Purification and characterization of an endotoxin-binding protein with protease inhibitory activity from Limulus amebocytes

Post-translational protein deimination signatures and extracellular vesicles (EVs) in the Atlantic horseshoe crab (Limulus polyphemus)

The horseshoe crab is a living fossil and a species of marine arthropod with unusual immune system properties which are also exploited commercially. Given its ancient status dating to the Ordovician period (450 million years ago), its standing in phylogeny and unusual immunological characteristics, the horseshoe crab may hold valuable information for comparative immunology studies. Peptidylarginine deiminases (PADs) are calcium dependent enzymes that are phylogenetically conserved and cause protein deimination via conversion of arginine to citrulline. This post-translational modification can lead to structural and functional protein changes contributing to protein moonlighting in health and disease. PAD-mediated regulation of extracellular vesicle (EV) release, a critical component of cellular communication, has furthermore been identified to be a phylogenetically conserved mechanism. PADs, protein deimination and EVs have hitherto not been studied in the horseshoe crab and were assessed in the current study. Horseshoe crab haemolymph serum-EVs were found to be a poly-dispersed population in the 20-400 nm size range, with the majority of EVs falling within 40-123 nm. Key immune proteins were identified to be post-translationally deiminated in horseshoe crab haemolymph serum, providing insights into protein moonlighting function of Limulus and phylogenetically conserved immune proteins. KEGG (Kyoto encyclopaedia of genes and genomes) and GO (gene ontology) enrichment analysis of deiminated proteins identified in Limulus revealed KEGG pathways relating to complement and coagulation pathways, Staphylococcus aureus infection, glycolysis/gluconeogenesis and carbon metabolism, while GO pathways of biological and molecular pathways related to a range of immune and metabolic functions, as well as developmental processes. The characterisation of EVs, and post-translational deimination signatures, revealed here in horseshoe crab, contributes to current understanding of protein moonlighting functions and EV-mediated communication in this ancient arthropod and throughout phylogeny.

Chromosome-level assembly of the horseshoe crab genome provides insights into its genome evolution

The evolutionary history of horseshoe crabs, spanning approximately 500 million years, is characterized by remarkable morphological stasis and a low species diversity with only four extant species. Here we report a chromosome-level genome assembly for the mangrove horseshoe crab (Carcinoscorpius rotundicauda) using PacBio reads and Hi-C data. The assembly spans 1.67 Gb with contig N50 of 7.8 Mb and 98% of the genome assigned to 16 chromosomes. The genome contains five Hox clusters with 34 Hox genes, the highest number reported in any invertebrate. Detailed analysis of the genome provides evidence that suggests three rounds of whole-genome duplication (WGD), raising questions about the relationship between WGD and species radiation. Several gene families, particularly those involved in innate immunity, have undergone extensive tandem duplication. These expanded gene families may be important components of the innate immune system of horseshoe crabs, whose amebocyte lysate is a sensitive agent for detecting endotoxin contamination.

Horseshoe crabs have been morphologically stable across evolutionary time. Here, the authors generate a chromosome-level assembly for the mangrove horseshoe crab, with implications for innate immunity, and challenging assumptions about the role of genome duplication in adaptive radiation.

Amide-mediated hydrogen bonding at organic crystal/water interfaces enables selective endotoxin binding with picomolar affinity

Since the discovery of endotoxins as the primary toxic component of Gram-negative bacteria, researchers have pursued the quest for molecules that detect, neutralize, and remove endotoxins. Selective removal of endotoxins is particularly challenging for protein solutions and, to this day, no general method is available. Here, we report that crystals of the purine-derived compound allantoin selectively adsorb endotoxins with picomolar affinity through amide-mediated hydrogen bonding in aqueous solutions. Atom force microscopy and chemical inhibition experiments indicate that endotoxin adsorption is largely independent from hydrophobic and ionic interactions with allantoin crystals and is mediated by hydrogen bonding with amide groups at flat crystal surfaces. The small size (500 nm) and large specific surface area of allantoin crystals results in a very high endotoxin-binding capacity (3 × 10(7) EU/g) which compares favorably with known endotoxin-binding materials. These results provide a proof-of-concept for hydrogen bond-based molecular recognition processes in aqueous solutions and establish a practical method for removing endotoxins from protein solutions.

