Elucidating the structural chemistry of glycosaminoglycan recognition by protein C inhibitor

A common protein C inhibitor exosite partially controls the heparin induced activation and inhibition of serine proteases

Protein C inhibitor (PCI) maintains hemostasis by inhibiting both procoagulant and anticoagulant serine proteases, and plays important roles in coagulation, fibrinolysis, reproduction, and anti-angiogenesis. The reactive site loop of PCI traps and irreversibly inhibits the proteases like APC (activating protein C), thrombin (FIIa) and factor Xa (FXa). Previous studies on antithrombin (ATIII) had identified Tyr253 and Glu255 as functional exosites that interact and aid in the inhibition of factor IXa and FXa. Presence of exosite in PCI is not known, however a sequence comparison with the PCI from different vertebrate species and ATIII identified Glu239 to be absolutely conserved. PCI residues analogous to ATIII exosite residues were mutated to R238A and E239A. Purified variant PCI in the presence of heparin (10 μg/ml) showed a 2-4 fold decrease in the rate of inhibition of the proteases. However, the stoichiometry of inhibition of FIIa, APC, and FXa by native PCI, R238A and E239A variants were found to be close to 1.0, which also indicated the formation of stable complexes based on SDS-PAGE and western blot analysis with thrombin and APC. Our findings revealed the possible presence of an exosite in PCI that influences the protease inhibition rates.

Investigation of the structure of regulatory proteins interacting with glycosaminoglycans by combining NMR spectroscopy and molecular modeling - the beginning of a wonderful friendship

The interaction of regulatory proteins with extracellular matrix or cell surface-anchored glycosaminoglycans (GAGs) plays important roles in molecular recognition, wound healing, growth, inflammation and many other processes. In spite of their high biological relevance, protein-GAG complexes are significantly underrepresented in structural databases because standard tools for structure determination experience difficulties in studying these complexes. Co-crystallization with subsequent X-ray analysis is hampered by the high flexibility of GAGs. NMR spectroscopy experiences difficulties related to the periodic nature of the GAGs and the sparse proton network between protein and GAG with distances that typically exceed the detection limit of nuclear Overhauser enhancement spectroscopy. In contrast, computer modeling tools have advanced over the last years delivering specific protein-GAG docking approaches successfully complemented with molecular dynamics (MD)-based analysis. Especially the combination of NMR spectroscopy in solution providing sparse structural constraints with molecular docking and MD simulations represents a useful synergy of forces to describe the structure of protein-GAG complexes. Here we review recent methodological progress in this field and bring up examples where the combination of new NMR methods along with cutting-edge modeling has yielded detailed structural information on complexes of highly relevant cytokines with GAGs.

So you think computational approaches to understanding glycosaminoglycan-protein interactions are too dry and too rigid? Think again!

Glycosaminoglycans (GAGs) play key roles in virtually all biologic responses through their interaction with proteins. A major challenge in understanding these roles is their massive structural complexity. Computational approaches are extremely useful in navigating this bottleneck and, in some cases, the only avenue to gain comprehensive insight. We discuss the state-of-the-art on computational approaches and present a flowchart to help answer most basic, and some advanced, questions on GAG-protein interactions. For example, firstly, does my protein bind to GAGs?; secondly, where does the GAG bind?; thirdly, does my protein preferentially recognize a particular GAG type?; fourthly, what is the most optimal GAG chain length?; fifthly, what is the structure of the most favored GAG sequence?; and finally, is my GAG-protein system 'specific', 'non-specific', or a combination of both? Recent advances show the field is now poised to enable a non-computational researcher perform advanced experiments through the availability of various tools and online servers.

Computational drill down on FGF1-heparin interactions through methodological evaluation

Glycosaminoglycans (GAGs) exhibit a key role in cellular communication processes through interactions with target proteins of the extracellular matrix (ECM). The sandwich-like interaction established between Fibroblast growth factor (FGF) and heparin (HE) represents quite a peculiar protein-GAG-protein system, which has been both structurally and functionally intensively studied. The molecular recognition characteristics of this system have been exploited in various computational studies in order to deepen understanding of GAG-protein interactions. Here, we drill down on the interactions established in this peculiar macromolecular complex by analyzing the applicability of docking techniques and molecular dynamics (MD)-based approaches, and we dissect the molecular recognition properties exhibited by FGF towards a series of HE derivatives. We examine the sensitivity of MM-GBSA free energy calculations in terms of receptor conformational space sampling and changes in the ligand structures. Furthermore, we investigate its predictive power in combination with other computational methods, namely the well-established Autodock3 (AD3) and dynamic molecular docking (DMD), a targeted MD-based docking method specifically developed to account for flexibility and solvent in computer simulations of protein-GAG systems. Our results show that a site-mapping approach can be effectively combined with AD3 and DMD calculations to accurately reproduce available experimental data and, furthermore, to determine specific GAG recognition patterns. This study deepens our understanding of the applicability of available theoretical approaches to the investigation of molecular recognition in protein-GAG systems.

Computational analysis of interactions in structurally available protein-glycosaminoglycan complexes

Glycosaminoglycans represent a class of linear anionic periodic polysaccharides, which play a key role in a variety of biological processes in the extracellular matrix via interactions with their protein targets. Computationally, glycosaminoglycans are very challenging due to their high flexibility, periodicity and electrostatics-driven nature of the interactions with their protein counterparts. In this work, we carry out a detailed computational characterization of the interactions in protein-glycosaminoglycan complexes from the Protein Data Bank (PDB), which are split into two subsets accounting for their intrinsic nature: non-enzymatic-protein-glycosaminoglycan and enzyme-glycosaminoglycan complexes. We apply molecular dynamics to analyze the differences in these two subsets in terms of flexibility, retainment of the native interactions in the simulations, free energy components of binding and contributions of protein residue types to glycosaminoglycan binding. Furthermore, we systematically demonstrate that protein electrostatic potential calculations, previously found to be successful for glycosaminoglycan binding sites prediction for individual systems, are in general very useful for proposing protein surface regions as putative glycosaminoglycan binding sites, which can be further used for local docking calculations with these particular polysaccharides. Finally, the performance of six different docking programs (Autodock 3, Autodock Vina, MOE, eHiTS, FlexX and Glide), some of which proved to perform well for particular protein-glycosaminoglycan complexes in previous work, is evaluated on the complete protein-glycosaminoglycan data set from the PDB. This work contributes to widen our knowledge of protein-glycosaminoglycan molecular recognition and could be useful to steer a choice of the strategies to be applied in theoretical studies of these systems.

