Use of annexin-V to demonstrate the role of phosphatidylserine exposure in the maintenance of haemostatic balance by endothelial cells

Annexin-V (PAP-I, lipocortin-V) acts as a potent anticoagulant in vitro by binding to negatively charged phospholipids with higher affinity than vitamin K-dependent proteins, with a Kd in the 10(-10) M range. The purpose of the present study was to use annexin-V as a probe to assess the catalytic potential of phospholipids in pro- and anti-coagulant reactions in purified systems and at the surface of endothelial cells in culture after stimulation. Procoagulant tissue factor and anticoagulant thrombomodulin activities were compared by using specific two-stage amidolytic assays performed with purified proteins. Procoagulant activity was estimated by the generation of Factor Xa by the Factor VII(a)-tissue factor complex. Anticoagulant activity was estimated by the generation of activated protein C by either the thrombin-thrombomodulin complex or Factor Xa. Annexin-V induced a decrease of 70% of thrombomodulin activity when thrombomodulin (5.4-214 nM) was reconstituted into phosphatidylcholine/phosphatidylserine (1:1, mol/mol) vesicles at 37.5 or 75 microM-phospholipid concentration, the apparent Ki being 0.5 microM at 75 microM-lipid. The saturating concentration of annexin-V was dependent on phospholipid concentration, but was independent of the phospholipid/thrombomodulin ratio. By contrast, when thrombomodulin was not reconstituted in vesicles, annexin-V had no effect. At 2 microM, annexin-V totally inhibited the generation of activated protein C by Factor Xa in the presence of 75 microM-lipid, the saturating inhibitory concentration being dependent on phospholipid concentration. At 0.1 microM, annexin-V totally inhibited tissue-factor activity present in crude brain thromboplastin. In the absence of stimulation, human endothelial cells in culture expressed significant thrombomodulin activity and no detectable tissue-factor activity. Basal thrombomodulin activity was only slightly inhibited (less than 15%) by 0.5 microM-annexin-V. Phorbol myristate acetate (PMA) induced the expression of tissue-factor activity and decreased thrombomodulin activity at the endothelial-cell surface. Annexin-V, at a concentration of 16 microM, caused an 80% decrease of tissue-factor activity induced by PMA at 10 ng/ml, whereas it inhibited thrombomodulin activity by only 15% on the same stimulated cells. Our results confirm that annexin-V inhibits, in vitro, procoagulant tissue-factor activity and anticoagulant activities (activation of protein C by the thrombin-thrombomodulin complex and by Factor Xa), through phospholipid-dependent mechanisms.(ABSTRACT TRUNCATED AT 400 WORDS)

Purification of annexin I and annexin II from human placental membranes by high-performance liquid chromatography

Annexin I and annexin II were extracted from human placental membranes with ethylene glycol bis(beta-amino-ethyl ether)-N,N'-tetraacetic acid (EGTA) and purified by high-performance liquid chromatography by measuring their ability to inhibit phospholipase A2 activity in vitro. Neither protein was capable of binding to a DEAE-5PW HPLC column at neutral pH; however, they were resolved through binding to a Mono S column and passage through size-exclusion HPLC columns. Annexin I and its covalently linked dimer (36 and 66 kDa, respectively, by sodium dodecyl sulfate (SDS)-gel electrophoresis) reacted in one-dimensional immunoblots with monoclonal antibodies to annexin I and calpactin II, and with monoclonal and polyclonal antibodies to lipocortin I, confirming that annexin I, calpactin II, and lipocortin I are the same or closely related proteins. Milligram amounts of monomeric annexin I containing negligible amounts of the cross-linked dimeric annexin I were selectively isolated from placental membranes by using buffers containing the sulfhydryl reagent iodoacetic acid. Milligram amounts of cross-linked annexin I were selectively isolated when placental membranes were initially treated with buffers that did not contain iodoacetic acid and then extracted with Triton X-100, suggesting that sulfhydryl-dependent transglutaminase activity contributes to the selective isolation of this protein. A third phospholipase A2-inhibitory protein (35 kDa by SDS-gel electrophoresis) that reacted in immunoblots with monoclonal antibodies to calpactin I and annexin II, indicating their similar identity, was isolated. The procedure employed allows the rapid purification of annexins I and II in milligram amounts from placental membranes within 2 days.

