Iodination of the Human Platelet Membrane

Lectin-based carbohydrate profile of megakaryocytes in murine fetal liver during development

Hematopoiesis is the process by which blood cells are generated. During embryonic development, these cells migrate through different organs until they reach the bone marrow, their definitive place in adulthood. Around E10.5, the fetal liver starts budding from the gut, where first hematopoietic cells arrive and expand. Hematopoietic cell migration occurs through cytokine stimulation, receptor expression, and glycosylation patterns on the cell surface. In addition, carbohydrates can modulate different cell activation states. For this reason, we aimed to characterize and quantify fetal megakaryocytic cells in mouse fetal liver according to their glycan residues at different gestational ages through lectins. Mouse fetuses between E11.5 and E18.5 were formalin-fixed and, paraffin-embedded, for immunofluorescence analysis using confocal microscopy. The results showed that the following sugar residues were expressed in proliferating and differentiating megakaryocytes in the fetal liver at different gestational ages: α-mannose, α-glucose, galactose, GlcNAc, and two types of complex oligosaccharides. Megakaryocytes also showed three proliferation waves during liver development at E12.5, E14.5, and E18.5. Additionally, the lectins that exhibited high and specific pattern intensities at liver capsules and vessels were shown to be a less time-consuming and robust alternative alternative to conventional antibodies for displaying liver structures such as capsules and vessels, as well as for megakaryocyte differentiation in the fetal liver.

Study on the phylogenetic tree of human platelet glycoproteins

Platelet glycoprotein is an important group of glycoproteins on platelets. Several types of platelet glycoproteins have been studied for their functions in the hemostasis system. A bioinformatic analysis was performed to find out how the platelet glycoproteins' genes are related to each other. A multiple sequence alignment phylogenetic tree was performed to present the family tree of the human platelet glycoproteins recorded in the genomic database, ExPASY. These derived sequences from the database were processed by ClustalW and subsequently used for preparation of the distance matrix by Phylip protdist. The final generated phylogenetic tree of human platelet glycoproteins was presented and discussed.

Procedure for quantification of platelet adhesion to biomaterials by radioscintigraphy

Detection of adhered platelets on biomaterial surface that has blood-contacting application is an important test to assess its thrombogenicity. Usually, for measurement of platelet adhesion, after exposure to platelet-rich plasma (PRP) under standardized conditions the test surface is rinsed to remove non-adherent cells and is analyzed under scanning electron microscopy (SEM) to detect morphology of adhered cell and degree of aggregate formation. However, being a qualitative test it is unlikely to give an accurate estimate of platelets adhered to the surface. On the other hand, use of radiolabels enables quantification of platelets deposited on a material or device. Because of high gamma emission of (111)In, it can be used for radioscintigraphy, however, its short half life (2.5 days) is a major hurdle in using it for quantification of platelet adhesion. (125)I is a relatively strong radiolabel that is easily tagged to most of the proteins and has a relatively long half-life (60 days). The major objectives of this study are to standardize the labeling conditions to get good (125)I activity on platelets, while maintaining normal cell function after they are labeled. Considering all possible uncertainties, quantity of isotope and platelets to be used and the conditions of iodination reaction are established to get repeatable and reproducible labeling of platelets. Further, it is demonstrated that (125)I-platelets can be used to determine total number of cells adhered to titanium surface, which is known to be used as a blood-contacting biomaterial.

The peanut-agglutinin (PNA)-binding surface components of malignant plasma cells

Plasma cells within bone marrow aspirates from multiple myeloma patients have been shown to be reactive with the lectin peanut agglutinin (PNA). This has been recently exploited by using PNA for purging bone marrow of malignant cells in autotransplantation therapy of the disease. The purpose of this investigation was to isolate and characterize the PNA-binding proteins of myeloma cells. We used the malignant plasma cell-derived line Karpas-620 (K620) as a model, and showed by affinity chromatography, SDS-PAGE, and immunoprecipitation that, among several PNA-binding proteins, a major one is an incompletely sialylated form of CD44. CD44 is a well-known homing receptor protein which is rich in carbohydrate and usually completely sialylated so that it does not react with PNA. We have then examined the PNA reactivity of myeloma cells from different patients and showed a clear difference in the profile of PNA-binding proteins from case to case. Moreover, in contrast to K620 cells, some of the patient plasma cells tested did not have a PNA-binding form of CD44. In conclusion, therefore, we have shown that a number of different proteins participate in PNA binding by malignant plasma cells. Moreover, we have demonstrated a novel, incompletely sialylated form of CD44 on a myeloma cell line. It is known that the level of glycosylation of CD44 and other proteins may affect their function, but how this relates to the malignant behaviour of plasma cells remains to be determined.