Ligand specificities and structural requirements of two Tachypleus plasma lectins for bacterial trapping

TPL (Tachypleus plasma lectin)-1 was purified by using a Sepharose column and TPL-2 was purified from an LPS-Sepharose (LPS coupled to Sepharose matrix) affinity column, as described previously [Chiou, Chen, Y.-W., Chen, S.-C., Chao and Liu (2000) J. Biol. Chem. 275, 1630-1634] and the corresponding genes were cloned [Chen, Yen, Yeh, Huang and Liu (2001) J. Biol. Chem. 276, 9631-9639]. In the present study, TPL-1 and -2 were produced in yeast, and the recombinant proteins secreted into the media were purified and characterized. The proteins show specific PGN (peptidoglycan)- and LPS-binding activity, suggesting a role in trapping Gram-positive and Gram-negative bacteria respectively in innate immunity. Using BIAcore assays, the dissociation constant for the TPL-1-PGN complex was measured as 8x10(-8) M. Replacement of Asn74, the N-glycosylation site of TPL-1, with Asp abolishes the PGN-binding affinity, whereas the unglycosylated TPL-2 N3D mutant retains LPS-binding activity. DTT (dithiothreitol) treatment to break disulphide linkages abrogates TPL-2 activity but does not interfere with TPL-1 function. Cys4 in TPL-2 may form an intermolecular disulphide bond, which is essential for activity. As a result, the TPL-2 C4S mutant is inactive and is eluted as a monomer on a non-reducing gel. TPL-2 C6S is active and forms a non-covalently linked dimer. A model describing TPL-2 binding with LPS is proposed. These two plasma lectins that have different ligand specificities can be used for the detection and discrimination of bacteria and removal of endotoxins.

Biochemical properties and cDNa cloning of two new lectins from the plasma of Tachypleus tridentatus: Tachypleus plasma lectin 1 and 2+

A Sepharose CL-4B-binding protein, Tachypleus plasma lectin 1 (TPL-1), and a lipopolysaccharide (LPS)-binding protein, Tachypleus plasma lectin-2 (TPL-2), have been isolated from the plasma of Tachypleus tridentatus and biochemically characterized. Each protein is coded by a homologous family of multigenes. TPL-1 binds to Sepharose CL-4B and was eluted with buffer containing 0.4 m GlcNAc. The deduced amino acid sequence of TPL-1 consisted of 232 amino acids with an N-glycosylation site, Asn-Gly-Ser at residues 74-76. It shares a 65% sequence identity and similar internal repeats of about 20 amino acid motifs with tachylectin-1. Tachylectin-1 was identified as a lipopolysaccharide-agarose binding nonglycosylated protein from the amebocytes of T. tridentatus. TPL-2 was eluted from the LPS-Sepharose CL-4B affinity column in buffer containing 0.4 m GlcNAc and 2 m KCl. The deduced amino acid sequence of TPL-2 consisted of 128 amino acids with an N-glycosylation site, Asn-Cys-Thr, at positions 3-5. It shares an 80% sequence identity with tachylectin-3, isolated from the amebocytes of T. tridentatus. TPL-2 purified by LPS-affinity column from the plasma predominantly exists as a dimer of a glycoprotein with an apparent molecular mass of 36 kDa. Tachylectin-3 is an intracellular nonglycosylated protein that also exists as a dimer in solution with an apparent molecular mass of 29 kDa. It recognizes Gram-negative bacteria through the 0-antigen of LPS. Western blot analyses showed that, in the plasma, TPL-1 and TPL-2 exist predominantly as oligomers with molecular masses above 60 kDa. They both bind to Gram-positive and Gram-negative bacteria, and this binding is inhibited by GlcNAc. Possible binding site of TPL-1 and TPL-2 to the bacteria could be at the NAc moiety of GlcNAc-MurNAc of the peptidoglycan. The physiological function of TPL-1 and TPL-2 is most likely related to their ability to form a cluster of interlocking molecules to immobilize and entrap invading organisms.

Definition of endotoxin binding sites in horseshoe crab factor C recombinant sushi proteins and neutralization of endotoxin by sushi peptides

