Acquisition of anti-phosphatidylserine IgM and IgG antibodies by infants and their mothers over time in Uganda.

Background: Production of anti-phosphatidylserine (anti-PS) antibodies has been associated with malaria and can aggravate pathology. How these autoantibodies develop during early childhood in a malaria context is not known. We examined levels of anti-PS IgG and IgM antibodies in a longitudinal cohort of mother-baby pairs during birth, in the infants at 2.5, 6 months, and in mothers and their babies at 9 months postpartum. Results: There was no difference between levels of anti-PS IgG in cord blood and the mothers' peripheral blood at birth. However, anti-PS IgM levels were significantly higher in the mothers compared to the infants' cord blood, and IgM levels were steadily increasing during the first 9 months of the infants' life. In infants that had the highest anti-PS IgM levels at birth, there was a decline until 6 months with a rise at 9 months. Infants that possessed high anti-PS IgG at birth also exhibited a progressive decline in levels. When anti-PS were correlated to different fractions of B-cells, there were several correlations with P. falciparum specific atypical B cells both at birth and at 2.5 months for the infants, especially for anti-PS IgM. Anti-PS also correlated strongly to C1q-fixing antibodies at birth. Conclusion: These results show that anti-PS IgG acquired by mothers could be transferred transplacentally and that IgM antibodies targeting PS are acquired during the first year of life. These results have increased the knowledge about autoimmune responses associated with infections in early life and is critical for a comprehensive understanding of malaria vaccine functionality in endemic areas.

Uncovering key steps in FGF12 cellular release reveals a common mechanism for unconventional FGF protein secretion.

FGF12 belongs to a subfamily of FGF proteins called FGF homologous factors (FHFs), which until recently were thought to be non-signaling intracellular proteins. Our recent studies have shown that although they lack a conventional signal peptide for secretion, they can reach the extracellular space, especially under stress conditions. Here, we unraveled that the long "a" isoform of FGF12 is secreted in a pathway involving the A1 subunit of Na(+)/K(+) ATPase (ATP1A1), Tec kinase and lipids such as phosphatidylinositol and phosphatidylserine. Further, we showed that the short "b" isoform of FGF12, which binds ATP1A1 and phosphatidylserine less efficiently, is not secreted from cells. We also indicated regions in the FGF12a protein sequence that are crucial for its secretion, including N-terminal fragment and specific residues, and proposed that liquid-liquid phase separation may be important in this process. Our results strongly suggest that the mechanism of this process is very similar for all unconventionally secreted FGF proteins.

Differential Lipid Binding Specificities of RAP1A and RAP1B are Encoded by the Amino Acid Sequence of the Membrane Anchors.

RAP1 proteins belong to the RAS family of small GTPases that operate as molecular switches by cycling between GDP-bound inactive and GTP-bound active states. The C-terminal anchors of RAP1 proteins are known to direct membrane localization, but how these anchors organize RAP1 on the plasma membrane (PM) has not been investigated. Using high-resolution imaging, we show that RAP1A and RAP1B form spatially segregated nanoclusters on the inner leaflet of the PM, with further lateral segregation between GDP-bound and GTP-bound proteins. The C-terminal polybasic anchors of RAP1A and RAP1B differ in their amino acid sequences and exhibit different lipid binding specificities, which can be modified by single-point mutations in the respective polybasic domains (PBD). Molecular dynamics simulations reveal that single PBD mutations substantially reduce the interactions of the membrane anchors with the PM lipid phosphatidylserine. In summary, we show that aggregate lipid binding specificity encoded within the C-terminal anchor determines PM association and nanoclustering of RAP1A and RAP1B. Taken together with previous observations on RAC1 and KRAS, the study reveals that the PBD sequences of small GTPase membrane anchors can encode distinct lipid binding specificities that govern PM interactions.

A change in metal cation switches selectivity of a phospholipid sensor from phosphatidic acid to phosphatidylserine.

Phosphatidic acid and phosphatidylserine are anionic phospholipids with emerging signalling roles in cells. Determination of how phosphatidic acid and phosphatidylserine change location and quantity in cells over time requires selective fluorescent sensors that can distinguish these two anionic phospholipids. However, the design of such synthetic sensors that can selectively bind and respond to a single phospholipid within the complex membrane milieu remains challenging. In this work, we present a simple and robust strategy to control the selectivity of synthetic sensors for phosphatidic acid and phosphatidylserine. By changing the coordination metal of a dipicolylamine (DPA) ligand from Zn(II) to Ni(II) on the same synthetic sensor with a peptide backbone, we achieve a complete switch in selectivity from phosphatidic acid to phosphatidylserine in model lipid membranes. Furthermore, this strategy was largely unaffected by the choice and the position of the fluorophores. We envision that this strategy will provide a platform for the rational design of targeted synthetic phospholipid sensors to probe plasma and intracellular membranes.