Acute megakaryoblastic leukemia in one cat and two dogs

The clinical, hematologic, and histologic features of acute megakaryoblastic leukemia are described for an 8-year-old female Domestic Shorthair cat, a 3-year-old female mixed-breed dog, and a 3-year-old male German Shepherd Dog. The neoplastic cells were characterized as belonging to the megakaryocytic lineage. The following techniques were used: electron microscopy; detection of antibodies against human von Willebrand factor (vWF) and human platelet glycoprotein GP IIIa using a modified avidin biotin peroxidase complex technique on formalin-fixed paraffin sections; and enzyme histochemical methods on plastic sections for alkaline phosphatase, acid phosphatase, myeloperoxidase, alpha-naphthyl acetate esterase, alpha-naphthyl butyrate esterase, naphthol AS acetate esterase, and naphthol AS-D chloroacetate esterase. In addition, benign megakaryocytic cells, platelets, and neoplastic cells were labeled with lectins that have partially been shown to bind to platelet glycoproteins of other species. In healthy cats and dogs, the megakaryocytes and platelets reacted with lectins PSA, LCA, PHA-L, and WGA. Megakaryocytes and platelets from healthy cats were also labeled by lectin PNA. The lectins PHA-L and WGA reacted with neoplastic cells from the cat and both dogs. Lectin PNA bound to neoplastic cells from the cat, and lectins PSA, LCA, and SBA bound to neoplastic cells from both dogs. For the retrospective examination of paraffin-embedded material, the detection of vWF and GP IIIa appears to be the most reliable method for the identification of megakaryocytic cells.

Alterations of wheat-germ agglutinin binding pattern on cell surface of blood platelets after thrombin stimulation

An attempt was made to demonstrate wheat-germ agglutinin (WGA) binding sites on platelet surfaces after thrombin stimulation, by means of a post-embedding cytochemical technique using colloidal gold as marker at an ultrastructural level. In unstimulated platelets washed with EDTA, an intense uniform labeling of WGA-gold complexes was found on the surface membrane. When washed platelets were stimulated by thrombin in the absence of Ca2+, only a release reaction was induced. WGA labeling on the surface membranes of these platelets decreased dramatically. However, the labeling intensity of WGA-gold complexes on the surface membrane of aggregated platelets induced by thrombin in the presence of Ca2+ increased significantly compared to that of thrombin-stimulated platelets in the absence of Ca2+. In contrast to the uniform labeling on the surface membranes of unstimulated platelets, clusters of gold label were often found on the surface membrane of the aggregated platelets, although there was no significant quantitative difference in the labeling intensity between these two groups. Thus, we present direct morphological evidence demonstrating qualitative and quantitative alterations of WGA labeling on the surface membrane of platelets after thrombin stimulation. The possibility is considered that WGA-binding glycoproteins in the surface membrane are involved in the aggregation response after thrombin stimulation.

Molecular defects in interactions of platelets with the vessel wall

The objectives of this review have been to summarize the recent research on inherited defects involving abnormal platelet function and to illustrate how studies of hemorrhagic syndromes have led to an increased understanding of the molecular events involved in platelet adhesion and aggregation. Emphasis has been placed on the two primary hemostatic reactions: the interaction of platelets with von Willebrand factor to promote adhesion to the subendothelium, and the interaction of platelets with fibrinogen to promote platelet aggregation. Even as these events are more clearly defined, new concepts of molecular structure, function, and heterogeneity are emerging, and the variety of recognized genetic defects is becoming more complex.

The platelet contacts during aggregation

APD-stimulated and aggregated platelets show dense structures (DS) on their free surfaces and filament bridges within 40-50 nm wide spaces of contacts between aggregating platelets (bridge contacts). Within the bridge contacts tight contacts are observed. Adjacent to tight contacts plasmalemmal openings into a canalicular system are seen. Pits and the central ends of these membranes are coated as seen in serial sections. Cationized ferritin (CF) added before stimulation binds to the whole negatively charged surface. Closer packed CF particles were observed on the DS and in the contacts on the central part of the bridges. CF did not introduce qualitative changes to the formation of typical platelet contacts, in particular of the tight contacts. Surface bound CF was found in the adjacent plasmalemmal invaginations and in the canalicular endomembranes. This result suggests an endocytosis mechanism which cleans the platelets of surface material during formation of tight contacts in aggregates.

