consistent with protein-induced disruption of the MLVs and the formation of small unilamellar vesicles or micelles but not inverse hexagonal or cubic phases. Incorporation of cholesterol in the DMPC MLVs afforded a partial stabilization of the lamellar structure, consistent with previous reports of membrane stabilization by cholesterol. AFM results are consistent with the above findings and show that addition of PDC-109 leads to a complete breakdown of PC membranes. The fraction of isotropic signal in (31)P NMR spectra of DPPG in the presence of PDC-109 was less than that of DMPC under similar conditions, suggesting a significantly higher affinity of the protein for PC. Confocal microscopic studies showed that addition of PDC-109 to human erythrocytes results in a disruption of the plasma membrane and release of hemoglobin into the solution, which was dependent on the protein concentration and incubation time.     

Interaction of the major protein from bovine seminal plasma, PDC-109 with phospholipid membranes and soluble ligands investigated by fluorescence approaches

The major protein from bovine seminal plasma, PDC-109 binds selectively to choline phospholipids on the sperm plasma membrane and plays a crucial role in priming spermatozoa for fertilization. The microenvironment and accessibility of tryptophans of PDC-109 in the native state, in the presence of phosphorylcholine (PrC) and phospholipid membranes as well as upon denaturation have been investigated by fluorescence approaches. Quenching of the protein intrinsic fluorescence by different quenchers decreased in the order: acrylamide>succinimide>Cs(+)>I(-). Ligand binding afforded considerable protection from quenching, with shielding efficiencies following the order: dimyristoylphosphatidylcholine (DMPC)>lysophosphatidylcholine (Lyso-PC)>PrC. This has been attributed to a partial penetration of the protein into the DMPC membranes and Lyso-PC micelles, as well as a further stabilization of the binding due to the interaction of PDC-109 with lipid acyl chains and the resulting tightening of the protein structure, leading to a decreased accessibility of the tryptophan residues. Red-edge excitation shift (REES) studies yielded REES values of 4 nm for both native and denatured PDC-109, whereas reduced and denatured protein gave a REES of only 0.5 nm, clearly indicating that the structural and dynamic features of the microenvironment around the tryptophan residues are retained even after denaturation, presumably due to the constraints imposed on the protein structure by disulfide bonds. Upon binding of PDC-109 to DMPC membranes and Lyso-PC micelles the REES values were reduced to 2.5 and 1.0 nm, respectively, which could be due to the penetration of some parts of the protein, especially the segment containing Trp-90 into the membrane interior, where the red-edge effects are considerably reduced.     

Fluorescence studies on the interaction of choline-binding domain B of the major bovine seminal plasma protein,PDC-109 with phospholipid membranes

The microenvironment and accessibility of the tryptophan residues in domain B of PDC-109 (PDC-109/B) in the native state and upon ligand binding have been investigated by fluorescence quenching, time-resolved fluorescence and red-edge excitation shift (REES) studies. The increase in the intrinsic fluorescence emission intensity of PDC-109/B upon binding to lysophosphatidylcholine (Lyso-PC) micelles and dimyristoylphosphatidylcholine (DMPC) membranes was considerably less as compared to that observed with the whole PDC-109 protein. The degree of quenching achieved by different quenchers with PDC-109/B bound to Lyso-PC and DMPC membranes was significantly higher as compared to the full PDC-109 protein, indicating that membrane binding afforded considerably lesser protection to the tryptophan residues of domain B as compared to those in the full PDC-109 protein. Finally, changes in red-edge excitation shift (REES) seen with PDC-109/B upon binding to DMPC membranes and Lyso-PC micelles were smaller that the corresponding changes in the REES values observed for the full PDC-109. These results, taken together suggest that intact PDC-109 penetrates deeper into the hydrophobic parts of the membrane as compared to domain B alone, which could be the reason for the inability of PDC-109/B to induce cholesterol efflux, despite its ability to recognize choline phospholipids at the membrane surface.     

