The role of protein S in the activation of thrombin activatable fibrinolysis inhibitor (TAFI) and regulation of fibrinolysis

Thrombin activatable fibrinolysis inhibitor (TAFI) is a carboxy-peptidase B-like proenzyme that after activation by thrombin downregulates fibrinolysis. Thrombomodulin stimulates the activation of both TAFI and protein C whereas activated protein C inhibits the activation of TAFI by downregulation of thrombin formation, a process in which protein S acts as a cofactor. Here we determined the role of protein S in the activation of TAFI and regulation of fibrinolysis. Depletion of protein S from plasma or inhibition of protein S by specific antibodies resulted in an increased rate of TAFI activation and in an increased maximum of TAFIa activity generated. The effect on the rate of TAFI activation could be attributed to the APC-independent anticoagulant function of protein S whereas the effect on the maximum activity could be attributed to the APC cofactor function of protein S. Therefore it is concluded that protein S inhibits TAFI activation in two ways. On one hand, protein S functions as a cofactor for APC which results in a reduction of the maximum induced TAFI activity and on the other hand protein S inhibits the initial thrombin formation independently of APC which results in a decreased rate of TAFI activation. The effect of the APC-independent anticoagulant activity of protein S on the activation of TAFI provides a new mechanism for the regulation of fibrinolysis in the early stages of clot formation.

Activated protein C versus protein C in severe sepsis

OBJECTIVE

To delineate critical differences between activated protein C (APC) and its precursor, protein C, with regard to plasma levels in health and in severe sepsis, and to discuss the implications of these differences as they relate to treatment strategies in patients with severe sepsis. DATA SOURCE/STUDY SELECTION: Published literature including abstracts, manuscripts, and review articles reporting studies in both experimental animal models and humans that provide an understanding of the relationship and the critical differences between circulating levels of APC and protein C.

DATA EXTRACTION AND SYNTHESIS

The protein C pathway represents one of the major regulatory systems of hemostasis, exhibiting antithrombotic, profibrinolytic and anti-inflammatory properties. This pathway also plays a critical role in the pathophysiology of severe sepsis. Central to this pathway is the vitamin K-dependent serine protease, APC, and its precursor, protein C. The conversion of protein C to APC is dependent on the complex of thrombin and thrombomodulin, an integral endothelial surface receptor. The conversion of protein C to APC is further augmented by another endothelial surface protein, the endothelial protein C receptor. There are limited published data on APC levels in health and disease, probably due to the complexity of the assay methodology for measuring APC and the absence of commercially available diagnostic kits. In animals and humans with normal functioning endothelium, circulating levels of APC (1-3 ng/mL) are positively correlated with protein C (4000-5000 ng/mL) concentration and the amount of thrombin generated. In patients with severe sepsis, there is a generalized endothelial dysfunction, contributing to multiple organ failure with increased morbidity and mortality. Persistently low protein C levels are related to poor prognosis. Key to understanding the treatment strategy with APC or protein C is knowledge of the functional status of the endothelium and, specifically, whether the microvasculature in patients with severe sepsis can support the conversion of protein C to APC. To date, only APC (drotrecogin alfa [activated]) has been shown to reduce mortality in severe sepsis in a large, phase 3, placebo-controlled, double-blind international trial. In contrast, no data, other than open-label case studies, are available for evaluation of the effects of protein C in the treatment of severe sepsis.

CONCLUSION

The limited data available indicate that lower levels of protein C in sepsis occur in the absence of appreciable conversion to APC. These observations indicate that treatment with APC may be more efficacious than protein C in severe sepsis, where generalized endothelial dysfunction may impair conversion of protein C to APC. Additional research is required to confirm these observations.

The role of protein C, protein S, and resistance to activated protein C in Legg-Perthes disease

OBJECTIVES

It has been hypothesized that Legg-Perthes disease is caused by repeated vascular interruptions of the blood supply to the proximal femur, which are precipitated by coagulation system abnormalities. To test this theory, we conducted a case-control study among 57 patients with Legg-Perthes disease and an equal number of community controls. We measured protein C and protein S and resistance to activated protein C (APC-R) from plasma.

STUDY DESIGN

Participants were placed into 1 of 3 mutually exclusive categories based on the control distribution: 1) normal, defined as either above or within 1 standard deviation below the expected mean; 2) low normal, defined as between 1 and 2 standard deviations below the expected mean; and 3) low, defined as >2 standard deviations below the expected mean. DNA was analyzed to determine the presence of a point mutation in the factor V gene that causes APC-R.

