Serpin

Serpin (serine protease inhibitor)
File:Serpin (stressed).png A serpin (white) with its 'reactive centre loop' (blue) bound to a protease (grey). Once the protease attempts catalysis it will be irreversibly inhibited. (PDB: 2K9O​)
Identifiers
Symbol	Serpin
Pfam	PF00079
InterPro	IPR000215
PROSITE	PDOC00256
SCOP	1hle
SUPERFAMILY	1hle
CDD	cd00172
Available protein structures: Pfam structures PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

Serpins are a superfamily of proteins with similar structures that were first identified for their protease inhibition activity and are found in all kingdoms of life. The acronym serpin was originally coined because the first serpins to be identified act on chymotrypsin-like serine proteases (serine protease inhibitors). They are notable for their unusual mechanism of action, in which they irreversibly inhibit their target protease by undergoing a large conformational change to disrupt its active site. This contrasts with the more common competitive mechanism for protease inhibitors that bind to and block access to the protease active site.

Protease inhibition by serpins controls an array of biological processes, such as coagulation and inflammation, and consequently these proteins are the target of medical research. Their unique conformation change also makes them of interest to the structural biology and protein folding research communities. The conformational-change-based mechanism confers certain advantages, but it also has drawbacks: serpins are vulnerable to mutations that result in protein misfolding and the formation of inactive long-chain polymers (serpinopathies). Serpin polymerisation not only reduces the amount of active inhibitor, but also leads to accumulation of the polymers, causing cell death and organ failure.

While most serpins control proteolytic cascades, some proteins with a serpin structure are not enzyme inhibitors, but instead perform diverse functions such as storage (e.g. egg white ovalbumin), hormone carriage proteins (thyroxine-binding globulin, cortisol-binding globulin) and molecular chaperones (HSP47). The term serpin is used to describe these members as well, despite their noninhibitory function, since they are evolutionarily related.

History

Protease inhibitory activity in blood plasma was first reported in the late 1800s, however it wasn't until the 1950s that the serpins antithrombin and antitrypsin were isolated. Initial research focused on their role in human disease: α1-antitrypsin deficiency is one of the most common hereditary diseases, causing emphysema, and antithrombin deficiency results in thrombosis.

The name "Serpin" was first coined in the 1980s when it became clear that they formed a large superfamily of related proteins that also included non-inhibitory members such as ovalbumin from egg white. Around the same time, the first structures were solved for serpins (first in the relaxed, and later in the stressed conformation). The structures indicated that the inhibitory mechanism involved an unusual conformational change and prompted the structural focus of serpin studies.

Over 1000 serpins have now been identified, including 36 human proteins, as well as molecules in all kingdoms of life animals, plants, fungi, bacteria, archaea and some viruses. In the 2000s, a systematic nomenclature was introduced in order to categorise members of the serpin superfamily based on their evolutionary relationships. Serpins are therefore the largest and most diverse family of protease inhibitors.

Activity

File:Serpin and protease.png

A protease (grey) bound to a serpin reactive centre loop (RCL, blue). When the protease's catalytic triad (red) cleaves the RCL, it becomes trapped in an inactive conformation. (PDB: 1K9O​)

Most serpins are protease inhibitors, targeting Extracellular, chymotrypsin-like serine proteases. These proteases possess a nucleophilic serine residue in a catalytic triad in their active site. Examples include thrombin, trypsin, and human neutrophil elastase. Serpins act as irreversible, suicide inhibitors by trapping an intermediate of the protease's catalytic mechanism.

Some serpins inhibit other protease classes, typically cysteine proteases, and are termed "cross-class inhibitors". These enzymes differ from serine proteases in that they use a nucleophilic cysteine residue, rather than a serine, in their active site. Nonetheless, the enzymatic chemistry is similar, and the mechanism of inhibition by serpins is the same for both classes of protease. Examples of cross-class inhibitory serpins include squamous cell carcinoma antigen 1 (SCCA-1) and the avian serpin myeloid and erythroid nuclear termination stage-specific protein (MENT), which both inhibit papain-like cysteine proteases.