Bovine early pregnancy factor (EPF) activity dependent on a 67-kDa polypeptide

Maternal bovine EPF activity can be reduced to one single polypeptide enriched and identified from serum of cows in early pregnancy. The relative molecular weight of this active polypeptide was estimated at 67 kDa. This bovine EPF was labelled by 125I and peroxidase. In parallel investigations of non-pregnant animals a 67-kDa polypeptide was additionally identified in the last purification step, but without EPF activity in the rosette inhibition test. This indicated occurrence of an inactive pre-compound (or carrier protein) of the EPF in the non-pregnant state. On pre-incubation of lymphocytes with EPF analogues (inactive polypeptide from nonpregnancy serum) EPF retained its optimal activity, its lymphocyte receptors being unaffected. Monoclonal antibodies produced against HPLC-enriched EPF were reactive to the 67-kDa polypeptide in pregnancy material as well as in nonpregnancy material and were not able to differentiate between 'pregnant' and 'nonpregnant'. A mouse anti-EPF serum produced against highly purified EPF isolated from SDS PAGE showed reactivity only against the 67-kDa polypeptide of pregnancy serum but not against that of non-pregnancy serum. This is the first evidence for a difference in antigenic determinants of the two 67-kDa proteins found in pregnancy and non-pregnancy serum. Furthermore, a second higher molecular weight protein could be identified by this antiserum in pregnancy and non-pregnancy serum.

Phospholipid binding of antiphospholipid antibodies and placental anticoagulant protein

We evaluated the interaction of antiphospholipid antibodies (aPL) with placental anticoagulant protein I (PAP I), a calcium-dependent phospholipid binding protein which may act as a natural anticoagulant. Clotting assays showed additive prolongation of clotting times with aPL and PAP I. ELISA and vesicle phospholipid binding studies showed PAP I inhibition of aPL binding to phospholipid but no inhibition of PAP I-phospholipid binding by aPL. aPL and PAP I interact additively in anticoagulant activity in in vitro clotting systems and compete for phospholipid in ELISA system. These data support the hypotheses that aPL and PAP I may recognize similar phospholipid epitopes and that in vivo interaction may occur.

Cloning and expression in Saccharomyces cerevisiae of a synthetic gene for the type-I trophoblast interferon ovine trophoblast protein-1: purification and antiviral activity

Ovine trophoblast protein-1 (oTP-1) is a unique, Type I, trophoblast interferon (IFN) that possesses potent antiviral activity and is thought to be primarily responsible for maternal recognition of pregnancy in sheep. To provide sufficient amounts of protein for detailed studies, a synthetic gene for oTP-1 was designed and assembled in Escherichia coli, subcloned into a yeast expression plasmid, and used to overproduce recombinant oTP-1 in Saccharomyces cerevisiae. Recombinant oTP-1 was purified from soluble yeast extract using sequential ion-exchange and molecular sieve chromatography. Recombinant oTP-1 purified in this fashion exhibited potent antiviral activity (0.6 x 10(8) U/mg) similar to native oTP-1. This expression system will enable production of large quantities of soluble, biologically active, and correctly processed recombinant oTP-1. Furthermore, the synthetic gene construct facilitates introduction of mutations for ongoing structure/function studies of this unique, Type I, trophoblast IFN.