Monoclonal antibodies against antigens of the human platelet surface: preparation and properties

Three monoclonal antibodies, FMC24, 25 and 27, reactive with human platelets, are described. In normal blood the 3 antibodies are specific for platelets, but FMC27 reacts with leukemic blast cells in a proportion of acute leukemias. FMC25 precipitates the platelet membrane glycoprotein lb and a glycoprotein of molecular weight 22,000. The antibodies show different effects on platelet aggregation.

The role of cell surface proteins in platelet stimulation by monosodium urate crystals

To test whether urate crystal-membrane protein interactions mediate platelet stimulation, platelet membrane proteins were radiolabeled by lactoperoxidase, extracted with 1% Triton X-100, and incubated with urate crystals. The crystal-associated and supernate fractions were isolated and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and by 2-dimensional isoelectric focusing followed by SDS-PAGE. Four of the lactoperoxidase-radiolabeled polypeptides that associated with urate crystals had reduced molecular weights and pIs of Mr = 105,000, pI 4.9; Mr = 123,000, pI = 4.9; Mr = 123,000, pI = 5.3; and Mr = 132,000, pI = 4.8-6.3, respectively. These proteins were characterized with regard to their labeling intensities, apparent isoelectric points, apparent molecular weights (reduced and nonreduced), lectin-binding properties, carbohydrate- and protein-staining characteristic, and presence or absence in Glanzmann's thrombasthenia. They have been identified as derived from glycoproteins Ib, IIb, and III (Phillips-Agin nomenclature) and an unidentified membrane protein. To test whether removal of these proteins would result in a diminution of platelet response to urate, intact platelets were digested with chymotrypsin, resulting in cleavage of more than 80% of these proteins and a 5-fold reduction in secretory responsiveness to urate crystals. Response to a second platelet stimulus, collagen, was unaffected, indicating an intact secretory mechanism. In addition, when platelets were preincubated with F(ab')2 fragments of an antibody directed against these proteins, platelet secretory response to urate was inhibited by 50%, whereas the responses to collagen and thrombin were unaffected. Thus, membrane proteins appear to mediate platelet stimulation by urate crystals.

Antibodies against platelet membrane glycoproteins. II. Influence on ADP- and collagen-induced platelet aggregation, crossed immunoelectrophoresis studies and relevance to Glanzmann's thrombasthenia

In Glanzmann's thrombasthenia glycoproteins IIb and IIIa are missing or strongly reduced and aggregation to ADP, collagen and thrombin is impaired. Antibodies against glycoproteins IIb and IIIa did not entirely induce a thrombasthenia-like state in normal platelets. However, they did strongly inhibit collagen-induced aggregation and inhibited the second wave of aggregation induced by ADP. Crossed immunoelectrophoresis studies using Triton X-100 extracts of whole platelets with these antibodies gave a single immunoprecipitate. This immunoprecipitate was absent when similar studies were carried out with thrombasthenic platelets. Platelet antibodies gave a number of immunoprecipitates with normal platelets and differences were observed with thrombasthenic platelets, the most notable of which was a marked reduction in one of the major immunoprecipitates. These results provide further evidence that glycoproteins IIb and IIIa are involved in the latter stages of platelet aggregation.

Lactoperoxidase-125I localization of salt-extractable alkaline phosphatase on the cytoplasmic membrane of Bacillus licheniformis

Previous histochemical and biochemical localizations of alkaline phosphatase in Bacillus licheniformis MC14 have shown that the membrane-associated form of the enzyme is located on the inner surface of the cytoplasmic membrane, and soluble forms are located in the periplasmic space and in the growth medium. The distribution of salt-extractable alkaline phosphatase on the surfaces of the cytoplasmic membrane of B. licheniformis MC14 was determined by using lactoperoxidase-125I labeling techniques. Cells harvested during rapid alkaline phosphatase production were converted to protoplasts or lysed protoplasts and labeled. Analysis of the data obtained indicated that 30% of the salt-extractable, membrane-associated alkaline phosphatase was located on the outer surface of the cytoplasmic membrane, whereas 70% of the membrane-associated enzyme was localized on the inner surface. Controls for protoplast integrity (release of tritiated thymidine or examination of cytoplasmic proteins for label content) indicated excellent protoplast stability. Controls indicated that chemical labeling was not a factor in the apparent distribution of alkaline phosphatase on the membrane. These results support the previously reported histochemical localization of alkaline phosphatase on the membrane inner surface. The presence of alkaline phosphatase on the membrane outer surface is reasonable, considering the soluble forms of the enzyme found in the periplasmic region and in the culture medium.