Biological function and localization

Protease inhibition

Approximately two-thirds of human serpins perform extracellular roles, preventing modulating activity of proteases in the bloodstream. For example, extracellular serpins regulate the proteolytic cascades central to blood clotting (antithrombin), the inflammatory and immune responses (antitrypsin, antichymotrypsin, and C1 inhibitor) and tissue remodeling (PAI-1). By inhibiting signalling cascade proteases, they can also affect development. The table of human serpins (below) provides examples of the range of functions performed by human serpin, as well as some of the diseases that result from serpin deficiency.

The protease targets of intracellular inhibitory serpins have been difficult to identify, since many of these molecules appear to perform overlapping roles. Further, many human serpins lack precise functional equivalents in model organisms such as the mouse. Nevertheless, an important function of intracellular serpins may be to protect against the inappropriate activity of proteases inside the cell. For example, one of the best-characterised human intracellular serpins is SERPINB9, which inhibits the cytotoxic granule protease granzyme B. In doing so, SERPINB9 may protect against inadvertent release of granzyme B and premature or unwanted activation of cell death pathways.

Some viruses use serpins to disrupt protease functions in their host. The viral serpin crmA increases infectivity by suppressing its host's inflammatory response through inhibition of IL-1 and IL-18 processing by the cysteine protease caspase-1. In eukaryotes, a plant serpin inhibits both metacaspases and a papain-like cysteine protease.

Non-inhibitory roles

Non-inhibitory extracellular serpins also perform important roles. Thyroxine-binding globulin and transcortin transport the hormones thyroxine and cortisol, respectively. Similarly, heat shock serpin HSP47 is a chaperone, essential for proper folding of collagen.

Some serpins are both protease inhibitors and perform additional roles. For example, the avian nuclear cysteine protease inhibitor, MENT, also acts as a chromatin remodelling molecule in avian red blood cells.

Structure

File:Serpin equilibrium.png

The native state of serpins is an equilibrium between a fully-stressed state (left) and a partially relaxed state (right). The five-stranded A-sheet (light blue) contains two functionally important regions for the serpin's mechanism, the breach and the shutter. The reactive centre loop (RCL, blue) exists in a dynamic equilibrium between the fully exposed conformation (left) and a conformation where it is partially inserted into the breach of the A-sheet (right). (PDB: 1QLP, 1YXA​)

All serpins share a common structure (or fold), despite their varied functions. All typically have three β-sheets (named A, B and C) and eight or nine α-helices (named hA–hI). The most significant regions to serpin function are the A-sheet and the reactive centre loop (RCL). The A-sheet includes two β-strands that are in a parallel orientation with a region between them called the shutter, and upper region called the 'breach'. The RCL forms the initial interaction with the target protease in inhibitory molecules. Structures have been solved showing the RCL either fully exposed or partially inserted into the A-sheet, and serpins are thought to be in dynamic equilibrium between these two states.

The serpin structures that have been determined cover a number of different conformations, which has been necessary for the understanding of their multiple-step mechanism of action. Structural biology has therefore played a central role in the understanding of serpin function and biology.

Conformational change and inhibitory mechanism

Inhibitory serpins do not inhibit their target proteases by the typical competitive (lock-and-key) mechanism used by most small protease inhibitors (e.g. Kunitz-type inhibitors). Instead, serpins use an unusual conformational change, which disrupts the structure of the protease and prevents it from completing catalysis. The conformational change involves the RCL moving to the opposite end of the protein and inserting into β-sheet A, forming an extra antiparallel β-strand. This converts the serpin from a stressed state, to a lower-energy relaxed state (S to R transition).