Crystallization and preliminary X-ray diffraction analysis of bovine seminal plasma PDC-109, a protein composed of two fibronectin type II domains

PDC-109 is a 13 kDa glycoprotein and the major phosphorylcholine- and heparin-binding protein of bull seminal plasma. It is built by an acidic 23-residue N-terminal sequence followed by a tandem of fibronectin type II domains. Full-length PDC-109 was crystallized in complex with o-phosphorylcholine by vapor diffusion in sitting drops. Crystals grew to maximal size of 0.5 × 0.3 × 0.2 mm³, diffract x-rays beyond 2.6 Å resolution, and belong to space group P321 with unit cell dimensions a = b = 93.6 Å, c = 52.7 Å. Proteins 28:454–456, 1997. © 1997 Wiley-Liss, Inc.     

Conserved ram seminal plasma proteins bind to the sperm membrane and repair cryopreservation damage

Whole seminal plasma (SP) enhances the function and fertility of frozen/thawed ram sperm. The objective of the current study was to investigate whether SP proteins capable of binding to molecules from the sperm plasma membrane were conserved among ram breeds, and whether these proteins were sufficient to overcome cryopreservation-induced reductions in sperm quality. Whole ram SP, obtained from rams of various breeds, improved progressive motility of frozen/thawed sperm at all times evaluated (P < 0.05); however, it did not improve total motility (15 min, P = 0.480; 30 min, P = 0.764; and 45 min, P = 0.795). To identify SP proteins responsible for this effect, a new method was developed to retain SP proteins that bound specifically to the sperm membrane by immobilization of sperm membrane proteins. These proteins specifically bound to the sperm surface, especially the acrosomal region. Lactotransferrin, epididymal secretory protein E1, Synaptosomal-associated protein 29, and RSVP-20 were identified (mass spectrometry) in this fraction. The retained SP proteins fraction repaired ultrastructural damage of frozen/thawed sperm and, with the addition of fructose, significantly improved motility of frozen/thawed sperm. We concluded that SP proteins that bound to the sperm membrane were conserved among ram breeds, and that when added to frozen/thawed semen (along with an energy source), they repaired ram sperm damage and enhanced sperm motility.     

Localization and structural characterization of an oligosaccharide O-linked to bovine PDC-109 Quantitation of the glycoprotein in seminal plasma and on the surface of ejaculated and capacitated spermatozoa

PDC-109 (13 kDa) is the most abundant component, and the major heparin-binding protein, of bovine (Bos taurus) seminal plasma. Here, we show that PDC-109 contains a single O-linked oligosaccharide (NeuNAc(2–6)-Galβ(1–3)-GalNAc-) attached to Thr¹¹. Immunoquantitation of PDC-109 indicates that its concentration in seminal plasma is 15–20 mg/ml. Though PDC-109 is not present on epididymal sperm, ejaculated spermatozoa on average are coated with (9.5 ± 0.3) × 10⁶ molecules of PDC-109/cell. This value remained constant in swim-up sperm and decreased to (7.7 ± 0.4) × 10⁶/spermatozoon after incubation for 24 h in capacitation medium at 39°C. These data substantiate the hypothesis that PDC-109 may be one of the seminal plasma components that enhance the fertilizing capacity of bull spermatozoa upon interaction with heparin-like glycosaminoglycans present in the female genital tract.     

The PDC-109 protein from bovine seminal plasma is similar to the gelatin-binding domain of bovine fibronectin and a kringle domain of human tissue-type plasminogen activator

PDC-109, a protein of unknown function, is a major component of bovine seminal plasma. Using a computer program designed to detect evolutionary relationships between proteins, I find that the PDC-109 protein is similar to the gelatin-binding domain of bovine fibronectin and part of a kringle domain of human tissue-type plasminogen activator (t-PA). The computer-based comparison of the amino acid sequence of PDC-109 with that of the gelatin-binding domain of fibronectin and part of the second kringle domain of t-PA yields scores that are 15.5 standard deviations and 7.8 standard deviations higher, respectively, than were obtained with a comparison of randomized sequences of these proteins. The probability (p) of getting these scores by chance is less than 10(-50) and 3 X 10(-15), respectively. The similarity between the amino acid sequences of PDC-109 and the gelatin-binding domain in fibronectin and the kringle of t-PA suggests some approaches for identifying the functions of PDC-109. Both t-PA and the gelatin-binding domain of fibronectin have adhesive functions, and the gelatin-binding domain promotes viral transformation of fibroblasts in culture. These functions may be associated with the PDC-109 protein.