RESULTS

We observed a statistically significant increased risk of Legg-Perthes disease with decreasing levels of protein C and a nearly significant increased risk with decreasing levels of protein S. The factor V gene defect was present in 5 (9%) of 55 cases and 3 (5%) of 56 controls (odds ratio 1.8, 95% confidence interval: 0.4-7.7), but the mean level on the APC-R plasma test was similar for cases and controls. Nine cases and 1 control had 2 low normal or low test results (odds ratio 13.0, 95% confidence interval: 2.2-75).

CONCLUSIONS

Our results support the belief that abnormalities of the coagulation system leading to a thrombophilic state play a role in Legg-Perthes disease; however, larger studies are needed before definitive recommendations for coagulation testing can be made.

Plasma glucosylceramide deficiency as potential risk factor for venous thrombosis and modulator of anticoagulant protein C pathway

To assess the relationship between venous thrombosis and plasma glucosylceramide (GlcCer) or phosphatidylethanolamine (PE), plasma levels of GlcCer and PE were determined for 70 venous thrombosis patients referred for evaluation and 70 healthy blood donors. The mean GlcCer level, but not the PE level, was lower in patients versus controls (4.9 vs 6.5 microg/mL [P =.0007] and 66 vs 71 microg/mL [P =.48], respectively). As a measure of relative risk, the odds ratio for deep vein thrombosis in subjects with GlcCer levels below the 10th percentile of controls was 5.7 (95% CI, 2.3-14). To assess the influence of glycolipids on anticoagulant response to activated protein C (APC):protein S in modified prothrombin time assays, the effects of depleting endogenous plasma GlcCer by glucocerebrosidase treatment or of adding exogenous purified GlcCer or other neutral glycolipids to plasma were tested. Glucocerebrosidase treatment reduced plasma sensitivity to APC:protein S in parallel with GlcCer reduction. Exogenously added GlcCer and the homologous Glc-containing globotriaosylceramide (Gb3Cer), but not galactosylceramide, dose-dependently prolonged clotting times of normal plasma in the presence, but not absence, of APC:protein S, which suggests that GlcCer or Gb3Cer can enhance protein C pathway anticoagulant activity. In studies using purified proteins, inactivation of factor Va by APC:protein S was enhanced by GlcCer alone and by GlcCer in multicomponent vesicles containing phosphatidylserine and phosphatidylcholine. These results suggest that the neutral glycolipids GlcCer and Gb3Cer may directly contribute to the anticoagulant activity of the protein C pathway and that deficiency of plasma GlcCer may be a risk factor for venous thrombosis. (Blood. 2001;97:1907-1914)

Cellular effects and signalling pathways activated by the anti-coagulant factor, protein S, in vascular cells protein S cellular effects

The anticoagulant factor protein S is a secreted vitamin K-dependent gamma-carboxylated protein that is mainly synthesised in the liver but is also made by endothelial cells and megakaryocytes in culture. In previous studies we have shown that protein S acts as a mitogen for cultured human vascular smooth muscle cells. The synthesis and secretion of protein S by endothelial cells suggests that in addition to its role in the coagulation cascade, protein S may be an important autocrine factor implicated in the pathophysiology of the vascular system. The effects of protein S on hVSMC proliferation, migration and survival are discussed. The activation of the components of the MAP kinase pathway, ERK1/2, JNK/SAPK and p38 is also summarised. Binding and chemical cross-linking experiments provided evidence for the existence of a cell surface protein S receptor(s). By virtue of its many cellular effects, it is suggested here that the anticoagulant factor protein S plays an important role in the pathophysiology of the vasculature.

Activated protein C resistance in Turkish women with severe preeclampsia

The purpose of this study was to investigate the occurrence rate of APC resistance (APC-R) with severe preeclampsia in Turkish women. Thirty-two consecutive women having severe preeclampsia were included in the study. Thirty-two healthy pregnant women served as the control group. APC-R assays were performed in the third trimester of pregnancy, and 3 and 9 months after delivery. APC-R was demonstrated in the third trimester, 3 months and 9 months after delivery in 27 (84.4%), 23 (71.9%) and 5 (15.6%) of 32 preeclamptic patients, respectively. APC-R rates were significantly higher in preeclamptic group than in normal pregnant women in the third trimester of pregnancy (p < 0.05). Decreased mean APC activity and also increased APC-R rate was still persisting in preeclamptic group for 3 months after delivery. Nine months after delivery, the mean APC activity and also APC-R rates approached to the normal pregnant women; however, there was a significant difference between both groups (p < 0.05). Our results indicate that acquired APC-R may be a contributory factor in the pathogenesis of preeclampsia.