A+-helix of protein C inhibitor (PCI) is a cell-penetrating peptide that mediates cell membrane permeation of PCI

Protein C inhibitor (PCI) is a serpin with broad protease reactivity. It binds glycosaminoglycans and certain phospholipids that can modulate its inhibitory activity. PCI can penetrate through cellular membranes via binding to phosphatidylethanolamine. The exact mechanism of PCI internalization and the intracellular role of the serpin are not well understood. Here we showed that testisin, a glycosylphosphatidylinositol-anchored serine protease, cleaved human PCI and mouse PCI (mPCI) at their reactive sites as well as at sites close to their N terminus. This cleavage was observed not only with testisin in solution but also with cell membrane-anchored testisin on U937 cells. The cleavage close to the N terminus released peptides rich in basic amino acids. Synthetic peptides corresponding to the released peptides of human PCI (His¹–Arg¹¹) and mPCI (Arg¹–Ala¹⁸) functioned as cell-penetrating peptides. Because intact mPCI but not testisin-cleaved mPCI was internalized by Jurkat T cells, a truncated mPCI mimicking testisin-cleaved mPCI was created. The truncated mPCI lacking 18 amino acids at the N terminus was not taken up by Jurkat T cells. Therefore our model suggests that testisin or other proteases could regulate the internalization of PCI by removing its N terminus. This may represent one of the mechanisms regulating the intracellular functions of PCI.

Protein C inhibitor (PCI) binds to phosphatidylserine exposing cells with implications in the phagocytosis of apoptotic cells and activated platelets

Protein C Inhibitor (PCI) is a secreted serine protease inhibitor, belonging to the family of serpins. In addition to activated protein C PCI inactivates several other proteases of the coagulation and fibrinolytic systems, suggesting a regulatory role in hemostasis. Glycosaminoglycans and certain negatively charged phospholipids, like phosphatidylserine, bind to PCI and modulate its activity. Phosphatidylerine (PS) is exposed on the surface of apoptotic cells and known as a phagocytosis marker. We hypothesized that PCI might bind to PS exposed on apoptotic cells and thereby influence their removal by phagocytosis. Using Jurkat T-lymphocytes and U937 myeloid cells, we show here that PCI binds to apoptotic cells to a similar extent at the same sites as Annexin V, but in a different manner as compared to live cells (defined spots on ∼10-30% of cells). PCI dose dependently decreased phagocytosis of apoptotic Jurkat cells by U937 macrophages. Moreover, the phagocytosis of PS exposing, activated platelets by human blood derived monocytes declined in the presence of PCI. In U937 cells the expression of PCI as well as the surface binding of PCI increased with time of phorbol ester treatment/macrophage differentiation. The results of this study suggest a role of PCI not only for the function and/or maturation of macrophages, but also as a negative regulator of apoptotic cell and activated platelets removal.

Potentiation of C1-esterase inhibitor by heparin and interactions with C1s protease as assessed by surface plasmon resonance

BACKGROUND

Human C1-esterase inhibitor (C1-INH) is a multifunctional plasma protein with a wide range of inhibitory and non-inhibitory properties, mainly recognized as a key down-regulator of the complement and contact cascades. The potentiation of C1-INH by heparin and other glycosaminoglycans (GAGs) regulates a broad spectrum of C1-INH activities in vivo both in normal and disease states. SCOPE OF RESEARCH: We have studied the potentiation of human C1-INH by heparin using Surface Plasmon Resonance (SPR), circular dichroism (CD) and a functional assay. To advance a SPR for multiple-unit interaction studies of C1-INH we have developed a novel (consecutive double capture) approach exploring different immobilization and layout.

MAJOR CONCLUSIONS

Our SPR experiments conducted in three different design versions showed marked acceleration in C1-INH interactions with complement protease C1s as a result of potentiation of C1-INH by heparin (from 5- to 11-fold increase of the association rate). Far-UV CD studies suggested that heparin binding did not alter C1-INH secondary structure. Functional assay using chromogenic substrate confirmed that heparin does not affect the amidolytic activity of C1s, but does accelerate its consumption due to C1-INH potentiation.

GENERAL SIGNIFICANCE

This is the first report that directly demonstrates a significant acceleration of the C1-INH interactions with C1s due to heparin by using a consecutive double capture SPR approach. The results of this study may be useful for further C-INH therapeutic development, ultimately for the enhancement of current C1-INH replacement therapies.

A computational modeling and molecular dynamics study of the Michaelis complex of human protein Z-dependent protease inhibitor (ZPI) and factor Xa (FXa)

Protein Z-dependent protease inhibitor (ZPI) and antithrombin III (AT3) are members of the serpin superfamily of protease inhibitors that inhibit factor Xa (FXa) and other proteases in the coagulation pathway. While experimental structural information is available for the interaction of AT3 with FXa, at present there is no structural data regarding the interaction of ZPI with FXa, and the precise role of this interaction in the blood coagulation pathway is poorly understood. In an effort to gain a structural understanding of this system, we have built a solvent equilibrated three-dimensional structural model of the Michaelis complex of human ZPI/FXa using homology modeling, protein-protein docking and molecular dynamics simulation methods. Preliminary analysis of interactions at the complex interface from our simulations suggests that the interactions of the reactive center loop (RCL) and the exosite surface of ZPI with FXa are similar to those observed from X-ray crystal structure-based simulations of AT3/FXa. However, detailed comparison of our modeled structure of ZPI/FXa with that of AT3/FXa points to differences in interaction specificity at the reactive center and in the stability of the inhibitory complex, due to the presence of a tyrosine residue at the P1 position in ZPI, instead of the P1 arginine residue in AT3. The modeled structure also shows specific structural differences between AT3 and ZPI in the heparin-binding and flexible N-terminal tail regions. Our structural model of ZPI/FXa is also compatible with available experimental information regarding the importance for the inhibitory action of certain basic residues in FXa.