Expression of a biologically active ovine trophoblastic interferon using a baculovirus expression system

Ovine trophoblast protein (oTP) an embryonic interferon, which plays a key role in maternal recognition of pregnancy, has been expressed in insect cells using a baculovirus expression system. A cDNA coding for oTP was inserted downstream of the strong polyhedrin promoter. Cells infected with recombinant virus produced biologically active oTP and greater than 90% was secreted into the culture medium during infection. High amount of antiviral activity were produced (up to 5 x 10(5) IU per ml of culture medium). Recombinant oTP (roTP) was purified by immunoaffinity chromatography and found to be identical to authentic oTP with respect to molecular mass and N-terminal amino acid sequence.

Identification of the major pregnancy-specific antigens of cattle and sheep as inactive members of the aspartic proteinase family

Pregnancy in cattle and sheep can be diagnosed by the presence of a conceptus-derived antigen in maternal serum that is secreted by trophoblast and placental tissue primarily as an acidic component of Mr 67,000. Molecular cloning of its cDNA reveals that the antigen belongs to the aspartic proteinase family and has greater than 50% amino acid sequence identity to pepsin, cathepsin D, and cathepsin E. The inferred sequences of the ovine and bovine polypeptides show approximately 73% identity to each other. Critical amino acid substitutions at the active site regions suggest that both proteins are enzymatically inactive. The antigen is a product of trophoblast binucleate cells that invade maternal endometrium at implantation sites.

Identification of an ectopic developmental antigen that appears in malignant ascitic fluid

A tumor-associated antigen that shares antigenicity with a pregnancy-associated protein has been detected in ascitic fluid of patients with advanced ovarian cancer. The protein, prepared from malignant ascitic fluid by the combination of poly(ethylene glycol) 4000 fractionation, DEAE-Sepharose column chromatography and preparative polyacrylamide gel electrophoresis (PAGE), was subsequently separated on sodium dodecyl sulfate-PAGE, as a single band corresponding to an approximate molecular mass of 91 kDa. Immunological analysis showed that this protein was a circulating species of an ectopic developmental antigen that markedly increases in biological fluids in tumor-bearing statuses.

Purification and characterization of acidic form of glutathione S-transferase in human fetal livers: high similarity to placental form

An acidic form of glutathione S-transferase (GST) was purified from human fetal livers by means of affinity chromatography and chromatofocusing. The major peak of the acidic form of GST was focused between pH 4.8 and 4.9. Judging by SDS-PAGE, the purified acidic GST was apparently homogeneous; the subunit molecular weight was estimated to be 23,000. The acidic GST catalyzed the conjugations of glutathione (GSH) with 1-chloro-2,4-dinitrobenzene (CDNB) and ethacrynic acid (EA). The immunochemical properties of the purified acidic GST were indistinguishable from those of human placental GST-pi. The N-terminal amino acid sequence of the acidic GST was identical with that of GST-pi from human placenta. The level of expression of the acidic form of GST was clearly different between human adult and fetal livers as examined on the levels of mRNA and protein.

Annexins V and VI: major calcium-dependent atrial secretory granule-binding proteins

Atrial natriuretic peptide is stored by atrial myocytes in secretory granules, known as atrial specific granules, and is released from these granules by exocytosis. We have isolated a group of atrial proteins by affinity chromatography that bind to atrial specific granules in a calcium-dependent manner. The two major proteins isolated (32.5 kd and 67 kd) are calcium-binding proteins and have been identified as annexins V and VI by immunoblotting with specific antisera. The calcium dependence of their binding to atrial specific granules has been characterized in vitro and indicates that this interaction takes place at micromolar levels of calcium. In addition, the group of proteins isolated includes another calcium-binding protein of 20 kd, as well as GTP-binding proteins of 22 to 26 kd. Membrane interactions during exocytosis are presumably mediated by the interaction of specific proteins with the granule membrane. The properties of the proteins described here, and their ability to bind to atrial specific granules in a calcium-dependent manner, make them likely candidates in the search for regulatory proteins mediating atrial natriuretic peptide secretion.