Serine and cysteine proteases catalyse peptide bond cleavage by a two-step process. Initially, the catalytic residue of the active site triad performs a nucleophilic attack on the peptide bond of the substrate. This releases the new N-terminus and forms a covalent ester-bond between the enzyme and the substrate. This covalent complex between enzyme and substrate is called an acyl-enzyme intermediate. For standard substrates, the ester bond is hydrolysed and the new C-terminus is released to complete catalysis. However, when a serpin is cleaved by a protease, it rapidly undergoes the S to R transition before the acyl-enzyme intermediate is hydrolysed. Since the RCL is still covalently attached to the protease via the ester bond, the S to R transition pulls protease from the top to the bottom of the serpin and distorts the catalytic triad. The distorted protease can only hydrolyse the acyl enzyme intermediate extremely slowly and so the protease remains covalently attached. Serpins are classed as irreversible inhibitors and as suicide inhibitors since each serpin protein permanently inactivates a single protease, and can only function once.

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The inhibitory mechanism of serpins involves a large conformational change (S to R transition). The serpin (white) first binds a protease (grey) with the exposed reactive centre loop (blue). When this loop is cleaved by the protease, it rapidly inserts into the A-sheet (light blue), deforming and inhibiting the protease. (PDB: 1K9O, 1EZX​)

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serine and cysteine proteases operate by a two-step catalytic mechanism. First, the substrate (blue) is attacked by the cysteine or serine of the catalytic triad (red) to form an acyl-enzyme intermediate. For typical substrates, the intermediate is resolved by hydrolysis by water. However, when the reactive centre loop (RCL) of a serpin is attacked, the conformational change (blue arrow) pulls the catalytic triad out of position, preventing it from completing catalysis. (Based on PDB: 1K9O, 1EZX​)

Allosteric activation

File:Serpin activation by heparin (unannotated).png

Some serpins are activated by cofactors. The the serpin antithrombin has an RCL (blue) where the P1 arginine (blue sticks) points inwards, preventing protease binding. Binding of heparin (green sticks) causes the P1 arginine residue to flip to an exposed position. The target protease (grey) then binds to both the exposed P1 arginine as well as the heparin. The serpin then activates and heparin is released. (PDB: 2ANT, 1TB6, 1EZX​)

The conformational mobility of serpins provides a key advantage over static lock-and-key protease inhibitors. In particular, the function of inhibitory serpins can be controlled by allosteric interactions with specific cofactors. The X-ray crystal structures of antithrombin, heparin cofactor II, MENT and murine antichymotrypsin reveal that these serpins adopt a conformation wherein the first two amino acids of the RCL are inserted into the top of the A β-sheet. The partially inserted conformation is important because co-factors are able to conformationally switch certain partially inserted serpins into a fully expelled form. This conformational rearrangement makes the serpin a more effective inhibitor.

The archetypal example of this situation is antithrombin, which circulates in plasma in a partially inserted relatively inactive state. The primary specificity determining residue (the P1 arginine) points toward the body of the serpin and is unavailable to the protease. Upon binding a high-affinity pentasaccharide sequence within long-chain heparin, antithrombin undergoes a conformational change, RCL expulsion, and exposure of the P1 arginine. The heparin pentasaccharide-bound form of antithrombin is, thus, a more effective inhibitor of thrombin and factor Xa. Furthermore, both of these coagulation proteases also contain binding sites (called exosites) for heparin. Heparin, therefore, also acts as a template for binding of both protease and serpin, further dramatically accelerating the interaction between the two parties. After the initial interaction, the final serpin complex is formed and the heparin moiety is released. This interaction is physiologically important. For example, after injury to the blood vessel wall, heparin is exposed, and antithrombin is activated to control the clotting response. Understanding of the molecular basis of this interaction enabled the development of Fondaparinux, a synthetic form of Heparin pentasaccharide used as an anti-clotting drug.

Latent conformation

File:Serpin latent state (unannotated).png

Some serpins can spontaneously convert to an inactive latent state. The serpin PAI-1 remains in the active conformation when bound to vitronectin (green). However, in the absence of vitronectin, PAI-1 can change to the inactive latent state. The uncleaved RCL (blue; disordered regions as dashed lines) inserts into the A-sheet, pulling a β-strand off the C-sheet (yellow). (PDB: 1OC0, 1DVM, 1LJ5​)

Certain serpins spontaneously undergo the S to R transition without having been cleaved by a protease, to form a conformation termed the latent state. Latent serpins are unable to interact with proteases and so are no longer protease inhibitors. The conformational change to latency is not exactly the same as the S to R transition of a cleaved serpin. Since the RCL is still intact, the first strand of the C-sheet has to peel off to allow full RCL insertion.