Membrane asymmetry. A survey and critical appraisal of the methodology. I. Methods for assessing the asymmetric orientation and distribution of proteins

This and the companion article are aimed at surveying the methods used for the study of membrane asymmetry. The techniques employed for the assessment of the asymmetric distribution and orientation of membrane proteins are reviewed in this article, whereas those pertaining to the unequal distribution of lipids are detailed in the companion paper. The use of immunological techniques and lectins, functions of proteins and their perturbations, chemical reagents, enzymatic isotopic labeling and enzymatic cleavage of membrane proteins and physical techniques are discussed and illustrated using recent examples of their application. Whenever appropriate, problems involving crypticity and non-availability or non-reactivity of functional sites, relevant chemical functions or protein fragments to appropriate ligands, reagents or modifying enzymes are envisaged and possible modification of the exposure of proteins during preparation of ghosts and other drawbacks are discussed, the use of different techniques and control experiments in conjunction is recommended for a more realistic assessment of the distribution and orientation of proteins.

Visualisation of lectin binding sites on the surface of human platelets using lectins adsorbed to gold granules

Washed human platelets have been incubated with the lectins WGA, ConA and RCA1, adsorbed to different-sized gold particles. Plasma membrane receptors for each lectin were then located by scanning and transmission electron microscopy.

Isolation of human platelet glycoproteins

Human platelet glycoproteins were isolated from whole platelets by two methods. The first method, that of affinity chromatography on wheat germ agglutinin, is based on the known affinity of lectins for cell surface glycoproteins. When solubilized whole platelets are used as starting material for this procedure, elution with N-acetylglucosamine yields primarily a glycoprotein of Mr approximately 150 000 as estimated by sodium dodecyl sulfate-acrylamide gel electrophoresis. The second method is based on the ability of the chaotropic salt lithium diiodosalicylate to extract glycoprotein from particulate cell fractions in water-soluble form. This method yields three major glycopeptides with apparent molecular weights after sulfhydryl reduction of 145 000, 125 000, and 95 000 as estimated on 5.6% sodium dodecyl sulfate-acrylamide gels. Carboxymethylation of these preparations in the presence of sulfhydryl-reducing agent further resolves a glycoprotein of Mr approximately 165 000. Treatment of whole platelets by periodate oxidation and sodium[3H]-borohydride reduction labels the three major glycoproteins extracted by lithium diiodosalicylate and the glycoprotein of Mr approximately 150 000 isolated on wheat germ agglutinin confirming their surface orientation. However, glycoprotein with Mr approximately 165 000 resolved by carboxymethylation of the lithium diiodosalicylate extracted glycoprotein mixture was not labelled by this method, suggesting that it represents the granule protein with similar electrophoretic characteristics described by others. Phosphorylation of intact platelets with 32Pi also results in labelling of glycoproteins isolated by both methods, suggesting that these molecules traverse the bilipid layer of the platelet membrane, bearing reactive groups on both outer and cytoplasmic surfaces.

Isolation of human platelet and red blood cell plasma membrane proteins by preparative detergent electrophoresis

High resolution polyacrylamide gel electrophoretic techniques have been applied to the preparative isolation and analysis of plasma membrane proteins and glycoproteins from human platelets and red blood cells. The techniques presented allow relatively simple, direct, rapid and quantitative purification of a broad molecular weight range of membrane proteins, by means of continuous elution preparative gel electrophoresis of protein solubilized with sodium dodecyl sulfate. Spectrophotometric and fluorophotometric (fluorescamine) profiling, and high resolution gel electrophoretic analysis (SDS-acrylamide gradient slab gels, and gel electrofocusing) of eluted protein species indicate that purified membrane proteins of a broad molecular weight range may be obtained in a one step procedure, and in quantities and concentrations sufficient for further analytical or experimental procedures.