The multi-functional serpin, protein C inhibitor: beyond thrombosis and hemostasis

Protein C inhibitor (PCI) is a member of the serine protease inhibitor (serpin) family. PCI was initially found to be an inhibitor of activated protein C, and later shown to be a potent inhibitor of blood coagulation and fibrinolysis such as that mediated by urokinase type-plasminogen activator. Therefore, the protein came to be known as plasminogen activator inhibitor-3. It also inhibits proteases involved in fertilization. PCI is broadly conserved, and is found in human, rhesus monkey, cow, rabbit, rat, mouse and chicken. The human PCI gene is located on chromosome 14q32.1 in a cluster of genes encoding related serpins. Sp1- and AP2-binding sites in the 5'-flanking region act as promoter and enhancer, respectively, for its expression in the liver. PCI mRNA is expressed in many organs in primates, but only in the reproductive organs in rodents. Recent studies using transgenic mice expressing the human gene have suggested that PCI is also involved in regulation of lung remodeling, tissue regeneration, vascular permeability, proteolysis in the kidney and tumor cell invasion. A protease inhibitor-independent activity of PCI, the prevention of anti-angiogenesis and metastasis of tumor cells, has also been observed. Thus, PCI is a unique multi-functional serpin playing diverse roles in the thrombosis and hemostasis in multiple organs and tissues of a variety of species.

N-glycans and the N terminus of protein C inhibitor affect the cofactor-enhanced rates of thrombin inhibition

Protein C inhibitor (PCI) is a serine protease inhibitor, displaying broad protease specificity, found in blood and other tissues. In blood, it is capable of inhibiting both procoagulant and anticoagulant proteases. Mechanisms that provide specificity to PCI remain largely unrevealed. In this study we have for the first time provided a full explanation for the marked size heterogeneity of blood-derived PCI and identified functional differences between naturally occurring PCI variants. The heterogeneity was caused by differences in N-glycan structures, N-glycosylation occupancy, and the presence of a Delta6-N-cleaved form. Bi-, tri-, and tetra-antennary complex N-glycans were identified. Fucose residues were identified both on the core GlcNAc and as parts of sialyl-Le(a/x) epitopes. Moreover, a glycan with a composition that implied a di-sialyl antenna was observed. PCI was N-glycosylated at all three potential N-glycosylation sites, Asn-230, Asn-243, and Asn-319, but a small fraction of PCI lacked the N-glycan at Asn-243. The overall removal of N-glycans affected the maximal heparin- and thrombomodulin-enhanced rates of thrombin inhibition differently in different solution conditions. In contrast, the Delta6-N-region increased both the heparin- and the thrombomodulin-enhanced rates of thrombin inhibition at all conditions examined. These results thus demonstrate that the N-linked glycans and the N-terminal region of blood-derived PCI in different ways affect the cofactor-enhanced rates of thrombin inhibition and provide information on the mechanisms by which this may be achieved. The findings are medically important, in view of the documented association of PCI with atherosclerotic plaques and the promising effect of PCI on reducing hypercoagulability states.

Molecular basis of thrombin recognition by protein C inhibitor revealed by the 1.6-A structure of the heparin-bridged complex

Protein C inhibitor (PCI) is a serpin with many roles in biology, including a dual role as pro- and anticoagulant in blood. The protease specificity and local function of PCI depend on its interaction with cofactors such as heparin-like glycosaminoglycans (GAGs) and thrombomodulin (TM). Both cofactors significantly increase the rate of thrombin inhibition, but GAGs serve to promote the anticoagulant activity of PCI, and TM promotes its procoagulant function. To gain insight into how PCI recognition of thrombin is aided by these cofactors, we determined a crystallographic structure of the Michaelis complex of PCI, thrombin, and heparin to 1.6 A resolution. Thrombin interacts with PCI in an unusual fashion that depends on the length of PCI's reactive center loop (RCL) to align the heparin-binding sites of the two proteins. The principal exosite contact is engendered by movement of thrombin's 60-loop in response to the unique P2 Phe of PCI. This mechanism of communication between the active site of thrombin and its recognition exosite is previously uncharacterized and may relate to other thrombin substrate-cofactor interactions. The cofactor activity of heparin thus depends on the formation of a heparin-bridged Michaelis complex and substrate-induced exosite contacts. We also investigated the cofactor effect of TM, establishing that TM bridges PCI to thrombin through additional direct interactions. A model of the PCI-thrombin-TM complex was built and evaluated by mutagenesis and suggests distinct binding sites for heparin and TM on PCI. These data significantly improve our understanding of the cofactor-dependent roles of PCI in hemostasis.

Regulation of protein C inhibitor (PCI) activity by specific oxidized and negatively charged phospholipids

Protein C inhibitor (PCI) is a serpin with affinity for heparin and phosphatidylethanolamine (PE). We analyzed the interaction of PCI with different phospholipids and their oxidized forms. PCI bound to oxidized PE (OxPE), and oxidized and unoxidized phosphatidylserine (PS) immobilized on microtiter plates and in aqueous suspension. Binding to OxPE and PS was competed by heparin, but not by the aminophospholipid-binding protein annexin V or the PCI-binding lipid retinoic acid. PS and OxPE stimulated the inhibition of activated protein C (aPC) by PCI in a Ca++-dependent manner, indicating that binding of both, aPC (Ca++ dependent) and PCI (Ca++ independent), to phospholipids is necessary. A peptide corresponding to the heparin-binding site of PCI abolished the stimulatory effect of PS on aPC inhibition. No stimulatory effect of phospholipids on aPC inhibition was seen with a PCI mutant lacking the heparin-binding site. A heparin-like effect of phospholipids (OxPE) was not seen with antithrombin III, another heparin-binding serpin, suggesting that it is specific for PCI. PCI and annexin V were found to be endogenously colocalized in atherosclerotic plaques, supporting the hypothesis that exposure of oxidized PE and/or PS may be important for the local regulation of PCI activity in vivo.

Structure of native protein C inhibitor provides insight into its multiple functions

Protein C inhibitor (PCI) is a multifunctional serpin with wide ranging protease inhibitory functions, unique cofactor binding activities, and potential non-inhibitory functions akin to the hormone-transporting serpins. To gain insight into the molecular mechanisms utilized by PCI we developed a robust expression system in Escherichia coli and solved the crystal structure of PCI in its native state. The five monomers obtained from our two crystal forms provide an NMR-like ensemble revealing regions of inherent flexibility. The reactive center loop (RCL) of PCI is long and highly flexible with no evidence of hinge region incorporation into beta-sheet A, as seen for other heparin-binding serpins. We adapted an extrinsic fluorescence method for determining dissociation constants for heparin and find that the N-terminal tail of PCI and residues adjacent to helix H are not involved in heparin binding. The minimal heparin length capable of tight binding to PCI was determined to be chains of eight monosaccharide units. A large hydrophobic pocket occupied by hydrophobic crystal contacts was found in an analogous position to the hormone-binding site in thyroxine-binding globulin. In conclusion, the data presented here provide important insights into the mechanisms by which PCI exercises its multiple inhibitory and non-inhibitory functions.