Regulation of the latency transition can act as a control mechanism in some serpins, such as PAI-1. Although PAI-1 is produced in the inhibitory S conformation, it "auto-inactivates" by changing to the latent state unless it is bound to the cofactor vitronectin. Similarly, antithrombin can also spontaneously convert to the latent state, as an additional modulation mechanism to its allosteric activation by heparin. Finally, the N-terminus of tengpin, a serpin from Thermoanaerobacter tengcongensis, is required to lock the molecule in the native inhibitory state. Disruption of interactions made by the N-terminal region results in spontaneous conformational change of this serpin to the latent conformation.

Conformational change in non-inhibitory functions

Certain non-inhibitory serpins also use the serpin conformational change as part of their function. For example, the native (S) form of thyroxine-binding globulin has high affinity for thyroxine, whereas the cleaved (R) form has low affinity. In similar manner, transcortin has higher affinity for cortisol when in its native (S) state, than its cleaved (R) state. Thus, in these serpins, RCL cleavage and the S to R transition has been commandeered to allow for ligand release, rather than protease inhibition.

In some serpins, the S-to -R transition can activate cell signalling events. In these cases, a serpin that has formed a complex with its target protease, is then recognised by a receptor. The binding event then leads to downstream signalling by the receptor. The S to R transition is therefore used to alert cells to the presence of protease activity. This differs from the usual mechanism whereby serpins affect signalling simply by inhibiting proteases involved in a signalling cascade.

Degradation

When a serpin inhibits a target protease, it forms a permanent complex, which needs to be disposed of. For extracellular serpins, the final serpin-enzyme complexes are rapidly cleared from circulation. One mechanism by which this occurs in mammals is via the low-density lipoprotein receptor-related protein (LRP), which binds to inhibitory complexes made by antithrombin, PA1-1, and neuroserpin, causing cellular uptake. Similarly, the Drosophila serpin, necrotic, is degraded in the lysosome after being trafficked into the cell by the Lipophorin Receptor-1 (homologous to the mammalian LDL receptor family).

Disease and serpinopathies

Serpins are involved in a wide array of physiological functions, and so mutations in genes encoding them can cause a range of diseases. Mutations that change the activity, specificity or aggregation properties of serpins all affect how they function. The majority of serpin-releated diseases are the result of serpin polymerisation into aggregates, however several other types of disease-linked mutations also occur. The disorder α-Antitrypsin deficiency is one of the most common hereditary diseases.

Inactivity or absence

File:Serpin delta conformation.png

The inactive δ-conformation of the disease-linked antichymotrypsin mutant (L55P). Four residues of the RCL (blue; disordered region as dashed line) are inserted into the top of the A-sheet. Part of the F α-helix (yellow) has unwound and fills the bottom half of the A-sheet. (PDB: 1QMN​)

Since the stressed serpin fold is high-energy, mutations can cause them to incorrectly change into their lower-energy conformations (e.g. relaxed or latent) before they have correctly performed their inhibitory role.

Mutations that affect the rate or the extent of RCL insertion into the A-sheet can cause the serpin to undergo its S to R conformational change before having engaged a protease. Since a serpin can only make this conformation change once, the resulting misfired serpin is inactive and unable to properly control its target protease. Similarly, mutations that promote inappropriate transition to the monomeric latent state cause disease by reducing the amount of active inhibitory serpin. For example, the disease-linked antithrombin variants wibble and wobble, both promote formation of the latent state.

The structure of the disease-linked mutant of antichymotrypsin (L55P) revealed an additional, inactive "δ-conformation". In the δ-conformation, four residues of the RCL are inserted into the top of β-sheet A. The bottom half of the sheet is filled as a result of one of the α-helices (the F-helix) partially switching to a β-strand conformation, completing the β-sheet hydrogen bonding. It is unclear whether other serpins can adopt this conformer, and whether this conformation has a functional role. However, it is speculated that the δ-conformation may be adopted by Thyroxine-binding globulin during thyroxine release.