Radio-iodination of plasma membranes of toad bladder epithelium

The present report describes high yield enzymatic radio-iodination of the apical and basal-lateral plasma membranes of toad bladder epithelium, by a procedure that does not breach the functional integrity of the epithelium, as assessed by the basal and vasopressin-sensitive short-circuit current (SCC). Restriction of the label to the membrane surface, was ascertained by light and electron-microscopic autoradiographs. On the apical surface, the grains were over the glycocalyx and the plasma membrane. Analysis of the labeled glycocalyx by agarose gel filtration, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), as well as enzymatic and pH-dependent hydrolysis indicated that the glycocalyx is a trichloro-acetic acid-soluble macromolecular complex of high molecular weight composed of a peptide moiety attached to large prosthetic groups (presumably carbohydrates) by O-glycosidic bonds. Analysis of the labeled apical plasma membrane components by agarose gel filtration and SDS-PAGE disclosed the presence of six major species of apparent molecular weights: 23,000, 28,000, 37,000, 44,000, 68,000, and 95,000. More than half of the membrane-associated radio-iodine was in two bands of molecular weights 37,000 and 44,000. Concentrations of vasopressin and cyclic AMP sufficient to increase the SCC significantly did not modify the extent of membrane labeling or the distribution of the label among the apical membrane components (presumably proteins) as assessed by SDS-PAGE. Iodination in the presence of amiloride inhibited incorporation but did not change the pattern of the distribution of the label among the components resolved by SDS-PAGE. Iodination of basal-lateral plasma membranes, at a yield comparable to that obtained with apical labeling, was attained after about 30 min of exposure of the intact bladder to the labeling solutions. Approximately 25% of the basal-lateral labeling was lost when the epithelial cells were harvested after collagenase treatment, implying that some iodination of the basement membrane had taken place. Less than 10% of iodination of the apical or basal-lateral surfaces was accounted for by lipid-labeling. Analysis of the labeled apical and basal-lateral species by enzymatic digestion and thin layer chromatography disclosed that virtually all the radioactivity was present as mono-iodotyrosine (MIT).

Neutrophilic leukocyte membrane proteins. I. Isolation

Rabbit exudate-derived PMN were homogenized and the cell membranes isolated on a two-phase aqueous system. Glycoproteins were extracted from cell membranes with lithium diiodosalicylate. SDS polyacrylamide gel electrophoretic analysis showed a consistent pattern of three major glycoprotein entities. Cells radioiodinated supravitally showed most of the radioactivity associated with larger glycoprotein entities whereas PMN membranes radiolabeled after isolation yielded a single major peak of radioactivity associated with a much smaller protein entity. Heterologous antisera against rabbit PMN, PMN membranes, and membrane glycoproteins were all cytotoxic for PMN in the presence of complement, and all bound to the PMN surface as demonstrated with immunocolloidal gold on electron microscopy. The data suggest that one or more glycoprotein entities are membrane-associated ectoglycoproteins which can be radiolabeled supravitally.

Membrane asymmetry

The components of biological membranes are asymmetrically distributed between the membrane surfaces. Proteins are absolutely asymmetrical in that every copy of a polypeptide chain has the same orientation in the membrane, and lipids are nonabsolutely asymmetrical in that almost every type of lipid is present on both sides of the bilayer, but in different and highly variable amounts. Asymmetry is maintained by lack of transmembrane diffusion. Two types of membrane proteins, called ectoproteins and endoproteins, are distinguished. Biosynthetic pathways for both types of proteins and for membrane lipids are inferred from their topography and distribution in the formed cells. Note added in proof. A cell-free system has now been developed which permits the mechanisms of membrane protein assembly to be studied (108). The membrane glycoprotein of vesicular stomatitis virus has been synthesized by wheat germ ribosomes in the presence of rough endoplasmic reticulum from pancreas. The resulting polypeptide is incorporated into the membrane, spans the lipid bilayer asymmetrically, and is glycosylated (108). The amino terminal portion of this transmembrane protein is found inside the endoplasmic reticulum vesicle, while the carboxyl terminal portion is exposed on the outer surface of the vesicle. Furthermore, addition of the glycoprotein to membranes after protein synthesis does not result in incorporation of the protein into the membrane in the manner described above (108). Consequently, protein synthesis and incorporation into the membrane must be closely coupled. Indeed, using techniques to synchronize the growth of nascent polypeptides, it has been shown (109) that no more than one-fourth of the glycoprotein chain can be made in the absence of membranes and still cross the lipid bilayer when chains are subsequently completed in the presence of membranes. These findings demonstrate directly that the extracytoplasmic portion of an ectoprotein can cross the membrane only during biosynthesis, and not after.