Control of the coagulation system by serpins. Getting by with a little help from glycosaminoglycans

Members of the serine protease inhibitor (serpin) superfamily play important roles in the inhibition of serine proteases involved in complex systems. This is evident in the regulation of coagulation serine proteases, especially the central enzyme in this system, thrombin. This review focuses on three serpins which are known to be key players in the regulation of thrombin: antithrombin and heparin cofactor II, which inhibit thrombin in its procoagulant role, and protein C inhibitor, which primarily inhibits the thrombin/thrombomodulin complex, where thrombin plays an anticoagulant role. Several structures have been published in the past few years which have given great insight into the mechanism of action of these serpins and have significantly added to a wealth of biochemical and biophysical studies carried out previously. A major feature of these serpins is that they are under the control of glycosaminoglycans, which play a key role in accelerating and localizing their action. While further work is clearly required to understand the mechanism of action of the glycosaminoglycans, the biological mechanisms whereby cognate glycosaminoglycans for each serpin come into contact with the inhibitors also requires much further work in this important field.

Crystal structure of protein C inhibitor provides insights into hormone binding and heparin activation

Protein C inhibitor (PCI) is a member of the serpin family that has many biological functions. In blood it acts as a procoagulant, and, in the seminal vesicles, it is required for spermatogenesis. The activity of PCI is affected by heparin binding in a manner unique among the heparin binding serpins, and, in addition, PCI binds hydrophobic hormones with apparent specificity for retinoids. Here we present the 2.4 A crystallographic structure of reactive center loop (RCL) cleaved PCI. A striking feature of the structure is a two-turn N-terminal shortening of helix A, which creates a large hydrophobic pocket that docking studies indicate to be the retinoid binding site. On the basis of surface electrostatic properties, a novel mechanism for heparin activation is proposed.

Protein C inhibitor (plasminogen activator inhibitor-3) and the risk of venous thrombosis

Protein C inhibitor (PCI), also known as plasminogen activator inhibitor-3, is a serine proteinase inhibitor that can inhibit enzymes in blood coagulation, fibrinolysis and fertility. The role of PCI in regulating the blood coagulation mechanism is not known, as it can inhibit both procoagulant (thrombin, factor Xa, factor XIa) and anticoagulant (activated protein C, thrombin-thrombomodulin, urokinase) enzymes. To determine the relevance of this inhibitor in thrombosis, PCI levels were assessed in the Leiden Thrombophilia Study, a case-control study of venous thrombosis in 473 patients with a first deep-vein thrombosis and 474 age- and sex-matched control subjects. PCI levels above the 95th percentile of the controls (136.1%) increased the risk 1.6-fold compared with PCI levels below the 95th percentile (95% confidence interval 0.9-2.8). There was a gradual increase in risk of thrombosis with further increasing levels of PCI. Adjustment for a number of possible confounders led to a reduction of the risk estimates associated with PCI. However, it is unclear whether adjustment for such factors in the risk models is justified. These results indicate that high levels of PCI may constitute a mild risk factor for venous thrombosis.

Localization of functional domains of the mitogenic toxin of Pasteurella multocida

The locations of the catalytic and receptor-binding domains of the Pasteurella multocida toxin (PMT) were investigated. N- and C-terminal fragments of PMT were cloned and expressed as fusion proteins with affinity tags. Purified fusion proteins were assessed in suitable assays for catalytic activity and cell-binding ability. A C-terminal fragment (amino acids 681 to 1285) was catalytically active. When microinjected into quiescent Swiss 3T3 cells, it induced changes in cell morphology typical of toxin-treated cells and stimulated DNA synthesis. An N-terminal fragment with a His tag at the C terminus (amino acids 1 to 506) competed with full-length toxin for binding to surface receptors and therefore contains the cell-binding domain. The inactive mutant containing a mutation near the C terminus (C1165S) also bound to cells in this assay. Polyclonal antibodies raised to the N-terminal PMT region bound efficiently to full-length native toxin, suggesting that the N terminus is surface located. Antibodies to the C terminus of PMT were microinjected into cells and inhibited the activity of toxin added subsequently to the medium, confirming that the C terminus contains the active site. Analysis of the PMT sequence predicted a putative transmembrane domain with predicted hydrophobic and amphipathic helices near the N terminus over the region of homology to the cytotoxic necrotizing factors. The C-terminal end of PMT was predicted to be a mixed alpha/beta domain, a structure commonly found in catalytic domains. Homology to proteins of known structure and threading calculations supported these assignments.

Disruption of the protein C inhibitor gene results in impaired spermatogenesis and male infertility

Protein C inhibitor (PCI) is a nonspecific, heparin-binding serpin (serine protease inhibitor) that inactivates many plasmatic and extravascular serine proteases by forming stable 1:1 complexes. Proteases inhibited by PCI include the anticoagulant activated protein C, the plasminogen activator urokinase, and the sperm protease acrosin. In humans PCI circulates as a plasma protein but is also present at high concentrations in organs of the male reproductive tract. The biological role of PCI has not been defined so far. However, the colocalization of high concentrations of PCI together with several of its target proteases in the male reproductive tract suggests a role of PCI in reproduction. We generated mice lacking PCI by homologous recombination. Here we show that PCI(-/-) mice are apparently healthy but that males of this genotype are infertile. Infertility was apparently caused by abnormal spermatogenesis due to destruction of the Sertoli cell barrier, perhaps due to unopposed proteolytic activity. The resulting sperm are malformed and are morphologically similar to abnormal sperm seen in some cases of human male infertility. This animal model might therefore be useful for analyzing the molecular bases of these human conditions.

Contribution of basic residues of the D and H helices in heparin binding to protein C inhibitor