Caspase-8 promotes scramblase-mediated phosphatidylserine exposure and fusion of osteoclast precursors.

Efficient cellular fusion of mononuclear precursors is the prerequisite for the generation of fully functional multinucleated bone-resorbing osteoclasts. However, the exact molecular factors and mechanisms controlling osteoclast fusion remain incompletely understood. Here we identify RANKL-mediated activation of caspase-8 as early key event during osteoclast fusion. Single cell RNA sequencing-based analyses suggested that activation of parts of the apoptotic machinery accompanied the differentiation of osteoclast precursors into mature multinucleated osteoclasts. A subsequent characterization of osteoclast precursors confirmed that RANKL-mediated activation of caspase-8 promoted the non-apoptotic cleavage and activation of downstream effector caspases that translocated to the plasma membrane where they triggered activation of the phospholipid scramblase Xkr8. Xkr8-mediated exposure of phosphatidylserine, in turn, aided cellular fusion of osteoclast precursors and thereby allowed generation of functional multinucleated osteoclast syncytia and initiation of bone resorption. Pharmacological blockage or genetic deletion of caspase-8 accordingly interfered with fusion of osteoclasts and bone resorption resulting in increased bone mass in mice carrying a conditional deletion of caspase-8 in mononuclear osteoclast precursors. These data identify a novel pathway controlling osteoclast biology and bone turnover with the potential to serve as target for therapeutic intervention during diseases characterized by pathologic osteoclast-mediated bone loss. Proposed model of osteoclast fusion regulated by caspase-8 activation and PS exposure. RANK/RANK-L interaction. Activation of procaspase-8 into caspase-8. Caspase-8 activates caspase-3. Active capase-3 cleaves Xkr8. Local PS exposure is induced. Exposed PS is recognized by the fusion partner. FUSION. PS is re-internalized.

Chikungunya virus release is reduced by TIM-1 receptors through binding of envelope phosphatidylserine.

T-cell immunoglobin and mucin domain protein-1 (TIM-1) mediates entry of chikungunya virus (CHIKV) into some mammalian cells through the interaction with envelope phospholipids. While this interaction enhances entry, TIM-1 has been shown to tether newly formed HIV and Ebola virus particles, limiting their efficient release. In this study, we investigate the ability of surface receptors such as TIM-1 to sequester newly budded virions on the surface of infected cells. We established a luminescence reporter system to produce chikungunya viral particles that integrate nano-luciferase and easily quantify viral particles. We found that TIM-1 on the surface of host cells significantly reduced CHIKV release efficiency in comparison to other entry factors. Removal of cell surface TIM-1 through direct cellular knock-out or altering the cellular lipid distribution enhanced CHIKV release. Over the course of infection, CHIKV was able to counteract the tethering effect by gradually decreasing the surface levels of TIM-1 in a process mediated by the nonstructural protein 2. This study highlights the importance of phosphatidylserine receptors in mediating not only the entry of CHIKV but also its release and could aid in developing cell lines capable of enhanced vaccine production. IMPORTANCE: Chikungunya virus (CHIKV) is an enveloped alphavirus transmitted by the bites of infectious mosquitoes. Infection with CHIKV results in the development of fever, joint pain, and arthralgia that can become chronic and last for months after infection. Prevention of this disease is still highly focused on vector control strategies. In December 2023, a new live attenuated vaccine against CHIKV was approved by the FDA. We aimed to study the cellular factors involved in CHIKV release, to better understand CHIKV's ability to efficiently infect and spread among a wide variety of cell lines. We found that TIM-1 receptors can significantly abrogate CHIKV's ability to efficiently exit infected cells. This information can be beneficial for maximizing viral particle production in laboratory settings and during vaccine manufacturing.

Interplay between Copper, Phosphatidylserine, and α-Synuclein Suggests a Link between Copper Homeostasis and Synaptic Vesicle Cycling.

