Ixodes ricinus Salivary Serpin Iripin-8 Inhibits the Intrinsic Pathway of Coagulation and Complement

Tick saliva is a rich source of antihemostatic, anti-inflammatory, and immunomodulatory molecules that actively help the tick to finish its blood meal. Moreover, these molecules facilitate the transmission of tick-borne pathogens. Here we present the functional and structural characterization of Iripin-8, a salivary serpin from the tick Ixodes ricinus, a European vector of tick-borne encephalitis and Lyme disease. Iripin-8 displayed blood-meal-induced mRNA expression that peaked in nymphs and the salivary glands of adult females. Iripin-8 inhibited multiple proteases involved in blood coagulation and blocked the intrinsic and common pathways of the coagulation cascade in vitro. Moreover, Iripin-8 inhibited erythrocyte lysis by complement, and Iripin-8 knockdown by RNA interference in tick nymphs delayed the feeding time. Finally, we resolved the crystal structure of Iripin-8 at 1.89 Å resolution to reveal an unusually long and rigid reactive center loop that is conserved in several tick species. The P1 Arg residue is held in place distant from the serpin body by a conserved poly-Pro element on the P′ side. Several PEG molecules bind to Iripin-8, including one in a deep cavity, perhaps indicating the presence of a small-molecule binding site. This is the first crystal structure of a tick serpin in the native state, and Iripin-8 is a tick serpin with a conserved reactive center loop that possesses antihemostatic activity that may mediate interference with host innate immunity.

[Amyloid-like fibrils forming and fibroblasts destruction in Tenon's capsule in progressive myopia as a result of pigment epithelium derived factor resistance to restricted proteolysis]

We have shown previously the presence of full length (50 kD) and truncated proteolytic form (45 kD) of pigment epithelium derived factor (PEDF) in the eye Tenon's capsule in progressive myopia. The full length PEDF is prevalent in myopia that correlates with breach in collagen fibrils forming. Immunohistochemical analysis of Tenon's capsule with polyclonal antibodies to PEDF revealed PEDF in control group being exclusively inside fibroblasts, whereas in myopia, PEDF was distributed extracellularly as halo around blasted fibroblasts. By means of atomic force microscopy and immunodot analysis with anti amyloid fibrils antibodies the ability was studied of recombinant PEDF fragments to form fibrils. Only full length PEDF was shown to form amyloid like fibril structures, but not the truncated form. Accumulation offibrils results in fibroblasts destruction and might be the cause of changes in biochemical and morphological structure of Tenon's capsule observed in myopia.

Two latent and two hyperstable polymeric forms of human neuroserpin

Human neuroserpin (hNS) is a serine protease inhibitor that belongs to the serpin superfamily and is expressed in nervous tissues. The serpin fold is generally characterized by a long exposed loop, termed the reactive center loop, that acts as bait for the target protease. Intramolecular insertion of the reactive center loop into the main serpin β-sheet leads to the serpin latent form. As with other known serpins, hNS pathological mutants have been shown to accumulate as polymers composed of quasi-native protein molecules. Although hNS polymerization has been intensely studied, a general agreement about serpin polymer organization is still lacking. Here we report a biophysical characterization of native hNS that is shown to undergo two distinct conformational transitions, at 55°C and 85°C, both leading to distinct latent and polymeric species. The latent and polymer hNS forms obtained at 45°C and 85°C differ in their chemical and thermal stabilities; furthermore, the hNS polymers also differ in size and morphology. Finally, the 85°C polymer shows a higher content of intermolecular β-sheet interactions than the 45°C polymer. Together, these results suggest a more complex conformational scenario than was previously envisioned, and, in a general context, may help reconcile the current contrasting views on serpin polymerization.

Serpin crystal structure and serpin polymer structure

Serpins are a family of structurally homologous proteins having metastable native structures. As a result, a serpin variant destabilized by mutation(s) has a tendency to undergo conformational changes leading to inactive forms, e.g., the latent form and polymer. Serpin polymers are involved in a number of conformational diseases. Although several models for polymer structure have been proposed, the actual structure remains unknown. Here, we provide a comprehensive list of serpins, both free and in complexes, deposited in the Protein Data Bank. Our discussion focuses on structures that potentially can contribute to a better understanding of polymer structure.

The human serpin proteinase inhibitor-9 self-associates at physiological temperatures

The metastable serpin architecture is perturbed by extremes of temperature, pH, or changes in primary sequence resulting in the formation of inactive, polymeric conformations. Polymerization of a number of human serpins in vivo leads to diseases such as emphysema, thrombosis, and dementia, and in these cases mutations are present within the gene encoding the aggregating protein. Here we show that aggregation of the human serpin, proteinase inhibitor-9 (PI-9), occurs under physiological conditions, and forms aggregates that are morphologically distinct from previously characterized serpin polymers. Incubation of monomeric PI-9 at 37 degrees C leads to the rapid formation of aggregated PI-9. Using a variety of spectroscopic methods we analyzed the nature of the structures formed after incubation at 37 degrees C. Electron microscopy showed that PI-9 forms ordered circular and elongated-type aggregates, which also bind the fluorescent dye Thioflavin T. Our data show that in vitro wild-type PI-9 forms aggregates at physiological temperatures. The biological implications of PI-9 aggregates at physiological temperatures are discussed.

Putative dimeric organization of nuclear receptor hormone-binding domains, deduced from hydrophobic cluster analysis

2D hydrophobic cluster analysis (HCA) protein sequence processing, efficient at low levels of sequence identity, leads to a coherent scheme for the structural organization of the hormone-binding domains (HBDs) of nuclear receptors. The typical serine protease inhibitor (serpin) fold, previously proposed, is confirmed as a likely framework for the hormone-binding domain and leads to a logical dimerization. Furthermore, homo- or hetero-dimerization creates sites where hormone could likely be bound, itself being an active component of the dimerization. This model fulfils many of the biochemical and biological data.

Mobile reactive centre of serpins and the control of thrombosis

Two protease inhibitors in human plasma play a key part in the control of thrombosis: antithrombin inhibits coagulation and the plasminogen activator inhibitor PAI-1 inhibits fibrinolysis, the dissolving of clots. Both inhibitors are members of the serpin family and both exist in the plasma in latent or inactive forms. We show here that the reactive centre of the serpins can adopt varying conformations and that mobility of the reactive centre is necessary for the function of antithrombin and its binding and activation by heparin; the identification of a new locked conformation explains the latent inactive state of PAI-1. This ability to vary conformation not only allows the modulation of inhibitory activity but also protects the circulating inhibitor against proteolytic attack. Together these findings explain the retention by the serpins of a large and unconstrained reactive centre as compared to the small fixed peptide loop of other families of serine protease inhibitors.