Protein C inhibitor (PCI) is a heparin-binding serine protease inhibitor (serpin) that regulates hemostatic proteases such as activated protein C (APC) and thrombin. The work described here provides further evidence that the PCI H helix, but not the D helix, has a major role in heparin-accelerated inhibition of APC and thrombin. We previously identified Arg-269 and Lys-270 of the H helix [R269A/K270A "H1" recombinant PCI (rPCI)] as important residues both for heparin-accelerated inhibition of thrombin and APC and for heparin-Sepharose binding (Shirk, R. A., Elisen, M. G. L. M., Meijers, J. C. M., and Church, F. C. (1994) J. Biol. Chem. 269, 28690-28695). H1 rPCI was used as a template for Ala-scanning mutagenesis of other H helix basic residues (H1-K266A, H1-K273A, and H1-K266A/K273A) and of the D helix basic residues (H1-K82A, H1-K86A, H1-R90A, and H1-K82A/K86A/R90A). Compared to wild-type rPCI/heparin (k2 = 2.2 x 10(7) M-1 min-1 for thrombin), heparin-accelerated thrombin inhibition was decreased 2.4-fold by H1 rPCI, 4.4-fold by H1-K266A rPCI, and 8-fold by H1-K273A rPCI. H1-K266A/K273A rPCI thrombin inhibition was essentially not accelerated by heparin. A similar trend was found for APC-heparin inhibition using these H helix rPCI mutants. In contrast, the D helix rPCI mutants did not have further reduced heparin-stimulated thrombin or APC inhibition compared to H1 rPCI. Interestingly, all of the H and D helix rPCI mutants had reduced heparin-Sepharose binding activity (ranging from 180 to 360 mM NaCl) compared to wild-type rPCI and H1 rPCI, which eluted at 650 and 430 mM NaCl, respectively. These data suggest that all four basic residues (Lys-266, Arg-269, Lys-270, Lys-273) in the H helix of PCI form a heparin binding site. Our results also imply that while the D helix basic residues (Lys-80, Lys-86, and Arg-90) contribute to overall heparin binding, they are not necessary for heparin-accelerated activity. We conclude that the primary heparin binding site of PCI is the H helix and not the D helix as found in other homologous heparin-binding serpins such as antithrombin III, heparin cofactor II, and protease nexin 1.

Sequence and expression analysis of bovine pigment epithelium-derived factor

PEDF, a member of the serpin superfamily of proteins related through their highly conserved folded conformation, has neurotrophic properties, including promotion of neurite-outgrowth and neuronal survival. Previously, we have purified and characterized PEDF protein from extracellular matrixes of bovine eyes. Here, we show the cDNA sequence and expression analysis of bovine PEDF. Northern analysis of RNA from bovine retinal pigment epithelium (RPE) and neural retina using a human PEDF cDNA fragment reveals expression of the PEDF gene only for RPE. Sequence analysis of a cDNA clone isolated from bovine RPE predicts a polypeptide of 416 amino acid residues that shares 88.6% and 85% amino acid identity with human and mouse PEDF, respectively. It has an N-terminal signal peptide, a consensus glycosylation site and homology with serpins including the conserved residues required for maintaining the serpin tertiary structure. Cell-free expression of the bovine PEDF cDNA by in vitro transcription and translation yields a precursor polypeptide of 45,000-Mr that immunoprecipitates with an antibody to human PEDF. Expression analysis in stably transfected baby hamster kidney cells shows that the recombinant bovine protein is secreted to the culture media as a mature 50,000-Mr protein, which induces neurite-outgrowth on retinoblastoma cells, like the naturally-occurring PEDF protein. Thus, the bovine PEDF cDNA isolated here codes for a functional soluble secreted PEDF glycoprotein.

Molecular cloning, tissue distribution and androgen regulation of rat protein C inhibitor

Protein C inhibitor (PCI) is the plasma serine protease inhibitor of activated protein C, the active enzyme of the anticoagulant protein C pathway. Recently, PCI was also detected in human seminal plasma and reproductive organs (testis, seminal vesicle and prostate) suggesting that PCI may also play an important role in the reproductive system. In this study, we cloned the full length of rat PCI cDNA, and determined its amino acid sequence and tissue distribution. We also evaluated the effect of androgen on PCI mRNA expression in seminal vesicles and testes. The isolated 2074-bp rat PCI cDNA was composed of a 47-bp 5'-non-coding region, a 1218-bp coding region of a 406-amino acid precursor protein, a stop codon and a 806-bp 3'-non-coding region. The deduced amino acid sequence of rat PCI showed 85.7%, 64.1% and 62.2% homology with that of mouse, rhesus monkey and human PCIs, respectively. Northern blot analysis showed that the rat PCI mRNA is expressed strongly in the seminal vesicle, moderately in the testis, but not in the liver. PCI mRNA expression in seminal vesicles and testes was found to increase during the process of development, suggesting that it is under androgen control. Subsequently, we examined the effect of castration and/or treatment with 17beta-estradiol or testosterone on PCI mRNA expression in the mature rat seminal vesicles. The PCI mRNA expression in seminal vesicles was significantly decreased after castration or 17beta-estradiol treatment. Testosterone itself did not affect PCI mRNA expression, but treatment in castrated rats significantly enhanced its mRNA expression. These findings suggest that the PCI gene expression in rat seminal vesicles is regulated by androgen.

Heparin binding of protein-C inhibitor--analysis of the effect of heparin on the interaction of protein-C inhibitor with tissue kallikrein

The non-specific serine-protease inhibitor protein-C inhibitor (PCI) inactivates its target enzymes by forming stable 1:1 complexes. Heparin stimulates most PCI/protease reactions, but interferes with the inhibition of tissue kallikrein by PCI by a hitherto unknown mechanism. In this study we analyzed the inhibitory effect of heparin on the tissue-kallikrein-PCI interaction. Free PCI and tissue-kallikrein x PCI complexes but not free tissue kallikrein bound to heparin-Sepharose, implying that the inhibitory effect of heparin cannot be caused by a tissue-kallikrein-heparin interaction. Heparin did not dissociate tissue-kallikrein x PCI complexes, making it unlikely that in the presence of heparin PCI becomes a substrate for, rather than an inhibitor of, tissue kallikrein. However, heparin-bound PCI, which was able to form complexes with 125I-urokinase, did not form complexes with 125I-tissue-kallikrein. This suggests that the inhibitory effect of heparin is either based on the neutralization of positive charges in the PCI molecule, which might be required for the interaction of PCI with the acidic protease tissue kallikrein, or on a change in reactivity of PCI upon heparin binding, making heparin-bound PCI no longer a tissue-kallikrein inhibitor. Neutralization of basic amino acids in the PCI molecule by glutamic acid, which prevented in a dose-dependent way the inhibitory effect of heparin, did not have any effect on the tissue-kallikrein-PCI interaction. Therefore, direct involvement of basic amino acid residues present in the heparin-binding site of PCI in the tissue-kallikrein-PCI interaction can be excluded. Heparin binding might rather cause a change in reactivity of PCI (e.g. by inducing a conformational change or by steric interference), thereby preventing its interaction with tissue kallikrein.