Immunochemically detected placental proteins and their biological functions

During the last 20 years a systematic search for proteins occurring in human term placenta (afterbirth) has been performed in our laboratory. As a result more than 30 soluble placental proteins and at least 20 different solubilized antigens apparently derived from the placental membranes have been identified by immunochemical methods in extracts from human term placentas. Most of these proteins have already been isolated to purity and characterized by their physicochemical parameters. Specific antisera to these proteins were obtained by immunizing animals with the corresponding purified proteins. They were used detect and localize these antigens by immunochemical methods in the placenta and in other human tissues. Sensitive immunochemical assays have been developed to exactly quantitate the new proteins in body fluids and to find out the diagnostic significance of measurement of these proteins in pregnant women and in patients with tumors and other diseases. Another aim was to elucidate the biological functions of our immunochemically detected proteins. The results obtained thus far are reported.

Structure of soluble and membrane-bound human annexin V

Annexins are a family of water-soluble proteins that bind to membranes in a calcium-dependent manner. Some members have been shown to exhibit voltage-dependent calcium channel activity, a property characteristic of integral membrane proteins. The structures of human annexin V in crystals obtained from aqueous solution and in two-dimensional crystals when bound to phospholipid layers have been determined by X-ray and electron crystallography, respectively. They are compared here. Both structures show close correspondence, suggesting that annexins attach to phospholipid membranes without substantial structural change. These observations, together with biochemical data, lead to the conclusion that annexin V interacts with phospholipid membranes with its convex face. We propose that binding is mediated by direct interaction between the phosphoryl headgroups and the calcium bound to polypeptide loops protruding from the convex face. The membrane area covered by annexin may thus become disordered and permeable allowing calcium flux through the membrane and the central channel-like structure found in annexin molecules.

NF-kappa B contacts DNA by a heterodimer of the p50 and p65 subunit

We recently reported that the apparently non-DNA-binding 65 kd subunit (p65) of the NF-kappa B transcription factor can modulate the DNA-binding specificity of the 50 kd subunit (p50) of NF-kappa B. In this study we provide an explanation for this property of p65. In electrophoretic mobility shift assays and upon UV cross-linking to DNA, gel-purified p65 is shown to be a kappa B-specific DNA-binding protein on its own. The binding activity was only detectable if high amounts of p65 were used for the analyses and after the application of a modified renaturation protocol. DNA-binding of the p65 dimer, in contrast to that of p50, was inhibited by I kappa B-alpha and -beta. This finding is consistent with a receptor function of p65 for both inhibitory subunits. Direct UV cross-linking of NF-kappa B to DNA probes which were photoreactive within only one half-site and a binding competition analysis with p65 showed that p65 has a strong preference for binding to the less conserved half site of kappa B motifs whereas p50 has a moderate preference for the more highly conserved half site. In electrophoretic mobility shift assays and upon sedimentation through glycerol gradients, NF-kappa B appears to exist as a heterodimer composed of one p50 and one p65 subunit whereas data from gel filtration suggest a higher order complex.

Identity between the placental protein PP10 and the specific plasminogen activator inhibitor of placental type PAI-2

The highly specific plasminogen activator inhibitor of placental type, PAI-2, occurs in the placenta in a low molecular mass form of 46.6 kDa, and in pregnancy plasma in a (possibly glycosylated) high molecular mass form of 60 kDa. Extensive knowledge is available about the functional properties of PAI-2 as a plasminogen activator inhibitor and about its molecular biology and regulation. Of the several placenta proteins (PP) isolated, one of them, PP10, has a molecular mass of 48 kDa and its occurrence in malignancy and in complications during pregnancy has been the topic of a number of studies, though its properties and physiological significance are unknown. The present findings constitute evidence of immunological identity between PP10 and PAI-2. The sections of the amino acid sequence of PP10 analysed here were found to have identical counterparts in the sequence of the low molecular mass form of PA1-2, but in several preparations PP10 was found to occur in an inactive two-chain form due to cleavage of an Arg-Thr bond, the two peptide chains being linked to each other by a disulphide bridge. The cleavage site is identical to that observed in the reaction between PAI-2 and urokinase. The results make it possible to coordinate and correlate the findings of many separate studies and our own observations on PP10 and PAI-2.