Three truncated fragments, harboring different sushi domains, namely, sushi123, sushi1, and sushi3 domains, of Factor C were produced as biologically active secreted recombinant proteins. Sushi1 and 3 each has a high-affinity LPS binding site with K:(d) of 10(-9) to 10(-10) M. Positive cooperativity in sushi123 resulted in a 1000-fold increase in K:(d)2. The core LPS binding region of sushi1 and 3 reside in two 34-mer peptides, S1 and S3. A rigidly held disulfide-bonded structure is not essential but is important for LPS binding, as confirmed by a 100- to 10000-fold decrease in affinity. Both S1 and S3 can inhibit LAL reaction and LPS-induced hTNF-alpha secretion with different potency. LAL assay revealed that at least two molecules of S1 bind cooperatively to one LPS molecule, with Hill's coefficient of 2.42. The LPS binding by S3 is independent and noncooperative. The modified SDelta1 and SDelta3 peptides exhibited increased LPS neutralization potential although its LPS binding affinities indicated only a 10-fold improvement. Hence, the structural difference of the four sushi peptides conferred different efficiencies in LPS neutralization without altering their binding affinity for LPS. Circular dichroism spectrometry revealed that the four peptides underwent conformational change in the presence of lipid A, transitioning from a random coil to either an alpha-helical or beta-sheet structure. Two factors are critical for the sensitivity of Factor C to LPS: 1) the presence of multiple binding sites for LPS on a single Factor C molecule; and 2) high positive cooperativity in LPS binding. The results showed that in the design of an improved LPS binding and neutralizing peptide, charge balance of the peptide is a critical parameter in addition to its structure.

A newly identified horseshoe crab lectin with binding specificity to O-antigen of bacterial lipopolysaccharides

We identified a novel horseshoe crab hemocyte-derived lectin, which we named tachylectin-4. It has more potent hemagglutinating activity against human A-type erythrocytes than a previously identified hemocyte lectin with an affinity to N-acetylglucosamine, tachylectin-2. The purified tachylectin-4 is an oligomeric glycoprotein of 470 kDa, composed of subunits of 30 and 31.5 kDa. Ca2+ at 10 mM enhanced the hemagglutinating activity 4-fold, and the activity was inhibited by EDTA and o-phenanthroline. L-Fucose and N-acetylneuraminic acid at 100 mM completely inhibited the activity of tachylectin-4. The activity was also inhibited more strongly by bacterial S-type lipopolysaccharides (LPS) but not by R-type LPS lacking O-antigen. The most effective S-type LPS was from Escherichia coli O111:B4, and the minimum concentration required for inhibiting agglutination against human A-type erythrocytes (0.1 microg/ml) was 160-fold lower than those of S-type LPS from Salmonella minnesota. Therefore, colitose (3-deoxy-L-fucose), a unique sugar present in the O-antigen of E. coli O111:B4 with structural similarity to L-fucose, is the most probable candidate for a specific ligand of tachylectin-4. A cDNA coding for tachylectin-4 was isolated from a hemocyte cDNA library. The open reading frame of the 1344-base pair cDNA coded for the mature protein with 232 amino acids. There is no significant sequence similarity to any other known LPS-binding lectins, whereas tachylectin-4 is homologous to the NH2-terminal domain with unknown functions of Xenopus laevis pentraxin 1.

LPS (lipopolysaccharide)-activated immune responses in a hemocyte cell line from Estigmene acraea (Lepidoptera)

The suitability of the hemocyte cell line BTI-EA-1174-A from Estigmene acraea (Lepidoptera) to serve as a tool for studying insect immune reactions in vitro was investigated. Addition of bacterial lipopolysaccharides to the cultures caused enhanced phagocytosis of silica beads, as well as increased lysozyme activity in the cell culture supernatants. Addition of fungal beta 1,3-glucans did not result in any activation. The LPS-influenced (1 mg/mL) increase of phagocytic reactions against the silica beads was at its highest within 24 h after LPS-addition. Activated cells exhibited drastic changes in their morphology in connection with reduced cell numbers in the cultures but without increased mortality rates. LPS-dosages higher than 10 micrograms/mL LPS provoked significantly enhanced lysozyme activities. A maximal induction took place with 1 mg/mL LPS. The lysozyme activity started to rise 2 days after LPS-addition, further increase was observed up to the seventh day. The responsible protein was isolated from cell culture supernatants and N-terminally sequenced. The exact molecular mass was determined by mass spectrometry as 14.080 kDa. The amino acid sequence of the analysed portion revealed high sequence-similarity to the lysozymes of other lepidopteran insects as well as to hen egg lysozyme. Further results presented in this paper give indications for the existence of soluble molecules which are released by the cells and which enhance the LPS-triggered activation.