Molecular cloning and sequence analysis of the mouse protein C inhibitor gene

The gene encoding mouse protein C inhibitor (mPCI) was isolated and its nucleotide sequence determined. Alignment of the genomic sequence with that of a cDNA obtained from mouse testis revealed that the mPCI gene (like the human counterpart) is composed of five exons and four introns with highly conserved exon/intron boundaries. It encodes a pre-polypeptide of 405 amino acids, which shows 63% identity with human PCI (hPCI). The putative reactive site is identical to that of hPCI from P5 to P3', suggesting a similar protease specificity. Also the putative heparin binding sites and 'hinge' regions are highly homologous in mouse and hPCI.

The exon 3 encoded sequence of the intracellular serine proteinase inhibitor plasminogen activator inhibitor 2 is a protein binding domain

We have used a combination of biochemical and immunological methods to probe for proteins that interact with the cytoplasmic form of plasminogen activator inhibitor 2 (PAI-2) and to identify the structure in PAI-2 that mediates the binding. By affinity chromatography on immobilized PAI-2, we purified a collection of PAI-2-binding proteins. These proteins bound 125I-labeled PAI-2 in vitro (IC50, approximately 10-100 nM) in a calcium-independent reaction that did not abrogate the proteinase inhibitory function of PAI-2. Annexin I was identified among the eluted proteins, and purified annexins I, II, IV, and V, but not III and VI, possessed 125I-labeled PAI-2 binding activity. Immune precipitation by anti-PAI-2 monoclonal and polyclonal antibodies of metabolically labeled melanoma cells treated with a cleavable cross-linker prior to analysis revealed three prominent proteins with apparent masses of 100, 70, and 50 kDa. We localized the protein binding domain in PAI-2 between amino acid residues 66 and 98, as determined by using a PAI-2 mutant lacking this domain and a synthetic peptide spanning this region. This region of PAI-2 corresponds to exon 3 of the gene sequence thought to be critical for PAI-2 functions.

Structural investigation of the alpha-1-antichymotrypsin: prostate-specific antigen complex by comparative model building

Prostate-specific antigen (PSA), produced by prostate cells, provides an excellent serum marker for prostate cancer. It belongs to the human kallikrein family of enzymes, a second prostate-derived member of which is human glandular kallikrein-1 (hK2). Active PSA and hK2 are both 237-residue kallikrein-like proteases, based on sequence homology. An hK2 model structure based on the serine protease fold is presented and compared to PSA and six other serine proteases in order to analyze in depth the role of the surface-accessible loops surrounding the active site. The results show that PSA and hK2 share extensive structural similarity and that most amino acid replacements are centered on the loops surrounding the active site. Furthermore, the electrostatic potential surfaces are very similar for PSA and hK2. PSA interacts with at least two serine protease inhibitors (serpins): alpha-1-antichymotrypsin (ACT) and protein C inhibitor (PCI). Three-dimensional model structures of the uncleaved ACT molecule were developed based upon the recent X-ray structure of uncleaved antithrombin. The serpin was docked both to PSA and hK2. Amino acid replacements and electrostatic complementarities indicate that the overall orientation of the proteins in these complexes is reasonable. In order to investigate PSA's heparin interaction sites, electrostatic computations were carried out on PSA, hK2, protein C, ACT, and PCI. Two heparin binding sites are suggested on the PSA surface and could explain the enhanced complex formation between PSA and PCI, while inhibiting the formation of the ACT-PSA complex, PSA, hK2, and their preliminary complexes with ACT should facilitate the understanding and prediction of structural and functional properties for these important proteins also with respect to prostate diseases.

Cellular receptor structures for pseudorabies virus are blocked by antithrombin III

Pseudorabies virus (PrV), an alphaherpesvirus of swine, uses cellular heparan sulfate residues as a receptor for attachment. Interaction of the virus with its receptor is mediated by the envelope glycoprotein C (PrV-gC), a protein with heparin-binding properties. We have previously shown that a region of this protein shows structural similarities to the high-affinity heparin-binding site of the serum protease-inhibitor antithrombin III (ATII). In this publication, we describe the effect of ATIII on interaction of PrV with its cellular receptor. ATIII bound specifically to heparan sulfate residues on the surface of herpesvirus-permissive RK13 cells. Binding of ATIII to RK13 cells interfered with adsorption of radioactively labelled PrV to these cells. Enzymatic treatment using heparinase I (E.C. 4.2.2.7) removed the receptor for PrV as well as the receptor for ATIII. Since amino acids 130-137 of the high affinity heparin-binding site of ATIII show structural similarities to amino acids 134-141 of PrV-gC, both sequences were synthesized as synthetic peptides. Although interaction of the peptide derived from ATIII with heparin was significantly stronger, both peptides interacted specifically with heparin in assays in vitro. These results suggest that PrV and ATIII interact with the same structure on the cellular surface.

Rapid inhibition of the sperm protease acrosin by protein C inhibitor

Heparin was found to be an allosteric modulator of the amidolytic activity of the protease acrosin. In the presence of saturating concentrations of heparin, there was a 4.9-fold decrease in the value of the Michaelis constant for the substrate D-Ile-Pro-Arg-p-nitroanilide and the value of kcat was 2.5-fold lower. Analysis of the data yielded a dissociation constant of 0.22 +/- 0.04 microM for the heparin-acrosin complex. The presence of relatively high concentrations of protein C inhibitor in seminal plasma [Laurell, M., Christensson, A., Abrahamson, P., Stenflo, J., & Lilja, H. (1992) J. Clin. Invest. 89, 1094-1101] suggests that this serpin may be involved in the control of the activity of acrosin. Acrosin was found to be rapidly inhibited by protein C inhibitor with the association rate constant (kass) for the formation of the complex being (2.41 +/- 0.03) x 10(5) M-1 s-1. The value of kass showed a bell-shaped dependence on the concentration of heparin; it was maximal at concentrations of heparin between 0.08 and 3 microM and decreased at lower and higher concentrations. At the optimal heparin concentration, the value of kass for the acrosin-protein C inhibitor reaction was 230-fold higher ((5.6 +/- 0.1) x 10(7) M-1 s-1) than in the absence of heparin. The results suggest that protein C inhibitor may be important in the physiological control of acrosin activity, particularly where the presence of heparin-like glycosaminoglycans would stimulate the acrosin-protein C inhibitor reaction.