The non-inhibitory proteins related to serpins can also cause diseases when mutated. For example, mutations in SERPINF1 cause osteogenesis imperfecta type VI in humans.

In the absence of a required serpin, the protease that it normally would regulate is over-active, leading to pathologies. Consequently, simple deficiency of a serpin (e.g. a null mutation) can result in disease. Gene knockouts, particularly in mice, are used to determine the normal functions of serpins by the effect of their absence.

Specificity change

In some rare cases, a single amino acid change in a serpin's RCL alters its specificity to target the wrong protease. For example, the Antitrypsin-Pittsburgh mutation (M358R) causes the Antitrypsin serpin to inhibit thrombin, causing a bleeding disorder.

Polymerisation and aggregation

Serpin polymerisation by domain swapping

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A domain-swapped serpin dimer. (PDB: 2ZNH​)

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A domain-swapped serpin trimer. Each monomer's RCL is inserted into its own structure (shown in red of the green monomer). (PDB: 3T1P​)

The majority of serpin diseases are due to protein aggregation and are termed "serpinopathies". Serpins are vulnerable to disease-causing mutations that promote formation of misfolded polymers due to their inherently unstable structures. Well-characterised serpinopathies include α1-antitrypsin deficiency (alpha-1), which may cause familial emphysema and sometimes liver cirrhosis, certain familial forms of thrombosis related to antithrombin deficiency, types 1 and 2 hereditary angioedema (HAE) related to deficiency of C1-inhibitor, and familial encephalopathy with neuroserpin inclusion bodies (FENIB; a rare type of dementia caused by neuroserpin polymerisation).

Each monomer of the serpin aggregate exists in the inactive, relaxed conformation (with the RCL inserted into the A-sheet). The polymers are therefore hyperstable to temperature and unable to inhibit proteases. Serpinopathies therefore cause pathologies similarly to other proteopathies (e.g. prion diseases) via two main mechanisms. First, the lack of active serpin results in uncontrolled protease activity and tissue destruction. Second, the hyperstable polymers themselves clog up the endoplasmic reticulum of cells that synthesize serpins, eventually resulting in cell death and tissue damage. In the case of antitrypsin deficiency, antitrypsin polymers cause the death of liver cells, sometimes resulting in liver damage and cirrhosis. Within the cell, serpin polymers are slowly removed via endoplasmic reticulum associated degradation. However, the details of how serpin polymers cause cell death remains to be fully understood.

Physiological serpin polymers are thought to form via domain swapping events, where a segment of one serpin protein inserts into another. Domain-swaps occur when mutations or environmental factors interfere with the final stages of serpin folding to the native state, causing high-energy intermediates to misfold. Both dimer and trimer domain-swap structures have been solved. In the dimer (of antithrombin), the RCL and part of the A-sheet incorporates into the A-sheet of another serpin molecule. The domain-swapped trimer (of antitrypsin) forms via the exchange of an entirely different region of the structure, the B-sheet (with each molecule's RCL inserted into its own A-sheet). It has also been proposed that serpins may form domain-swaps by inserting the RCL of one protein into the A-sheet of another (A-sheet polymerisation). These domain-swapped dimer and trimer structures are though to be the building blocks of the disease-causing polymer aggregates, however the exact mechanism is still unclear.

Therapeutic strategies

Several therapeutic approaches are in use or under investigation to treat the most common serpinopathy: antitrypsin deficiency. Antitrypsin augmentation therapy is approved for severe antitrypsin deficiency-related pulmonary emphysema. In this therapy, antitrypsin is purified from the plasma of blood donors and administered intravenously (first marketed as Prolastin). To treat severe antitrypsin deficiency-related disease, lung and liver transplantation has proven effective. In animal models, gene targeting in induced pluripotent stem cells has been successfully used to correct an antitrypsin polymerisation defect and to restore the ability of the mammalian liver to secrete active antitrypsin. Small molecules have also been developed that block antitrypsin polymerisation in vitro.