Limulus intracellular coagulation inhibitor type 3. Purification, characterization, cDNA cloning, and tissue localization

We reported that limulus intracellular coagulation inhibitor type-1 (LICI-1) (Miura, Y., Kawabata, S., and Iwanaga, S. (1994) J. Biol. Chem. 269, 542-547) and LICI type-2 (LICI-2) (Miura, Y., Kawabata, S. , Wakamiya, Y., Nakamura, T., and Iwanaga, S. (1995) J. Biol. Chem. 270, 558-565) found in the hemocyte lysate belong to the serpin family. The LICI-1 specifically inhibits limulus lipopolysaccharide-sensitive serine protease, factor C (k1 = 2.5 x 10(6) M-1 s-1), whereas LICI-2 inhibits preferentially limulus clotting enzyme (k1 = 4.3 x 10(5) M-1 s-1). In our ongoing studies on limulus serpin, we found another inhibitor, named LICI type-3 (LICI-3), which strongly inhibits (1,3)-beta-D-glucan-sensitive serine protease, factor G (k1 = 3.9 x 10(5) M-1 s-1). Thus, the limulus hemolymph coagulation cascade is effectively regulated by at least the three endogenous serpins. LICI-3, newly identified in hemocytes, is a single chain glycoprotein with an apparent Mr = 53,000, the largest one among known limulus serpins. A cDNA sequence for LICI-3 coded a mature protein of 392 amino acids, of which 68 residues were confirmed by peptide sequencing. LICI-3 showed significant sequence similarity to LICI-1 (45.8% identity) and LICI-2 (33.7% identity). LICI-3 contained a putative reactive site, -Arg-Ser-, distinct from that of LICI-2 (-Lys-Ser-) but the same as that of LICI-1. Expression of LICI-3 mRNA was detected only in hemocytes, and not in heart, brain, stomach, intestine, coxal gland, and skeletal muscle. Immunoblotting of the hemocyte-derived large and small granules with antiserum against LICI-3 suggested that it is stored specifically in large granules, as in the case of LICI-1 and LICI-2, and is released in response to external stimuli.

Limulus kexin: a new type of Kex2-like endoprotease specifically expressed in hemocytes of the horseshoe crab

A Kex2-like protease was identified in hemocytes of the horseshoe crab (Tachypleus tridentatus), named limulus kexin, and a full-length cDNA was obtained from a hemocyte cDNA library. The deduced amino acid sequence contains 752 residues, composed of five domains with a signal sequence, a propeptide, a catalytic domain, a Ser/Thr-rich domain, and a transmembrane domain. The domain organization is very similar to that of the yeast Kex2 except that limulus kexin does not have a cytoplasmic tail. The catalytic domain exhibits striking sequence identities with those of furins, especially Drosophila furin1 (79%). Northern blotting showed specific expression of limulus kexin in hemocytes, suggesting the involvement in proteolytic processing of the granule components of hemocytes.

Clotting and immune defense in Limulidae

The blue blood of the horseshoe crab contains a sophisticated defense system very sensitive to pathogens or foreign materials. The hemocytes circulating in the hemolymph detect trace amounts of LPS molecules on the invading microorganisms and respond quickly to release the granular components into the external milieu. The coagulation system composed of three serine protease zymogens, factor C, factor B, and proclotting enzyme, and a clottable protein, coagulogen, is activated by LPS to form insoluble coagulin gel. The coagulation system also responds to beta-(1,3) glucan through the activation of unique heterodimeric serine protease zymogen, factor G. The pathogens are, thus, engulfed in the gel and subsequently killed by antimicrobial substances with various specificities, which are also released from cells. The horseshoe crab has developed two kinds of serine protease zymogens as biological sensors, factor C and factor G, which are responsive to LPS and beta-(1,3) glucan on the surface of Gram-negative bacteria and fungi, respectively. These are possible invaders for horseshoe crabs and also for most animals including humans. This novel heterodimeric serine protease zymogen, factor G, may open a new way to develop an innovative assay system to quantitate beta-(1,3) glucans. Furthermore, these LPS and beta-(1,3) glucan sensitive factors could be utilized as a unique tool to analyze other biological reactions caused by LPS or the glucan. Although the coagulation reaction in horseshoe crabs is famous, it is not the only defense mechanism of this animal. Many agglutinins are present either in hemolymph plasma or in the cell. The hemolymph plasma also has cytolytic activity against foreign cells. These cellular and humoral defense systems, in concert, defend themselves from invading foreign organisms. Such a sophisticated defense system has allowed the horseshoe crab to survive for more than 200 million years on the earth. Horseshoe crabs are often called ¿living fossils." However, they are not fossils. They are living.