C4b-binding protein protects coagulation factor Va from inactivation by activated protein C

We investigated the effect of C4BP on APC-mediated inactivation of factor Va (FVa) in the absence and presence of protein S. FVa inactivation was biphasic (k(506) = 4.4 x 10(8) M(-)(1) s(-)(1), k(306) = 2.7 x 10(7) M(-)(1) s(-)(1)), and protein S accelerated Arg(306) cleavage approximately 10-fold. Preincubation of protein S with C4BP resulted in a total abrogation of protein S cofactor activity. C4BP also protected FVa from inactivation by APC in the absence of protein S. Control experiments with CLB-PS13, a monoclonal anti-protein S antibody, indicated that inhibition of FVa inactivation by C4BP was not mediated through contaminating traces of protein S in our reaction systems. Protection of FVa was prevented by a monoclonal antibody directed against the C4BP alpha-chain. Recombinant rC4BPalpha comprised of only alpha-chains also protected FVa, but in the presence of protein S, the level of protection was decreased, since rC4BPalpha lacks the beta-chain responsible for C4BP binding to protein S. A truncated C4BP beta-chain (SCR-1+2) inhibited protein S cofactor activity, but had no effect on FVa inactivation by APC in the absence of protein S. In conclusion, C4BP protects FVa from APC-catalyzed cleavage in a protein S-independent way through direct interactions of the alpha-chaims of C4BP with FVa and/or APC.

Blood coagulation

Under normal circumstances, the coagulation system is balanced in favour of anticoagulation. Thrombin is the key effector enzyme of the clotting cascade. Antagonists of vitamin K inhibit a vitamin-K-dependent post-translational modification of several coagulation proteins, which is required for these proteins to attain a phospholipid-binding conformation. Heparin stimulates the activity of antithrombin, a serine-protease inhibitor. Analysis of knock-out mice has shown the relative importance of the coagulation factors in vivo. Gene therapy may soon be a therapeutic option for inherited deficiencies of factors VIII and IX.

Regulation of factor VIIIa by human activated protein C and protein S: inactivation of cofactor in the intrinsic factor Xase

Factor VIIIa is a trimer of A1, A2, and A3-C1-C2 subunits. Inactivation of the cofactor by human activated protein C (APC) results from preferential cleavage at Arg336 within the A1 subunit, followed by cleavage at Arg562 bisecting the A2 subunit. In the presence of human protein S, the rate of APC-dependent factor VIIIa inactivation increased several-fold and correlated with an increased rate of cleavage at Arg562. (Active site-modified) factor IXa, blocked cleavage at the A2 site. However, APC-catalyzed inactivation of factor VIIIa proceeded at a similar rate independent of factor IXa, consistent with the location of the preferential cleavage site within the A1 subunit. Addition of protein S failed to increase the rate of cleavage at the A2 site when factor IXa was present. In the presence of factor X, cofactor inactivation was inhibited, due to a reduced rate of cleavage at Arg336. However, inclusion of protein S restored near original rates of factor VIIIa inactivation and cleavage at the A1 site, thus overcoming the factor X-dependent protective effect. These results suggest that in the human system, protein S stimulates APC-catalyzed factor VIIIa inactivation by facilitating cleavage of A2 subunit (an effect retarded in the presence of factor IXa), as well as abrogating protective interactions of the cofactor with factor X. (Blood. 2000;95:1714-1720)