Copper homeostasis is critical to the functioning of the brain, and its breakdown is linked with many brain diseases. Copper is also known to interact with the negatively charged lipid, phosphatidylserine (PS), as well as α-synuclein, an aggregation-prone protein enriched in the synapse, which plays a role in synaptic vesicle docking and fusion. However, the interplay between copper, PS lipid, and α-synuclein is not known. Herein, we report a detailed and predominantly kinetic study of the interactions among these three components pertinent to copper homeostasis and neurotransmission. We found that synaptic vesicle-mimicking small unilamellar vesicles (SUVs) can sequester any excess free Cu2+ within milliseconds, and bound Cu2+ on SUVs can be reduced to Cu+ by GSH at a nearly constant rate under physiological conditions. Moreover, we revealed that SUV-bound Cu2+ does not affect the binding between wild-type α-synuclein and SUVs but affect that between N-terminal acetylated α-synuclein and SUVs. In contrast, Cu2+ can effectively displace both types of α-synuclein from the vesicles. Our results suggest that synaptic vesicles may mediate copper transfer in the brain, while copper could participate in synaptic vesicle docking to the plasma membrane via its regulation of the interaction between α-synuclein and synaptic vesicle.

Cryo-EM architecture of a near-native stretch-sensitive membrane microdomain.

Biological membranes are partitioned into functional zones termed membrane microdomains, which contain specific lipids and proteins1-3. The composition and organization of membrane microdomains remain controversial because few techniques are available that allow the visualization of lipids in situ without disrupting their native behaviour3,4. The yeast eisosome, composed of the BAR-domain proteins Pil1 and Lsp1 (hereafter, Pil1/Lsp1), scaffolds a membrane compartment that senses and responds to mechanical stress by flattening and releasing sequestered factors5-9. Here we isolated near-native eisosomes as helical tubules made up of a lattice of Pil1/Lsp1 bound to plasma membrane lipids, and solved their structures by helical reconstruction. Our structures reveal a striking organization of membrane lipids, and, using in vitro reconstitutions and molecular dynamics simulations, we confirmed the positioning of individual PI(4,5)P2, phosphatidylserine and sterol molecules sequestered beneath the Pil1/Lsp1 coat. Three-dimensional variability analysis of the native-source eisosomes revealed a dynamic stretching of the Pil1/Lsp1 lattice that affects the sequestration of these lipids. Collectively, our results support a mechanism in which stretching of the Pil1/Lsp1 lattice liberates lipids that would otherwise be anchored by the Pil1/Lsp1 coat, and thus provide mechanistic insight into how eisosome BAR-domain proteins create a mechanosensitive membrane microdomain.

Correlation of Anti-Phosphatidylserine/Prothrombin and Anti-Phosphatidylserine Antibodies with Pregnancy Outcomes.

BACKGROUND: The objective of this study was to investigate the clinical relevance of anti-prothrombin antibodies (aPT) and anti-phosphatidylserine/prothrombin antibodies (aPS/PT) in relation to pregnancy outcomes and coagulation parameters, as well as immune markers. METHODS: We retrospectively analyzed 477 pregnant women with experienced at least one spontaneous miscarriage who were tested for aPT and aPS/PT antibodies, and compared their clinical characteristics, coagulation indicators, immune biomarkers, and pregnancy outcomes to assess the diagnostic accuracy of these antibodies. RESULTS: We found that the aPT IgG and the aPS/PT IgM were independently associated with increased risk of pregnancy loss, with odds ratios (ORs) of 1.055 (95% confidence interval [CI]: 1.009-1.103, p = 0.017) and 1.041 (95% CI: 1.015-1.067, p = 0.002), respectively. Moreover, we found that the aPS/PT IgM had a higher diagnostic performance than the aPT IgG, as indicated by the AUC of 0.663 and 0.593, respectively. The pregnancy loss rate was positively correlated with the level of aPS/PT IgM, while the aPT IgG is not. We also found that in the pregnancy loss group, aPT IgG showed negative correlations with prothrombin time (PT); aPS/PT IgM showed positive correlations with aPS/PT IgG. However, none of aPT IgG, aPT IgM, aPS/PT IgM, or aPS/PT IgG was related to other adverse pregnancy outcomes, such as preterm delivery, fetal growth restriction (FGR), or preeclampsia (PE). CONCLUSION: Our findings suggest that aPT IgG and aPS/PT IgM are independent risk factors for pregnancy loss, especially aPS/PT IgM, which has a positive linear correlation with pregnancy loss.

Enhancing Spatial Resolution in Tandem Mass Spectrometry Ion/Ion Reaction Imaging Experiments through Image Fusion.