Evolution

Serpins are the most widely distributed and largest family of protease inhibitors. They were initially believed to be restricted to eukaryote organisms, but have since been found in a number of bacteria, archaea and some viruses. It remains unclear whether these prokaryote genes are the descendants of an ancestral prokaryotic serpin or the product of horizontal gene transfer fom eukaryotes. Most intracellular serpins belong to a single phylogenetic clade. Exceptions include the intracellular heat shock serpin HSP47, which is a chaperone essential for proper folding of collagen, and cycles between the cis-Golgi and the endoplasmic reticulum.

Protease-inhibition is thought to be the ancestral function, with non-inhibitory members the results of evolutionary neofunctionalisation of the structure. The S to R conformational change has also been adapted by some binding serpins to regulate affinity for their targets.

Distribution

Animal

Human

The human serpin naming system is based upon a phylogenetic analysis of approximately 500 serpins from 2001. The human genome encodes 16 serpin clades, termed serpinA through serpinP, including 29 inhibitory and 7 non-inhibitory serpin proteins. The proteins are named serpinXY where X is the clade of the protein and Y the number of the protein within that clade.

Specialised mammalian serpins

Many mammalian serpins have been identified that share no obvious orthology with a human serpin counterpart. Examples include numerous rodent serpins (particularly some of the murine intracellular serpins) as well as the uterine serpins. The term uterine serpin refers to members of the serpin A clade that are encoded by the SERPINA14 gene. Uterine serpins are produced by the uterine endometrium of a restricted group of mammals in the Laurasiatheria clade under the influence of progesterone or estrogen. They are probably not functional proteinase inhibitors and may function during pregnancy to inhibit maternal immune responses against the conceptus or to participate in transplacental transport.

Insect

The Drosophila melanogaster genome contains 29 serpin encoding genes. Amino acid sequence analysis has placed 14 of these serpins in serpin clade Q and three in serpin clade K with the remaining twelve classified as orphan serpins not belonging to any clade. The clade classification system is difficult to use for Drosophila serpins and instead a nomenclature system has been adopted that is based on the position of serpin genes on the Drosophila chromosomes. Thirteen of the Drosophila serpins occur as isolated genes in the genome (including Serpin-27A, see below), with the remaining 16 organised into three gene clusters that occur at chromosome positions 28D (2 serpins), 42D (5 serpins), 43A (4 serpins), 77B (3 serpins) and 88E (2 serpins).

Studies on Drosophila serpins reveal that Serpin-27A inhibits the Easter protease (the final protease in the Nudel, Gastrulation Defective, Snake and Easter proteolytic cascade) and thus controls dorsoventral patterning. Easter functions to cleave Spätzle (a chemokine-type ligand), which results in toll-mediated signaling. In addition to its central role in embryonic patterning, toll signaling is also important for the innate immune response in insects. Accordingly, serpin-27A additionally functions to control the insect immune response. In Tenebrio molitor (a large beetle), a protein (SPN93) comprising two discrete tandem serpin domains functions to regulate the toll proteolytic cascade.

Worm

The genome of the nematode worm C. elegans contains nine serpins, all of which lack signal sequences and so are likely intracellular. However, only five of these serpins appear to function as protease inhibitors. One of these serpins, SRP-6, performs a protective function and guards against stress-induced calpain-associated lysosomal disruption. Further, SRP-6 inhibits lysosomal cysteine proteases released after lysosomal rupture. Accordingly, worms lacking SRP-6 are sensitive to stress. Most notably, SRP-6 knockout worms die when placed in water (the hypo-osmotic stress lethal phenotype or Osl). It has therefore been suggested that lysosomes play a general and controllable role in determining cell fate.

Plant

Plant serpins were amongst the first members of the superfamily that were identified. The serpin barley protein Z is highly abundant in barley grain, and one of the major protein components in beer. The genome of the model plant, Arabidopsis thaliana contain 18 serpin-like genes, although only eight of these are full-length serpin sequences.