Vitamin K-dependent proteins in the developing and aging nervous system

Although the warfarin embryopathy syndrome, with its neurologic and bone abnormalities, has been known for decades, the role of vitamin K in the brain has not been studied systematically. Recently, it was demonstrated that vitamin K-dependent carboxylase expression is temporally regulated in a tissue-specific manner with high expression in the nervous system during the early embryonic stages and with liver expression after birth and in adult animals. This finding, along with the discovery of wide distribution of the novel vitamin K-dependent growth factor, Gas6, in the central nervous system, provides compelling evidence of a biologic role of vitamin K during the development of the nervous system. In animals and bacteria, vitamin K was observed to influence the brain sulfatide concentration and the activity and synthesis of an important enzyme involved in brain sphingolipids biosynthesis. Taken together, previous research results point to a possible role of vitamin K in the nervous system, especially during its development. Hence, the knowledge of the biologic role of vitamin K in the brain may be important for unveiling the mechanisms of normal and pathologic development and aging of the nervous system. The role of the vitamin K-dependent protein Gas6 in activation of signal transduction events in the brain in light of the age-related changes in the nervous system is also discussed.

Clinically symptomatic central venous catheter-related deep venous thrombosis in newborns

The objective of this study was to evaluate the incidence of clinically symptomatic central venous catheter (CVC)-related deep venous thrombosis (DVT) in newborns and small infants and to try to identify clinical and genetic risk factors for catheter-related DVT among children with thrombotic complications. CVC was inserted in 44 consecutive infants (age range 0-90 d) during the period January 1990 to December 1995 in the neonatal intensive care unit (NICU) of Kuopio University Hospital in Kuopio. The symptoms of DVT were: syndrome of superior vena cava in 2, swelling at the CVC puncture site in 6 and repeated CVC obstructions in 2. The formation of DVT was verified by venography. Children with DVT (n = 10) had 26 (10-365, in total 623) catheter days compared with 9 d (1-155, in total 591) in patients without DVT (n = 26) (p < 0.005). The median (range) number of days from catheter insertion to diagnosis of DVT was 19 (7-210). CVC had to be removed from 11 (25%) children due to various complications. There was no DVT-related mortality. A positive family history with thromboembolic episodes at a young age was found in 3 of 10 families with a child suffering CVC-related DVT. The levels of coagulation inhibitors were evaluated at the age of 9-69 mo in all 10 (23%) children with CVC-related DVT. We detected no deficiencies in protein S, protein C or antithrombin III. One child was heterozygous for the point mutation (R506Q) in the factor V gene known to cause activated protein C resistance (APCR). We conclude that newborns with CVC are at great risk of DVT and that the aetiology of DVT can rarely be identified via measurements of coagulation inhibitors.

Interaction between protein S and complement C4b-binding protein (C4BP). Affinity studies using chimeras containing c4bp beta-chain short consensus repeats

Human C4b-binding protein (C4BP) is a regulator of the complement system and plays an important role in the regulation of the anticoagulant protein C pathway. C4BP can bind anticoagulant protein S, resulting in a decreased cofactor function of protein S for activated protein C. C4BP is a multimeric protein containing several identical alpha-chains and a single beta-chain (C4BPbeta), each chain being composed of short consensus repeats (SCRs). Previous studies have localized the protein S binding site to the NH2-terminal SCR (SCR-1) of C4BPbeta. To further localize the protein S binding site, we constructed chimeras containing C4BPbeta SCR-1, SCR-2, SCR-3, SCR-1+2, SCR-1+3, and SCR-2+3 fused to tissue-type plasminogen activator. Binding assays of protein S with these chimeras indicated that SCR-2 contributes to the interaction of protein S with SCR-1, since the affinity of protein S for SCR-1+2 was up to 5-fold higher compared with SCR-1 and SCR-1+3. Using an assay that measures protein S cofactor activity, we showed that cofactor activity was decreased due to binding to constructs that contain SCR-1. SCR-1+2 inhibited more potently than SCR-1 and SCR-1+3. SCR-3 had no additional effect on SCR-1, and therefore the effect of SCR-2 was specific. In conclusion, beta-chain SCR-2 contributes to the interaction of C4BP with protein S.

A plasma coagulation assay for an activated protein C-independent anticoagulant activity of protein S

To study the physiological importance of the activated protein C (APC)-independent anticoagulant activity of protein S, we developed an assay specific for this activity. The ability of protein S to prolong the clotting time in an APC-independent way was expressed as the ratio of the clotting time in a plasma sample divided by the clotting time in the presence of a polyclonal antibody against human protein S (both after 1:1 dilution in protein S-C4BP deficient plasma). The mean protein S-dependent anticoagulant ratio (PSdAR) was 1.53+/-0.18 in 34 healthy controls, and was significantly lower in 16 heterozygous protein S deficient patients (PSdAR=1.15+/-0.09). In plasmas from patients under oral anticoagulant therapy the mean PSdAR was within the range of the control population (1.50+/-0.18). The mean total protein S antigen level in these plasmas was 58%, suggesting a higher specific APC-independent anticoagulant activity of protein S in these patients than in normals. This functional protein S assay can be used for the laboratory diagnosis of protein S deficiency, and to study the mechanism of the APC-independent anticoagulant activity in plasma.