We have recently developed a charge inversion ion/ion reaction to selectively derivatize phosphatidylserine lipids via gas-phase Schiff base formation. This tandem mass spectrometry (MS/MS) workflow enables the separation and detection of isobaric lipids in imaging mass spectrometry, but the images acquired using this workflow are limited to relatively poor spatial resolutions due to the current time and limit of detection requirements for these ion/ion reaction imaging mass spectrometry experiments. This trade-off between chemical specificity and spatial resolution can be overcome by using computational image fusion, which combines complementary information from multiple images. Herein, we demonstrate a proof-of-concept workflow that fuses a low spatial resolution (i.e., 125 µm) ion/ion reaction product ion image with higher spatial resolution (i.e., 25 µm) ion images from a full scan experiment performed using the same tissue section, which results in a predicted ion/ion reaction product ion image with a 5-fold improvement in spatial resolution. Linear regression, random forest regression, and two-dimensional convolutional neural network (2-D CNN) predictive models were tested for this workflow. Linear regression and 2D CNN models proved optimal for predicted ion/ion images of PS 40:6 and SHexCer d38:1, respectively.

Phosphatidylserine enrichment in the nuclear membrane regulates key enzymes of phosphatidylcholine synthesis.

Phosphatidylserine (PS) is an important anionic phospholipid that is synthesized within the endoplasmic reticulum (ER). While PS shows the highest enrichment and serves important functional roles in the plasma membrane (PM) but its role in the nucleus is poorly explored. Using three orthogonal approaches, we found that PS is also uniquely enriched in the inner nuclear membrane (INM) and the nuclear reticulum (NR). Nuclear PS is critical for supporting the translocation of CCTα and Lipin1α, two key enzymes important for phosphatidylcholine (PC) biosynthesis, from the nuclear matrix to the INM and NR in response to oleic acid treatment. We identified the PS-interacting regions within the M-domain of CCTα and M-Lip domain of Lipin1α, and show that lipid droplet formation is altered by manipulations of nuclear PS availability. Our studies reveal an unrecognized regulatory role of nuclear PS levels in the regulation of key PC synthesizing enzymes within the nucleus.

TMEM16F exacerbates tau pathology and mediates phosphatidylserine exposure in phospho-tau-burdened neurons.

TMEM16F is a calcium-activated phospholipid scramblase and nonselective ion channel, which allows the movement of lipids bidirectionally across the plasma membrane. While the functions of TMEM16F have been extensively characterized in multiple cell types, the role of TMEM16F in the central nervous system remains largely unknown. Here, we sought to study how TMEM16F in the brain may be involved in neurodegeneration. Using a mouse model that expresses the pathological P301S human tau (PS19 mouse), we found reduced tauopathy and microgliosis in 6- to 7-mo-old PS19 mice lacking TMEM16F. Furthermore, this reduction of pathology can be recapitulated in the PS19 mice with TMEM16F removed from neurons, while removal of TMEM16F from microglia of PS19 mice did not significantly impact tauopathy at this time point. Moreover, TMEM16F mediated aberrant phosphatidylserine exposure in neurons with phospho-tau burden. These studies raise the prospect of targeting TMEM16F in neurons as a potential treatment of neurodegeneration.

Tubulin-binding region alters tau-lipid interactions and changes toxicity of tau fibrils formed in the presence of phosphatidylserine lipids.

Alzheimer's disease is the fastest-growing neurodegenerative disease that affects over six million Americans. The abnormal aggregation of amyloid ß peptide and Tau protein is the expected molecular cause of the loss of neurons in brains of AD patients. A growing body of evidence indicates that lipids can alter the aggregation rate of amyloid ß peptide and modify the toxicity of amyloid ß aggregates. However, the role of lipids in Tau aggregation remains unclear. In this study, we utilized a set of biophysical methods to determine the extent to which phospatidylserine (PS) altered the aggregation properties of Tau isoforms with one (1N4R) and two (2N4R) N terminal inserts that enhance the binding of Tau to tubulin. We found that the length and saturation of fatty acids (FAs) in PS altered the aggregation rate of 2N4R isoform, while no changes in the aggregation rate of 1N4R were observed. These results indicate that N terminal inserts play an important role in protein-lipid interactions. We also found that PS could change the toxicity of 1N4R and 2N4R Tau fibrils, as well as alter molecular mechanisms by which these aggregates exert cytotoxicity to neurons. Finally, we found that although Tau fibrils formed in the presence and absence of PS endocytosed by cells, only fibril species that were formed in the presence of PS exert strong impairment of the cell mitochondria.