Gravidin, an endogenous inhibitor of phospholipase A2 activity, is a secretory component of IgA

Gravidin, a phospholipase inhibitor characterised previously from amniotic fluid, was partially sequenced at the N-terminal and found to be identical to secretory component of human IgA. Inhibition of antiphospholipase activity was observed after incubation of gravidin with monoclonal antibody to human secretory component. Secretory component isolated from human saliva and breast milk was found to inhibit arachidonic acid release from human lymphocytes. It was concluded that gravidin is secretory component of IgA.

Immunoregulatory effects of ovine trophoblastin protein (oTP): all five isoforms suppress PHA-induced lymphocyte proliferation

The immunosuppressive properties of ovine trophoblastin protein (oTP) isoforms purified to homogeneity by DEAE HPLC have been studied within and across species barriers by in vitro assays. It has been demonstrated that not only the classical oTP 1, but in fact all 5 isoforms, are immunosuppressive in a PHA-induced proliferation assay, whilst being ineffective on IL-2 dependent CTL-L2 cell replication. The significance of these findings is discussed.

Isolation and characterization of membrane-associated placental proteins

Membrane-associated proteins (MPs) of the human term placenta (afterbirth) were obtained by extracting the insoluble part of the tissue with solubilizing agents, after the soluble material had been removed by washing with saline. The insoluble residue was subsequently exhaustively extracted first with the nonionic detergent Triton X-100 and then with 6 M urea. In the Triton extract eleven new different membrane-associated antigens could be detected by immunochemical methods; they were designated as MP2A to MP2L. One of these proteins (MP2C) was found to be immunochemically identical with the already described soluble placental protein PP21 [3]. MP1 another antigen detected in the Triton extract later was identified as heart stable alkaline phosphatase. In the urea extract eight different membrane-associated antigens could be identified by immunochemical methods; they were designated as MP3 to MP10. MP3 later was found to be immunochemically identical with laminin. All these membrane-associated proteins have now been isolated to purity and characterized by their physico-chemical properties. Specific antisera to the new proteins were obtained by immunizing animals with the corresponding purified proteins. They were used to detect and quantitate the new proteins in extracts of placentas and other human tissues by immunochemical methods such as gel diffusion tests. The immunocytochemical localization of the new proteins as well as measurement of their concentrations in body fluids by sensitive radioimmunoassays or enzyme immunoassays are presently under investigation.

Isolation and characterization of five new soluble placental tissue proteins (PP22, PP23, PP24, PP25, PP26)

Five new soluble placental tissue proteins (PP22, PP23, PP24, PP25, PP26) were isolated to purity from saline extracts of human term placentas and characterized by their physico-chemical properties. Specific antisera to the new proteins were obtained by immunizing animals with the corresponding purified proteins. They were used to detect and quantitate the new proteins in extracts of placentas and other human tissues by immunochemical methods such as gel diffusion tests. The immunohistochemical localization of the new proteins as well as measurement of their concentrations in body fluids by sensitive radioimmunoassays are presently under investigation.

Tissue anticoagulants in the human placenta: preliminary study with a heparin-like anticoagulant and review of the literature

In this study a new heparin-like anticoagulant in the human placenta is described and an extensive review of the literature is given. An anticoagulant with a heparin-like activity was isolated from an extract of the human placenta. A two-stage purification procedure was applied: affinity chromatography on heparin-Sepharose and ion-exchange chromatography on DEAE-trisacryl. The isolated anticoagulant fraction was found to prolong both thrombin clotting time and activated partial thromboplastin time and to increase the activity of antithrombin III and heparin cofactor II. The placental-tissue anticoagulants described in the literature are divided by the author into five groups: (1) acidic peptides of low molecular weight; (2) thrombomodulin; (3) phospholipid-binding protein anticoagulants; (4) heparin-like anticoagulants, and (5) others. The anticoagulant described in the present study has been classified to the 4th group.