The SHBG-like region of protein S is crucial for factor V-dependent APC-cofactor function

Activated protein C (APC) regulates blood coagulation by degrading factor Va (FVa) and factor VIIIa (FVIIIa). Protein S is a cofactor to APC in the FVa degradation, whereas FVIIIa degradation is potentiated by the synergistic APC-cofactor activity of protein S and factor V (FV). To elucidate the importance of the sex-hormone-binding globulin (SHBG)-like region in protein S for expression of anticoagulant activity, a recombinant protein S/Gas6 chimera was constructed. It comprised the amino-terminal half of protein S and the SHBG-like region of Gas6, a structurally similar protein having no known anticoagulant properties. The protein S/Gas6 chimera expressed 40-50%, APC-cofactor activity in plasma as compared to wild-type protein S. In the degradation of FVa by APC, the protein S/Gas6 chimera was only slightly less efficient than wild-type protein S. In contrast, the protein S/Gas6 chimera expressed no FV-dependent APC-cofactor activity in a FVIIIa-degradation system. This demonstrates the SHBG-like region to be important for expression of APC-cofactor activity of protein S and suggests that the SHBG-like region of protein S interacts with FV during the APC-mediated inactivation of FVIIIa.

Activated protein C sensitivity, protein C, protein S and coagulation in normal pregnancy

A prospective study of activated protein C sensitivity, protein C, protein S, and other coagulation factors in 239 women during normal pregnancy was carried out. Protein C activity appeared unaffected by gestation, although an elevation of protein C activity was observed in the early puerperium. A fall in total and free protein S with increasing gestation was observed. Activated protein C sensitivity ratio (APC:SR) showed a progressive fall through pregnancy. This fall correlated with changes in factor VIIIc, factor Vc and protein S. 38% of subjects, with no evidence of Factor V Leiden or anticardiolipin antibodies, showed a low APC:SR (APC:SR <2.6) in the third trimester of pregnancy. Aside from a significant reduction in birth weight, no difference in pregnancy outcome was observed between these subjects and those with a normal APC:SR. Activated protein C sensitivity ratio, modified by pre-dilution of patient samples with factor V depleted plasma, showed no consistent trend with gestation.

The inherited basis of venous thrombosis

Venous thrombosis represents a manifestation of disordered hemostatic balance. The classical presentation is of pain and swelling of the lower limb, although clinical history and examination are notoriously misleading in reaching a diagnosis. A number of acquired predispositions have been associated with a tendency to thrombosis, such as immobilisation, surgery, malignancy and certain types of oral contraception, but in at least half of the instances no predisposition can be identified. A variety of genetic risk factors have also been identified. Mutations within the genes for antithrombin, protein C and protein S are associated with a venous thromboembolic phenotype. The commonest thrombophilic predisposition however is a variant of coagulation factor V, factor V Leiden, which results from a single amino acid substitution rendering the factor V molecule resistant to activated protein C. Factor V Leiden is present in approximately 5% of individuals of European origin, and is found in up to 40% of those with confirmed venous thrombosis. Increasingly it is recognised that venous thrombosis should be considered a polygenic disorder, with interactions between the various single gene defects which predispose to thrombosis, as well as normal genetic variation between individuals in the levels of both procoagulant and anticoagulant proteins, all determining which individuals will express the phenotype of venous thrombosis.

A laboratory approach to the evaluation of hereditary hypercoagulability

The concept of hypercoagulability and especially its evaluation in the clinical laboratory has changed dramatically during the last few years. The genetic basis and the mechanisms of the various factors responsible for hypercoagulability are briefly reviewed with emphasis on the most common genetic deficiencies. The major thrust of this review centers on the cost-effective approach to examining patients with a personal or family history of venous thrombosis. Several new concepts dealing with thrombotic risk are presented with a focus on the theory that multiple factors cause thrombosis in affected patients. A proposal for a cost-effective sequential testing scheme for the accurate diagnosis of hereditary hypercoagulability is discussed. The knowledge of thrombotic risk factors is evolving rapidly, requiring the clinical laboratory to remain flexible. Ultimately, the clinical laboratory must take a leading role in the diagnosis of hereditary thrombotic disease by serving as the consultant to the primary caregiver by providing an up-to-date and cost-effective evaluation.