Phosphatidylserine and/or Sialic Acid Modified Liposomes Increase Uptake by Tumor-associated Macrophages and Enhance the Anti-tumor Effect.

DOX liposomes have better therapeutic effects and lower toxic side effects. The targeting ability of liposomes is one of the key factors affecting the therapeutic effect of DOX liposomes. This study developed two types of targeted liposomes. Sialic acid (SA)-modified liposomes were designed to target the highly expressed Siglec-1 receptor on tumor-associated macrophages surface. Phosphatidylserine (PS)-modified liposomes were designed to promote phagocytosis by monocyte-derived macrophages through PS apoptotic signaling. In order to assess and compare the therapeutic potential of different targeted pathways in the context of anti-tumor treatment, we compared four phosphatidylserine membrane materials (DOPS, DSPS, DPPS and DMPS) and found that liposomes prepared using DOPS as material could significantly improve the uptake ability of RAW264.7 cells for DOX liposomes. On this basis, normal DOX liposomes (CL-DOX) and SA-modified DOX liposomes (SAL-DOX), PS-modified DOX liposomes (PS-CL-DOX), SA and PS co-modified DOX liposomes (PS-SAL-DOX) were prepared. The anti-tumor cells function of each liposome on S180 and RAW264.7 in vitro was investigated, and it was found that SA on the surface of liposomes can increase the inhibitory effect. In vivo efficacy results exhibited that SAL-DOX and PS-CL-DOX were superior to other groups in terms of ability to inhibit tumor growth and tumor inhibition index, among which SAL-DOX had the best anti-tumor effect. Moreover, SAL-DOX group mice had high expression of IFN-γ as well as IL-12 factors, which could significantly inhibit mice tumor growth, improve the immune microenvironment of the tumor site, and have excellent targeted delivery potential.

Investigation of miltefosine-model membranes interactions at the molecular level for two different PS levels modeling cancer cells.

Miltefosine (MLT) is a broad-spectrum drug included in the alkylphospholipids (APL) used against leishmania and various types of cancer. The most crucial feature of APLs is that they are thought to only kill cancerous cells without harming normal cells. However, the molecular mechanism of action of APLs is not completely understood. The increase in the phosphatidylserine (PS) ratio is a marker showing the stage of cancer and even metastasis. The goal of this research was to investigate the molecular effects of miltefosine at the molecular level in different PS ratios. The effects of MLT on membrane phase transition, membrane orders, and dynamics were studied using DPPC/DPPS (3:1) and DPPC/DPPS (1:1) multilayer (MLV) vesicles mimicking DPPS ratio variation, Differential Scanning Calorimetry (DSC), and Fourier Transform Infrared spectroscopy (FTIR). Our findings indicate that miltefosine is evidence at the molecular level that it is directed towards the tumor cell and that the drug's effect increases with the increase of anionic lipids in the membrane depending on the stage of cancer.

Lipid Identification of Biomarkers in Esophageal Squamous Cell Carcinoma by Lipidomic Analysis.

Lipids participate in many important biological functions through energy storage, membrane structure stabilization, signal transduction, and molecular recognition. Previous studies have shown that patients with esophageal squamous cell carcinoma (ESCC) have abnormal lipid metabolism. However, studies characterizing lipid metabolism in ESCC patients through lipidomics are limited. Plasma lipid profiles of 65 ESCC patients and 42 healthy controls (HC) were characterized by lipidomics-based ultraperformance liquid chromatography quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS). Single-factor and multi-factor statistical analysis were used to screen the differences in blood lipids between groups, and combined with component ratio analysis and receiver operating characteristic (ROC) curve diagnostic efficiency assessment, to reveal the potential mechanisms and biomarkers of ESCC. There were significant differences in lipid profiles between the ESCC and HC groups. Thirty-six differential lipids (11 up-regulated and 25 down-regulated) were selected based on the criteria of p < .05 and fold change > 1.3 or < 0.77. Glycerophospholipids were the major differential lipids, suggesting that these lipid metabolic pathways exhibit a significant imbalance that may contribute to the development of esophageal squamous cell carcinoma. Among them, the seven candidate biomarkers for esophageal squamous cell carcinoma with the highest diagnostic value are three phosphatidylserine (PS), three fatty acids (FA) and one phosphatidylcholine (PC).

Overexpression of phosphatidylserine synthase IbPSS1 enhances salt tolerance by stimulating ethylene signaling-dependent lignin synthesis in sweetpotato roots.