Human placental protein methylase--I. Purification and characterization

1. Protein methylase I (S-adenosylmethionine[:]protein-arginine N-methyltransferase; EC 2.1.1.23) which methylates protein-bound arginine residues has been purified from human term placenta 400-fold with an approximate yield of 6%. 2. When histone was used as in vitro substrate, the methylation products were found to be NG-mono-, NG, NG-di- and NG, N'G-dimethylarginine. The enzyme was found to be sensitive toward Cu2+ with Ki value of 8 x 10(-5) M. The Km value for S-adenosyl-L-methionine was 5 x 10(-6) M. 3. When this partially purified protein methylase I was incubated with isolated human placental nuclei and S-adenosyl-L-[methyl-3H]methionine, the major endogenous [methyl-3H]-labeled proteins were protein species of 23, 38, 45 and 68 kDa, the 23 kDa species being the most predominant. 4. The endogenous enzyme activity during the pregnancy increased significantly, reaching more than 4 times the initial activity at the end of term.

Kinetics and mechanism of proteinase-binding of pregnancy zone protein (PZP). Appearance of sulfhydryl groups in reactions with proteinases

Proteinase binding by pregnancy zone protein (PZP), an alpha-macroglobulin involves bait region cleavages, association of dimeric-PZP into tetrameric and reaction of internal gamma-glutamyl-beta-cysteinyl thiol esters of PZP with proteinase side chains. The product is an equimolar enzyme-PZP(tetramer) covalently linked complex with four free sulfhydryl groups. The kinetics of the appearances of sulfhydryl groups during the reaction of PZP with chymotrypsin has been investigated using stopped-flow and conventional mixing techniques over a broad concentration range. Thiol ester cleavages followed double exponential decays corresponding with two steps. The faster one resulted in the appearance of three sulfhydryl groups with an observed rate constant, k(obs) = k1.1 + k1.2 delta E, dependent on the excess concentration of chymotrypsin, delta E, and k1.1 = 0.03 s-1 and k1.2 = 4 x 10(4) M-1 s-1. The last sulfhydryl group appeared in a slower step, with similar concentration dependence and k2.1 approximately 0.003 s-1 and k2.2 approximately 5 x 10(3) M-1s-1. Covalent binding of the enzyme apparently was simultaneous with the faster thiol ester cleavage step. Based on these and previous results a model of the reaction mechanism of the proteinase binding reaction of PZP is proposed. It consists of four major steps: (i) Bait region cleavage of PZP-dimers by the enzyme, (ii) fast association of enzyme-PZP(dimer) species with native PZP or with another enzyme-PZP(dimer) compound resulting in release of one of the associated enzyme molecules (iii) reaction of an average of three thiol esters of the enzyme-PZP(tetramer) intermediate with the associated internal enzyme molecule or with an external one. In this step one enzyme molecule becomes covalently linked to the PZP-(tetramer), three sulfhydryl groups appear and the enzymic activity of the bound enzyme molecule decreases to the level of that of the final complex. (iv) Hydrolysis of the last thiol ester and in the presence of excess enzyme, degradation of enzyme-PZP(tetramer) complexes and formation of fragments some of which are the size of PZP(dimer) with enzyme bound.

Purification and partial amino acid sequence of annexin V from porcine gastric mucosal membranes

1. Annexin V has been purified from Triton X-100 extracts of porcine gastric mucosal membranes by a combination of chromatography on concanavalin A-Sepharose and DEAE-Sepharose, and preparative gel electrophoresis. 2. No N-terminal amino acid sequence was detected. 3. The sequences of 11 tryptic peptides were determined, amounting to a total of 121 amino acids, or 38% of the molecule. 4. When the peptides were compared with the cDNA-derived sequence of human annexin V, only three substitutions were observed. 5. Human and porcine annexin V are 97% homologous within the sequenced regions.