Increased tissue factor-initiated prothrombin activation as a result of the Arg506 --> Gln mutation in factor VLEIDEN

The effect of the Arg506 --> Gln mutation in factor VLEIDEN on thrombin generation was evaluated in a reconstituted system using the purified components of the tissue factor (TF) pathway to thrombin and the components of the protein C pathway. Recombinant full-length tissue factor pathway inhibitor (RTFPI) was included in the system because of a previously observed synergistic inhibitory effect of TFPI and the protein C pathway on TF-initiated thrombin generation. Thrombin generation initiated by 1.25 pM factor VIIa.TF in the absence of the protein C pathway components occurs following an initiation phase, after which prothrombin is quantitatively converted to 1.4 microM thrombin. The factor VLEIDEN mutation did not influence thrombin generation in the reconstituted model in the absence of the protein C pathway. In the presence of 2.5 nM TFPI, 65 nM protein C, and 10 nM recombinant soluble thrombomodulin (Tm), thrombin generation catalyzed by normal factor V was abolished after the initial formation of 25 nM thrombin. In contrast, persistent thrombin generation was observed in the presence of factor VLEIDEN in the same system, although the rate of thrombin generation was slower compared with the reaction without protein C and Tm. The rate of thrombin generation with factor VLEIDEN increased with time and ultimately resulted in quantitative prothrombin activation. When the TFPI concentration was reduced to 1.25 nM, thrombin generation is still curtailed in the presence of normal factor V. In contrast, under similar conditions using factor VLEIDEN, the protein C pathway totally failed to down-regulate thrombin generation. The dramatic effect of a 50% reduction in TFPI concentration on the inhibitory potential of the protein C pathway on thrombin generation catalyzed by factor VLEIDEN suggests that the observed synergy between TFPI and the protein C pathway is directly governed by the TFPI concentration and by cleavage of the factor Va heavy chain at Arg506. This cleavage appears to have a dramatic regulatory effect in the presence of low concentrations of TFPI. Markedly increased thrombin generation in the presence of both 1.25 nM TFPI and factor VLEIDEN was also observed when antithrombin-III was added to the system to complete the natural set of coagulation inhibitors. Protein S (300 nM) had a minimal effect in the model on the inhibition of thrombin generation by protein C, Tm, and TFPI, with either normal factor V or factor VLEIDEN. Protein S also failed to significantly potentiate the action of the protein C pathway in the presence of antithrombin-III in reactions employing normal factor V or factor VLEIDEN. The absence of an effect of protein S in the model, which employs saturating concentrations of phospholipid, suggests that the reported interactions of protein S with coagulation factors are not decisive in the reaction. Altogether the data predict that TFPI levels in the lower range of normal values are a risk factor for thrombosis when combined with the Arg506 --> Gln mutation in factor VLEIDEN.

Sequence and functional relationships between androgen-binding protein/sex hormone-binding globulin and its homologs protein S, Gas6, laminin, and agrin

Androgen-binding protein/sex hormone-binding globulin (ABP/SHBG) is an extracellular binding protein that regulates the bioavailability of sex steroids. ABP/SHBG is closely related to the globular (G) domain of vitamin K-dependent protein S family of proteins and more distantly related to the G domains of several extracellular matrix proteins. ABP/SHBG appears to have evolved from the fusion of two ancestral G domains. Expanding evidence suggests that ABP/SHBG has other functions that are mediated through membrane binding, including signal transduction; however, the types of binding proteins (receptors) have not been identified. Sequence comparisons of ABP/SHBG with G domains of its homologs protein S, Gas6, laminin, and agrin have identified regions of ABP/SHBG that may bind receptors related to homolog receptors. These membrane receptors include beta-integrins, alpha-dystroglycan, and receptor tyrosine kinases. The G domains of laminin and related proteins have clearly evolved from a common ancestor to interact with specific receptors and binding proteins. It remains to be determined if ABP/SHBG followed this evolutionary pathway.