Phosphatidylserine (PS) is an important lipid signaling required for plant growth regulation and salt stress adaptation. However, how PS positively regulate plant salt tolerance is still largely unknown. In this study, IbPSS1-overexpressed sweetpotato plants that exhibited overproduction of PS was employed to explore the mechanisms underlying the PS stimulation of plant salt tolerance. The results revealed that the IbPSS1-overexpressed sweetpotato accumulated less Na+ in the stem and leaf tissues compared with the wild type plants. Proteomic profile of roots showed that lignin synthesis-related proteins over-accumulated in IbPSS1-overexpressed sweetpotato. Correspondingly, the lignin content was enhanced but the influx of Na + into the stele was significantly blocked in IbPSS1-overexpressed sweetpotato. The results further revealed that ethylene synthesis and signaling related genes were upregulated in IbPSS1-overexpressed sweetpotato. Ethylene imaging experiment revealed the enhancement of ethylene mainly localized in the root stele. Inhibition of ethylene synthesis completely reversed the PS-overproduction induced lignin synthesis and Na+ influx pattern in stele tissues. Taken together, our findings demonstrate a mechanism by which PS regulates ethylene signaling and lignin synthesis in the root stele, thus helping sweetpotato plants to block the loading of Na+ into the xylem and to minimize the accumulation of Na+ in the shoots.

Disruption of Transmembrane Phosphatidylserine Asymmetry by HIV-1 Incorporated SERINC5 Is Not Responsible for Virus Restriction.

Host restriction factor SERINC5 (SER5) incorporates into the HIV-1 membrane and inhibits infectivity by a poorly understood mechanism. Recently, SER5 was found to exhibit scramblase-like activity leading to the externalization of phosphatidylserine (PS) on the viral surface, which has been proposed to be responsible for SER5's antiviral activity. This and other reports that document modulation of HIV-1 infectivity by viral lipid composition prompted us to investigate the role of PS in regulating SER5-mediated HIV-1 restriction. First, we show that the level of SER5 incorporation into virions correlates with an increase in PS levels in the outer leaflet of the viral membrane. We developed an assay to estimate the PS distribution across the viral membrane and found that SER5, but not SER2, which lacks antiviral activity, abrogates PS asymmetry by externalizing this lipid. Second, SER5 incorporation diminished the infectivity of pseudoviruses produced from cells lacking a flippase subunit CDC50a and, therefore, exhibited a higher baseline level of surface-accessible PS. Finally, exogenous manipulation of the viral PS levels utilizing methyl-alpha-cyclodextrin revealed a lack of correlation between external PS and virion infectivity. Taken together, our study implies that the increased PS exposure to SER5-containing virions itself is not directly linked to HIV-1 restriction.

Binding of coagulation factor IXa to procoagulant platelets revisited: Low affinity and interactions with other factors.

Binding of activated factor IX (fIXa) to the phosphatidylserine-expressing procoagulant platelets is a critical step in blood coagulation, which is necessary for the membrane-dependent intrinsic tenase complex assembly and factor X activation. However, the nature and parameters of the fIXa binding sites on the procoagulant platelet surface remain unclear. We used flow cytometry to elucidate the quantitative details of the fluorescently labeled fIXa binding to gel-filtered activated platelets. FIXa bound to the procoagulant platelet subpopulation only, with the parameters (maximal number of binding sites at 58900 ± 3400, Kd at 1000 ± 170 nM) similar to binding observed with phospholipid vesicles. No specific high-affinity binding sites for fIXa were detected, and binding proceeded similarly for different methods of procoagulant platelet production (thrombin, thrombin receptor activation peptide, collagen-related peptide, their combinations, or calcium ionophore A23187). Factor VIII, known to form a high affinity complex with fIXa, enhanced fIXa binding to platelets. In contrast, only competition effects were observed for factor X, which binds fIXa with much lower affinity. Unexpectedly, fIXa itself, fIX, and prothrombin also dose-dependently enhance fIXa binding at concentrations below 1000 nM, suggesting the formation of membrane-bound fIXa dimers and fIXa-prothrombin complexes on platelets. These findings provide a novel perspective on the fIXa binding site on procoagulant platelets, which does not have any major differences from pure phospholipid-based model membranes, exhibits inherently low affinity (3-5 orders of magnitude below the physiologically relevant fIXa concentration) but is significantly enhanced by its cofactor VIII, and regulated by previously unknown membrane interactions.