Plant serpins are potent inhibitors of mammalian chymotrypsin-like serine proteases in vitro, the best-studied example being barley serpin Zx (BSZx), which is able to inhibit trypsin, chymotrypsin as well as several blood coagulation factors. However, close relatives of chymotrypsin-like serine proteases are absent in plants. The RCL of several serpins from wheat grain and rye contain poly-Q repeat sequences similar to those present in the prolamin storage proteins of the endosperm. It has therefore been suggested that plant serpins may function to inhibit proteases from insects or microbes that would otherwise digest grain storage proteins. In support of this hypothesis, specific plant serpins have been identified in the phloem sap of pumpkin (CmPS-1) and cucumber plants. However, while an inverse correlation between up-regulation of CmPS-1 expression and aphid survival was observed, in vitro feeding experiments revealed that recombinant CmPS-1 did not appear to affect insect survival.

Alternative roles and protease targets for plant serpins have been proposed. The Arabidopsis serpin, AtSerpin1 (At1g47710; 3LE2), mediates set-point control over programmed cell death by targeting the 'Responsive to Desiccation-21' (RD21) papain-like cysteine protease. AtSerpin1 also inhibits metacaspase-like proteases in vitro. Two other Arabidopsis serpins, AtSRP2 (At2g14540) and AtSRP3 (At1g64030) appear to be involved in responses to DNA damage.

Fungal

A single fungal serpin has been characterized to date: celpin from Piromyces sp. strain E2. Piromyces is an anaerobic fungus found in the gut of ruminants and is important for digesting plant material. Celpin is predicted to be an inhibitory molecule and contains two N-terminal dockerin domains in addition to the serpin domain. Dockerins are commonly found in proteins that localise to the fungal cellulosome, a large Extracellular multiprotein complex that breaks down cellulose. It is therefore suggested that celpin protects the cellulosome against plant proteases. Certain bacterial serpins also localize to the cellulosome.

Prokaryote

Predicted serpin genes are sporadically distributed in prokaryotes. In vitro studies on some of these molecules have revealed that they are able to inhibit proteases, and it is suggested that they function as inhibitors in vivo. Several prokaryote serpins are found in extremophiles. Accordingly, and in contrast to mammalian serpins, these molecules possess elevated resistance to heat denaturation. The precise role of most bacterial serpins remains obscure, however, Clostridium thermocellum serpin localises to the cellulosome. It is suggested that the role of cellulosome-associated serpins may be to prevent unwanted protease activity against the cellulosome.

Viral

Serpins are also expressed by viruses as a way to evade the host's immune defense. In particular, serpins expressed by pox viruses, including cow pox (vaccinia) and rabbit pox (myxoma), are of interest because of their potential use as novel therapeutics for immune and inflammatory disorders as well as transplant therapy. A study on Serp1 reveals this molecule suppresses the toll-mediated innate immune response and allows indefinite cardiac allograft survival in rats. Studies on Crma and Serp2, reveal both are cross-class inhibitor and targets both serine (Granzyme B; albeit weakly) and cysteine proteases (Caspase 1 and Caspase 8). In comparison to their mammalian counterparts, viral serpins contain significant deletions of elements of secondary structure. Specifically, structural studies on crmA reveals this molecule lacks the D-helix as well as significant portions of the A- and E-helices.

See also

• List of MeSH codes (D12.776) for serpins
• Proteopathy
• Protease inhibitor

External links

• PDB Molecule of the Month Serpin
• Serpins at the US National Library of Medicine Medical Subject Headings (MeSH)
• James Whisstock laboratory at Monash University
• Jim Huntington serpin laboratory at University of Cambridge
• Frank Church serpin group at University of North Carolina at Chapel Hill
• Paul Declerck serpin group at Katholieke Universiteit Leuven
• Tom Roberts laboratory at Macquarie University
• Robert Fluhr laboratory at Weizmann Institute of Science
• Merops protease inhibitor claudication (Family I4) at University of Cambridge