Crystal structure of native Anopheles gambiae Serpin-2, a negative regulator of melanization in mosquitoes

Division of Biology, Kansas State University, Manhattan, Kansas 66506, USA.
Proteins Structure Function and Bioinformatics (Impact Factor: 2.63). 06/2011; 79(6):1999-2003. DOI: 10.1002/prot.23002
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Available from: Kristin Michel
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    • "The predicted structure is similar to the native fold of other serpins consisting of three β-sheet domains, 8 α-helices and a RCL (Figure 1). A comparison with other structures suggested that SRPN6 overall shares highest similarity with human antithrombin III (PDB: 1E03, chain I) with a calculated RMSD of 1.47 Å. Similarity to AgSRPN2, the only mosquito serpin for which the protein structure was solved by X-ray crystallography, is lower, with a calculated RMSD of 1.69 Å (PDB: 3PZF, [40]). Similarity to M. sexta Serpin 1K (PDB: 1SEK, [51]) is comparable with an RMSD of 1.67 Å, and likewise to T. molitor Serpin48 (PDB: 3OZQ, [52]) with an RMSD of 1.63 Å. "
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    ABSTRACT: Serine proteinase inhibitors of the serpin family are well known as negative regulators of hemostasis, thrombolysis and innate immune responses. Additionally, non-inhibitory serpins serve functions as chaperones, hormone transporters, or anti-angiogenic factors. In the African malaria mosquito, Anopheles gambiae s.s., at least three serpins (SRPNs) are implicated in the innate immune response against malaria parasites. Based on reverse genetic and cell biological analyses, AgSRPN6 limits parasite numbers and transmission and has been postulated to control melanization and complement function in mosquitoes. This study aimed to characterize AgSRPN6 biophysically and determine its biochemical mode of action. The structure model of AgSRPN6, as predicted by I-Tasser showed the protein in the native serpin fold, with three central β-sheets, nine surrounding α-helices, and a protruding reactive center loop. This structure is in agreement with biophysical and functional data obtained from recombinant (r) AgSRPN6, produced in Escherichia coli. The physical properties of purified rAgSRPN6 were investigated by means of analytical ultracentrifugation, circular dichroism, and differential scanning calorimetry tools. The recombinant protein exists predominantly as a monomer in solution, is composed of a mixture of α-helices and β-sheets, and has a mid-point unfolding temperature of 56°C. Recombinant AgSRPN6 strongly inhibited porcine pancreatic kallikrein and to a lesser extent bovine pancreatic trypsin in vitro. Furthermore, rAgSRPN6 formed inhibitory, SDS-stable, higher molecular weight complexes with prophenoloxidase-activating proteinase (PAP)1, PAP3, and Hemolymph protein (HP)6, which are required for melanization in the lepidopteran model organism, Manduca sexta. Taken together, our results strongly suggest that AgSRPN6 takes on a native serpin fold and is an inhibitor of trypsin-like serine proteinases.
    Full-text · Article · Nov 2012 · PLoS ONE
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    • "However, none of the three sequenced mosquito genomes contains a nec ortholog (Bartholomay et al., 2010). While expression of several AaSRPNs is regulated by the Toll pathway (Bian et al., 2005; Shin et al., 2006; Zou et al., 2011), there is currently little evidence of which SRPN, if any, has taken on the function of Nec in mosquitoes. Interestingly, Toll pathway function during development in D. melanogaster is controlled by Spn27A (Ligoxygakis et al., 2003). "
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    ABSTRACT: In vector-borne diseases, the complex interplay between pathogen and its vector's immune system determines the outcome of infection and therefore disease transmission. Serpins have been shown in many animals to be key regulators of innate immune reactions. Their control over regulatory proteolytic cascades ultimately decides whether the recognition of a pathogen will lead to an appropriate immune response. In mosquitoes, serpins (SRPNs) regulate the activation of prophenoloxidase and thus melanization, contribute to malaria parasite lysis, and likely Toll pathway activation. Additionally, in culicine mosquitoes, SRPNs are able to regulate hemostasis in the vertebrate host, suggesting a crucial role during bloodfeeding. This review summarizes the annotation, transcriptional regulation, and current knowledge of SRPN function in the three mosquito species for which the complete genome sequence is available. Additionally, we give a brief overview of how SRPNs may be used to prevent transmission of vector-borne diseases.
    Full-text · Article · Sep 2012 · Journal of insect physiology
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    ABSTRACT: Serpins are ubiquitously distributed serine protease inhibitors that covalently bind to target proteases to exert their activities. Serpins regulate a wide range of activities, particularly those in which protease-mediated cascades are active. The Drosophila melanogaster serpin Spn43Ac negatively controls the Toll pathway that is activated in response to fungal infection. The entomopathogenic fungus Beauveria bassiana offers an environmentally friendly alternative to chemical pesticides for insect control. However, the use of mycoinsecticides remains limited in part due to issues of efficacy (low virulence) and the recalcitrance of the targets (due to strong immune responses). Since Spn43Ac acts to inhibit Toll-mediated activation of defense responses, we explored the feasibility of a new strategy to engineer entomopathogenic fungi with increased virulence by expression of Spn43Ac in the fungus. Compared to the 50% lethal dose (LD50) for the wild-type parent, the LD50 of B. bassiana expressing Spn43Ac (strain Bb::S43Ac-1) was reduced ∼3-fold, and the median lethal time against the greater wax moth (Galleria mellonella) was decreased by ∼24%, with the more rapid proliferation of hyphal bodies being seen in the host hemolymph. In vitro and in vivo assays showed inhibition of phenoloxidase (PO) activation in the presence of Spn43Ac, with Spn43Ac-mediated suppression of activation by chymotrypsin, trypsin, laminarin, and lipopolysaccharide occurring in the following order: chymotrypsin and trypsin > laminarin > lipopolysaccharide. Expression of Spn43Ac had no effect on the activity of the endogenous B. bassiana-derived cuticle-degrading protease (CDEP-1). These results expand our understanding of Spn43Ac function and confirm that suppression of insect immune system defenses represents a feasible approach to engineering entomopathogenic fungi for greater efficacy.
    Full-text · Article · May 2014 · Applied and Environmental Microbiology
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