[Altered platelet surface glycoproteins in chronic myeloid leukemia (author's transl)]

The present status of knowledge about the platelet membrane components suggests that glycoproteins (GP) are involved in the intercellular platelet specific reactions, such as adhesion and aggregation. Normal human platelets and platelet membranes solubilized under dissociating conditions produce characteristic polypeptide and GP electrophoretic patterns (PAGE). Platelets and isolated platelet membranes from patients with chronic myeloid leukemia (CML) presented dissimilarities of their glycoconjugates as compared to the normal platelets. The modification of the electrophoretic banding visualized by the periodic acid-Schiff's reagent (PAS) consisted in the decrease of the 155,000 GP L, the presence of two PAS positive bands in the area of the 135,000 normal PAS positive GP LL and a variable decrease of the 100,000 GP LLL (Apparent mol. wt. are indicated). In addition, preliminary data showed an increased catalytic transfer of the labelled galactosyl and N acetygalactosaminyl residues from exogenous nucleotide 14C-sugar precursors onto the CML-platelet endogenous sugar acceptors. These firstly reported data on molecular abnormalities of the platelet membrane GP in CML, suggested their possible relationship with the impaired platelet adhesion and aggregation occurring in this disease or the ability of leukemic platelets to express modified surface membrane components.

Isolation of the membrane glycoproteins of human blood platelets by lectin affinity chromatography

The major platelet membrane glycoproteins have been solubilized in 1.0% sodium deoxycholate and subjected to affinity chromatography on the lectins from Lens culinaris, wheat germ and Abrus precatorius. Polyacrylamide gel electrophoresis in the presence and absence of a reducing agent together with the differential binding of the lectins to the glycoproteins permitted the distinction of at least seven separate glycoprotein entities. A new nomenclature for the glycoproteins is proposed to accomodate the additional data. Using combinations of lectin columns, glycoproteins Ia and Ib could be prepared in a pure state and IIb and IIIa could be greatly purified. The binding of lectins to glycoprotein Ib has been strongly implicated as a necessary step in the aggregation response of platelets to lectins.

Plasma membrane phosphorylation by endogenous phosphate donors in human blood platelets. Selectivity of the action of dibutyryl cyclic AMP

Incubation of platelet-rich plasma with 32Pi leads to cellular uptake of the isotope and covalent incorporation into several cell constituents. Plasma membranes isolated from intact labelled platelets, delipidated and solubilized in sodium dodecyl sulfate, show, upon gel electorphoretic analysis, three main peaks of radioactivity: two in the molecular weight range 100 000-30 000 and an additional very slow migrating component strong positive by the peirodic acid-Schiff reaction. Treatment of the cells with dibutyryl cyclic AMP under conditions just sufficient to completely inhibit platelet aggregation leads to an increased isotope incorporation. Electrophoretic analysis of membranes isolated from dibutyryl cyclic AMP-treated cell reveals: (a) no change in the general pattern of distribution of the isotope, (b) no difference in the isotope incorporation to the two components of lower mol. wt. and (c) a marked increase (greater than 100%) in isotope incorporation in the slow migrating material as compared to membranes isolated from control cells. This material can be extracted from platelet plasma membranes after treatment of the membranes for 5 h with Triton X-100, at a detergent-to-protein ratio of 7.5. When the membrane material extracted with Triton X-100 is subjected to gel chromatography in Agarose (Biogel A-15m), the phosphorylated material that corresponds to the slow migrating band in polyacrylanide gel electrophoresis is eluted with or very close to the void volume of the column. Isoelectric focussing of this fraction, shows a single radioactive peak corresponding to an isolectric point of 3.78. The isolated component is pronase-sensitive, contains 52% of carbohydrate and 15% sialic acid. Analysis of the stability of the bound phosphate suggests that about 43% of it is bound as acyl-phosphate. The results reported, obtained through an approach that closely resembles physiological conditions are compatible with the participation of this membrane phosphoglycoprotein in the phenomena of platelet aggregation.