A novel type of limulus lectin-L6. Purification, primary structure, and antibacterial activity

Lipopolysaccharide (LPS)-binding protein(s) was first screened in the detergent extract of horseshoe crab (limulus) hemocytes using LPS-immobilized agarose. A protein, designated L6 (M(r) = 27,000), was found to bind to LPS-agarose and was eluted with EDTA or o-phenanthroline. The L6 protein, however, did not inhibit the LPS-mediated activation of a limulus serine protease zymogen factor C. L6 had an affinity to the matrix of Sepharose CL-6B itself, and it could be eluted with high concentrations of monosaccharides (0.5-1.0 M), such as glucose, mannose, and galactose, suggesting a lectin-like nature. The entire amino acid sequence of L6 was determined by sequencing peptides derived from CNBr and enzymatic cleavages. L6 contained 7 half-cystines, and 1 cysteine residue at position 201 had a free SH-group. In addition, positions of the remaining three intrachain disulfide bonds were assigned by amino acid and sequence analyses of three cystinyl peptides produced by lysyl endopeptidase digestion. These results indicated that the entire sequence of L6 consisted of 221 residues with no N-linked sugar and was composed of six tandem repeats, each consisting of 33-38 amino acid residues. Inductively coupled plasma spectrometry of L6 indicated the presence of 0.75 mol zinc/mol of protein. No significant sequence homology was observed between L6 and other proteins, including various animal lectins and LPS-binding proteins. However, L6 showed agglutinating activity on LPS-coated sheep erythrocytes and Gram-negative and Gram-positive bacteria, it inhibited the growth of Gram-negative bacteria, and thus it presumably recognizes carbohydrate components in the cell wall of bacteria.

A limulus intracellular coagulation inhibitor type 2. Purification, characterization, cDNA cloning, and tissue localization

We described in a foregoing report findings on serpin, a serine protease inhibitor, newly identified in horseshoe crab (Tachypleus tridentatus) hemocytes and we name it limulus intracellular coagulation inhibitor, LICI (Miura, Y., Kawabata, S., and Iwanaga, S. (1994) J. Biol. Chem. 269, 542-547). This serpin specifically inhibits limulus lipopolysaccharide-sensitive serine protease, factor C. In ongoing studies on limulus serpin, we have found another inhibitor, LICI type-2 (LICI-2), which inhibits not only factor C (k1 = 7.1 x 10(4) M-1 S-1) but also limulus clotting enzyme (k1 = 4.3 x 10(5) M-1 S-1). LICI-2 inhibits mammalian serine proteases, including alpha-thrombin, salivary kallikrein, plasmin, and tissue plasminogen activator. The inactivation of plasmin is the most rapid (k1 = 1.2 x 10(6) M-1 S-1). The purified LICI-2 is a single chain glycoprotein with an apparent M(r) = 42,000. A cDNA for LICI-2 was isolated and the open reading frame coded for a mature protein of 386 amino acids, of which 160 residues were confirmed by peptide sequencing. Although LICI-2 shows significant sequence similarity to the previous limulus serpin, LICI-1 (42% identity), LICI-2 contains a unique putative reactive site, -Lys-Ser-, distinct from that of LICI-1 (-Arg-Ser-). Northern blotting revealed expression of LICI-2 mRNA only in hemocytes, and not in heart, brain, stomach, intestine, coxal gland, and skeletal muscle. The immunoblot of large and small granule components with antiserum against purified LICI-2 suggests that LICI-2 is stored specifically in large granules, as in the case of LICI-1, and is released in response to external stimuli. We propose that the LICIs be classified into a new subfamily of intracellular serpins, regulated secretory serpins.

Role of hemocyte-derived granular components in invertebrate defense

Figure 2 illustrates an outline of the cellular and humoral defense systems in limulus. On the basis of the knowledge described above, it is suggested that granular components present in L and S granules in the hemocytes play a decisive role in the biological defense for this animal. The isolated L granules contain at least three clotting factors plus coagulogen as the major component. The known anti-LPS factor and a number of additional unknown protein components are also present in the L granules. On the other hand, the isolated S granules contain antimicrobial tachyplesins as the major component, in addition to six unidentified proteins. We speculate that the L-granule-derived protein components, which probably contain all the factors essential for the Limulus clotting system participate, in immobilizing invading microbes, and that the S-granule-derived tachyplesins contribute to a self-defense system against invaders. Although we have not mentioned hemolymph plasma components, there are many humoral factors, such as proteinase inhibitors, alpha 2-macroglobulin, various lectins, C-reactive protein, and polyphemin, all of which are important for antimicrobial defense. Furthermore, Liu and colleagues have reported several endotoxin-binding proteins and a cell-adhesion protein found in the Limulus hemocytes. Although the exact functions of these substances are unknown, they may act in concert with other components to provide biological defense for the animal. Nevertheless, compared to our knowledge of mammalian blood cells, much less remains to be learned of biological/physiological events in horseshoe crab hemocytes.