Characterization of feline conceptus proteins during pregnancy

This study characterized proteins secreted de novo by feline conceptuses collected on Days 10, 12, and 15 (n = 22, preimplantation blastocysts); Days 15, 16, 17, 19, 21, and 25 (n = 6, postimplantation zonary girdle [ZG] i.e. trophoblast and endometrium); and Days 30, 36, 39, and 50 (n = 5, postimplantation ZG and free chorioallantois [CA]) and cultured in Minimal Essential Medium. De novo secretion was shown by incorporation of 3H-leucine into proteins detected in culture media by 2D-PAGE and fluorography. Western blotting, and NH2-terminal amino acid microsequencing. Major radiolabeled proteins identified as they appeared temporally on fluorographs were as follows: feline conceptus protein 1 (fCP1), Mr = 20,000, pI 5.0-5.3; fCP2, Mr = 80,000, pI 6.5-7.2; fCP3a, Mr = 67,000, pI 6.3-6.5; fCP3b, Mr = 67,000, pI 5.9-6.3; fCP4, Mr = 56,000, pI 5.0-6.0; and fCP5, Mr = 29,000, pI 5.0-5.8. The fCP1 was produced by blastocysts on Days 10-15, ZG on Days 16-25, and CA on Day 30; on Days 39-50, CA synthesized 5 proteins, possibly fCP1 isomers. The fCP2, fCP3a and b, and fCP4 were produced by blastocysts on Day 15, ZG on Day 25, and CA on Days 30-50. The fCP5 was made by ZG on Days 16-36 and by CA on Days 30-39. Western blotting identified fCP1 as retinol-binding protein (RBP), fCP2 as alpha fetoprotein, fCP3a as albumin, and fCP3b as transferrin. Amino acid sequence homologies between fCP1 and rabbit and human plasma RBP and porcine conceptus RBP2 were 93, 96, and 100%, respectively, at the first 37 NH2-terminal amino acids. The identities of fCP4 and fCP5 have not been established. Antiviral activity detected in all media was less than 3 units/ml when tested with feline fibroblast cells infected with vesicular stomatitis virus.

Characterization of placental prolactin-like protein-A in intracellular and extracellular compartments

Prolactin-like protein-A (PLP-A) was purified from medium conditioned by junctional zone explants dissected from the rat chorioallantoic placenta via concanavalin A affinity chromatography, sodium dodecyl sulfate-gel electrophoresis and two-dimensional gel electrophoresis. Monomeric PLP-A was found to be heterogeneous and consisted of two major molecular sizes, 29 and 33 kDa. The charge and size heterogeneity attributed to monomeric PLP-A could be accounted for by the addition of N-linked carbohydrate moieties to the PLP-A structure. Monomeric PLP-A lacked lactogenic bioactivities and several attempts to sequence 29 and 33 kDa PLP-A monomers proved unsuccessful. Biochemical characterization of PLP-A species present in various intracellular and extracellular compartments indicated that PLP-A is normally secreted as high molecular weight complexes and that PLP-A species were also targeted to the nucleus. PLP-A species isolated from placental cytosol or from serum of pregnant rats predominantly circulated as disulfide-linked high molecular weight complexes. PLP-A antipeptide antisera showed limited but specific reactivity with the high molecular weight PLP-A species. We have specifically identified a 29 kDa protein species in placental cell nuclei with three antibodies directed to three different regions of the predicted amino acid sequence of PLP-A. The 29 kDa nuclear immunoreactive protein had an equivalent electrophoretic mobility to the 29 kDa PLP-A protein of junctional zone cytosol. The 33 kDa PLP-A protein was not identified in the nucleus. The biological significance of the circulating high molecular weight PLP-A species or the nuclear PLP-A species remains to be determined.