Inhibition of human secretory class II phospholipase A2 by heparin

By means of kinetic analyses using Triton X-100/deoxycholic acid/dilauroylglycerophosphoethanolamine (4:2:1, molar ratio) mixed micelles we examined the effects of heparin on the activity of several phospholipases A2 (PLA2). Heparin avidly bound cationic PLA2s including human secretory class II PLA2 and thereby inhibited their hydrolysis of phospholipids in the mixed micelles. Initial velocity measurements indicated that heparin behaved as a competitive inhibitor for human secretory class II PLA2 and closely related A.h. blomhoffii PLA2 and A.p. piscivorus PLA2. In particular, heparin showed the highest specificity for human secretory class II PLA2. In the absence of deoxycholic acid in mixed micelles, A.h. blomhoffii PLA2 was also strongly inhibited by heparin. The observed inhibition was not due to the interaction of heparin with the active site of PLA2 because heparin did not inhibit the hydrolysis of monomeric substrates by PLA2s. Both kinetic measurements and fluorescence measurements of PLA2-bound 8-anilino-1-naphthalene sulfonate in the presence of varying amounts of heparin showed that a heparin molecule bound about seven molecules of PLA2. When positive charges of four lysines in the amino-terminal region of A.h. blomhoffii PLA2 were neutralized by limited carbamoylation, heparin neither bound the carbamoylated A.h. blomhoffii PLA2 nor inhibited the hydrolysis of Triton X-100/dilauroylglycerophosphocholine mixed micelles by the carbamoylated A.h. blomhoffii PLA2 that retained 50% activity of native A.h. blomhoffii PLA2. Also, heparin did not inhibit the hydrolysis of mixed micelles by 7,10-bis(octanoyl)ated A.p. piscivorus PLA2 in which two lysines in the amino-terminal alpha-helix are acylated. These results indicate that the inhibition of human secretory class II PLA2 and related cationic PLA2s by heparin originates from the interaction of heparin with cationic residues in the amino-terminal region that forms a part of interfacial binding site. In addition, unique structural features of human secretory class II PLA2, together with its unique mode of interaction with heparin, suggest that this PLA2 might have an additional heparin-binding site. Although the heparin-PLA2 binding diminished as the ionic strength of reaction medium increased, the inhibition of human secretory class II PLA2 by heparin remained significant at the physiological ionic strength. An estimated value of inhibition constant (Ki) was 0.1 microM under physiological conditions, which suggests that a normal pharmaceutical dose of heparin might inhibit human secretory class II PLA2 and regulate its biological effects.

Models of the serine protease domain of the human antithrombotic plasma factor activated protein C and its zymogen

Three-dimensional structural analysis of physiologically important serine proteases is useful in identifying functional features relevant to the expression of their activities and specificities. The human serine protease anticoagulant protein C is currently the object of many genetic site-directed mutagenesis studies. Analyzing relationships between its structure and function and between naturally occurring mutations and their corresponding clinical phenotypes would be greatly assisted by a 3-dimensional structure of the enzyme. To this end, molecular models of the protease domain of protein C have been produced using computational techniques based on known crystal structures of homologous enzymes and on protein C functional information. The resultant models corresponding to different stages along the processing pathway of protein C were analyzed for structural and electrostatic differences arising during the process of protein C maturation and activation. The most satisfactory models included a calcium ion bound to residues homologous to those that ligate calcium in the trypsin structure. Inspection of the surface features of the models allowed identification of residues putatively involved in specific functional interactions. In particular, analysis of the electrostatic potential surface of the model delineated a positively charged region likely to represent a novel substrate recognition exosite. To assist with future mutational studies, binding of an octapeptide representing a protein C cleavage site of its substrate factor Va to the enzyme's active site region was modeled and analyzed.

Interaction of thrombin with antithrombin, heparin cofactor II, and protein C inhibitor

alpha-Thrombin is a trypsin-like serine proteinase involved in blood coagulation and wound repair processes. Thrombin interacts with many macromolecular substrates, cofactors, cell-surface receptors, and blood plasma inhibitors. The three-dimensional structure of human alpha-thrombin shows multiple surface "exosites" for interactions with these macromolecules. We used these coordinates to probe the interaction of thrombin's active site and two exosites, anion-binding exosite-I and -II, with the blood plasma serine proteinase inhibitors (serpins) antithrombin (AT), heparin cofactor II (HC), and protein C inhibitor (PCI). Heparin, a widely used anticoagulant drug, accelerates the rate of thrombin inhibition by AT, PCI, and HC. Thrombin Quick II is a dysfunctional thrombin mutant with a Gly 226-->Val substitution in the substrate specificity pocket. We found that thrombin Quick II was inhibited by HC, but not by AT or PCI. Molecular modeling studies suggest that the larger Val side chain protrudes into the specificity pocket, allowing room for the smaller P1 side chain of HC (Leu) but not the larger P1 side chain of AT and PCI (both with Arg). gamma T-Thrombin and thrombin Quick I (Arg 67-->Cys) are both altered in anion-binding exosite-I, yet bind to heparin-Sepharose and can be inhibited by AT, HC, and PCI in an essentially normal manner in the absence of heparin. In the presence of heparin, inhibition of these altered thrombins by HC is greatly reduced compared to both AT and PCI. alpha-Thrombin with chemically modified lysines in both anion-binding exosite-I and -II has no heparin accelerated thrombin inhibition by either AT or HC. Thrombin lysine-modified in the presence of heparin has protected residues in anion-binding exosite-II and the loss of heparin-accelerated inhibition by HC is greater than that by AT. Collectively, these results suggest differences in serpin reactive site recognition by thrombin and a more complicated mechanism for heparin-accelerated inhibition by HC compared to either AT or PCI.

Interaction of activated protein C with serpins

The inhibition of activated protein C by six different serine protease inhibitors (serpins) that have arginine residues in the P1 position has been investigated. Micromolar concentrations of C1-inhibitor failed to inhibit the enzyme, and it was inhibited only slowly by antithrombin III with an association rate constant (kass.) of 0.15 M-1.s-1. The kass. values for the other serpins tested (protease nexin I, protein C inhibitor, and mutants of alpha 1-antichymotrypsin and alpha 1-antitrypsin with P1 arginine residues) were at least 1000-fold higher, with P1-Arg-alpha 1-antitrypsin (kass. = 7 x 10(4) M-1.s-1) being the most effective inhibitor. The inhibition with these four serpins appeared to be reversible, with inhibition constants in the nanomolar range. The relatively high value of kass. for protease nexin I (5 x 10(3) M-1.s-1) suggested that it may be involved in the control of activated protein C on the surface of platelets where protein nexin I is present at relatively high concentrations. The value of kass. for protease nexin I, protein C inhibitor and antithrombin III showed a bell-shaped dependence on heparin concentration. At optimal concentrations, heparin accelerated the rate of inhibition by protease nexin I, protein C inhibitor and antithrombin III by 44-, 18- and 13-fold respectively. The kinetic constants for the inhibition of thrombin were also determined, and in all cases the serpins were more effective inhibitors of thrombin. Comparison of the sequences of the active-site regions of activated protein C and thrombin suggested that the more hydrophobic active site of thrombin may be more favourable for interactions with serpins.