C-reactive proteins, limunectin, lipopolysaccharide-binding protein, and coagulin. Molecules with lectin and agglutinin activities from Limulus polyphemus

In 1964, Levin and Bang discovered that gram-negative bacterial endotoxin could rapidly induce gelation of Limulus amebocyte lysate. This observation has led to the development of the most sensitive and specific method for the detection of bacterial endotoxin in pharmaceuticals and drugs intended for human use. Over 10 years ago, Bang injected endotoxin into young horseshoe crabs and observed a time and dose-dependent coagulation of the whole hemolymph. Limunectin, LEBP-PI, and Limulus CRP are found together with coagulin as part of the hemolymph clot at the time of endotoxin-induced exocytosis of amebocytes. In this manner, these molecules with agglutinin/lectin activities could work in concert to assist in the recognition and eventual removal of invading microorganisms from the circulating system. Although the mechanism of endotoxin-induced clot formation is to a large extent understood, the mechanism of clot dissolution and removal in the Limulus hemolymph remains to be clarified.

Bacterial endotoxins: extraordinary lipids that activate eucaryotic signal transduction

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Agglutination activity of Limulus polyphemus coagulogen following limited proteolysis

1. A 14 kDa protein with cell agglutination properties has been purified from endotoxin-activated L. polyphemus amebocyte lysate. Amino terminal sequence analysis indicates that this protein corresponds to a proteolytically cleaved product (coagulin) of coagulogen. 2. Similar cell agglutination activity can be generated, in vitro, by proteolytic cleavage of the coagulogen with either trypsin, endogenous protease or an alpha 2M/enzyme complex isolated from amebocytes. 3. Studies with [125I]-labeled coagulogen showed that only coagulin, not the intact coagulogen, binds to rabbit erythrocytes and formalin-fixed amebocytes. 4. The cell agglutination activity of coagulin towards erythrocytes was not inhibited by various sugars tested, and was not Ca(2+)-dependent. 5. These findings suggest that coagulogen and coagulin are reminiscent of their mammalian counterparts, fibrinogen and fibrin, in their clotting and relative adhesive properties.

The limulus clotting reaction

Our biochemical studies on the hemolymph coagulation-complement system using limulus indicate that the circulating hemocytes contain at least four serine protease zymogens and one clottable protein, coagulogen, which constitute a cascade triggered by bacterial endotoxins and (1,3)-beta-D-glucan. We also found several antimicrobial substances, tachyplesin peptides and anti-lipopolysaccharide factor, in the hemocytes. These clotting factors and antimicrobial substances are released into the hemolymph in response to lipopolysaccharide, where they cooperate in immobilization and killing of invading microorganisms as a host defense.

Molecular mechanism of hemolymph clotting system in Limulus

Limulus (horseshoe crab) hemolymph is known to be very sensitive to bacterial endotoxin (LPS), which causes a rapid coagulation response. Hemolymph contains a single type of hemocyte that undergoes aggregation, adhesion, and degranulation in response to LPS. The granule contents are released into the hemolymph, where they form an insoluble gel. We have characterized four components involved in this coagulation response that comprise a cascade of three serine protease zymogens (factor C, factor B, and proclotting enzyme) and one clottable protein (coagulogen). Of these components, factor C sensitive to LPS is a protein composed of five complement-related domains ("Sushi" or SCR), an EGF-like domain, and a C-type lectinlike domain as well as a putative amino-terminal LPS-binding domain. This domain structure is very similar to that of selectin family of cell adhesion molecules, suggesting that it might also function as a cell adhesion molecule after the release into the hemolymph. Factor B and the proclotting enzyme share a common Cys-rich motif ("cliplike" domain) in the amino-terminal portions. This domain is also found in a putative serine protease zymogen ("easter") in Drosophila, which is essential for normal embryonic development. All four of the components of the cascade and an antibacterial protein (anti-LPS factor) are localized to a specific type of the hemocyte granule. Another antibacterial peptide (tachyplesins I and II) is localized in a distinct granule population. The contents of both granule populations are released into the hemolymph in response to LPS, where they cooperate in immobilization and killing of Gram-negative bacteria.