Glycosaminoglycans and the regulation of blood coagulation

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Design and synthesis of a helix heparin-binding peptide

Elaboration of heparin-protein-binding interactions is necessary to understand how heparin modulates protein function. The heparin-binding domain of some proteins is postulated to be a helix structure which presents a surface of high positive charge density. Thus, a synthetic 19-residue peptide designed to be alpha-helical in character was synthesized, and its interaction with heparin was studied. The peptide was shown to be 75% helix by circular dichroism (CD) spectrometry in neutral pH buffer (at 2 degrees C); helicity increased to nearly 85% under high ionic strength conditions or to nearly 100% in 75% ethanol. Increasing the temperature of the solution caused a change in the spectral envelope consistent with a coil transition of the peptide. The midpoint of the transition (i.e., the temperature at which the helix content was determined to be 50%) was 25 degrees C, and the determined van't Hoff enthalpy change (delta HvH) was 3.2 kcal/mol of peptide. By CD, heparin increases the helix content of the peptide to 100% and increases the apparent thermal stability of the peptide by about 1 kcal/mol. The melting point for the helix/coil transition of the heparin-peptide complex was 50 degrees C. The thermal coefficient of the transition (approximately 300 deg.cm2.dmol-1.degree C-1) was essentially the same for the peptide alone or the peptide-heparin complex. Dissociation of the complex under high ionic strength conditions was also observed in the CD experiment. Biological assays showed less heparin-binding activity than expected (micromolar KD values), but this was attributed to the absence of critical lysyl residues in the peptide.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning of a novel hyaluronan receptor that mediates tumor cell motility

A cDNA encoding a unique hyaluronan receptor has been molecularly cloned from a lambda GT11 3T3 cDNA expression library. Immunoblot analyses of cell lysates, using antibodies to peptides encoded in the cDNA, specifically react with a 58-kD protein. This protein is regulated by the mutant H-ras gene in cells containing a metallothionein promoter H-ras hybrid gene. Further, antibodies to peptide sequences encoded in the cDNA block the increase in locomotion resulting from induction of the mutant H-ras gene in this cell line. In a transblot assay, the bacterially expressed protein binds to biotinylated hyaluronan. Antibodies to peptides encoded in the cDNA react in immunoblot assays with the 58- and 52-kD proteins of a novel hyaluronan receptor complex previously implicated in cell locomotion. Furthermore, antibodies specific to the 58- and 52-kD proteins, which block ras-induced locomotion, also cross-react with the expressed, encoded protein. The gene product described here appears to be a new type of hyaluronan receptor that is involved in cell locomotion. It is named RHAMM, an acronym for receptor for hyaluronan-mediated motility.

Discovery of a genetic polymorphism of human plasma protein C inhibitor (PCI): genetic survey utilizing isoelectric focusing followed by immunoblotting, immunological and biochemical characterization

The objectives of this study were to determine the genetic basis of the electrophoretic differences of human plasma protein C inhibitors (PCI) from 977 individuals. Three discrete antibodies were produced against the PCI purified from human plasma and peptides that corresponded to the N-terminal 15 amino acid residues and the C-terminal 15 residues of human PCI, the chemical structures of which were determined by cDNA sequence analysis. The combined techniques of polyacrylamide gel isoelectric focusing and immunoblotting with these three different antibodies resolved the plasma PCI into several isoprotein bands, with a pH range of 6-7. These PCI isoproteins, however, were not stained by anti-human kallikrein, anti-human protein C or anti-human urokinase antibodies. Therefore, each of the PCI bands, which were detected by immunoblotting with the anti-PCI antibody and the two different anti-peptide antibodies, were derived from free PCI, and not an inactive PCI species. Two common phenotypes, designated PCI 1 and 1-2, were recognized, and family studies showed that they represented homozygosity or heterozygosity for two autosomal codominant alleles, PCI*1 and PCI*2. A population study of plasma samples collected from 977 Japanese individuals indicated that the frequencies of the PCI*1 and PCI*2 alleles were 0.988 and 0.012, respectively.

Assessment of protein models with three-dimensional profiles

As methods for determining protein three-dimensional (3D) structure develop, a continuing problem is how to verify that the final protein model is correct. The revision of several protein models to correct errors has prompted the development of new criteria for judging the validity of X-ray and NMR structures, as well as the formation of energetic and empirical methods to evaluate the correctness of protein models. The challenge is to distinguish between a mistraced or wrongly folded model, and one that is basically correct, but not adequately refined. We show that an effective test of the accuracy of a 3D protein model is a comparison of the model to its own amino-acid sequence, using a 3D profile, computed from the atomic coordinates of the structure 3D profiles of correct protein structures match their own sequences with high scores. In contrast, 3D profiles for protein models known to be wrong score poorly. An incorrectly modelled segment in an otherwise correct structure can be identified by examining the profile score in a moving-window scan. The accuracy of a protein model can be assessed by its 3D profile, regardless of whether the model has been derived by X-ray, NMR or computational procedures.

Proteoglycans in cell regulation

Proteoglycans are a diverse group of proteins carrying one or more glycosaminoglycan side chains linked to the protein as O-glycosides. Our appreciation of these structures has matured from a curiosity about unusual structural glycoproteins, to confer upon them a central role in cell biology. The major classes of glycosaminoglycans are heparan sulfate and heparin, chondroitin and dermatan sulfates, keratan sulfate and hyaluronic acid. The latter is unique in that it does not contain sulfate residues, and appears to be synthesized, at least sometimes, free of a carrier protein. There is now a wealth of information on the ability of these structures to influence the growth and development of cells and tissues. Many direct and specific effects of proteoglycans will undoubtedly be found, and there are likely to be indirect effects of the glycosaminoglycans relating to their polyelectrolyte nature. Convincing arguments that biological activity resides in certain proteoglycan core proteins are also appearing. The following discussion concerns the role of proteoglycans in the regulation and action of autocrine and polypeptide growth factors, direct mitogenic and antimitogenic actions of glycosaminoglycans, the role of these structures in regulating gene expression, and the biological activities of proteoglycan core proteins. The probable role of proteoglycans in normal glomerular cell function, and in progressive renal disease, will be presented as a harbinger of the significant role we can expect them to play in diagnosis and therapy in the near future.