Molecular Basis for Genetic Resistance of Anopheles gambiae to Plasmodium: Structural Analysis of TEP1 Susceptible and Resistant Alleles

Binh V Le,Marni Williams,Richard H G Baxter,Shankar Logarajah,Kenneth D Vernick

doi:10.1371/journal.ppat.1002958

Abstract

Thioester-containing protein 1 (TEP1) is a central component in the innate immune response of Anopheles gambiae to Plasmodium infection. Two classes of TEP1 alleles, TEP1*S and TEP1*R, are found in both laboratory strains and wild isolates, related by a greater or lesser susceptibility, respectively to both P. berghei and P. falciparum infection. We report the crystal structure of the full-length TEP1*S1 allele which, while similar to the previously determined structure of full-length TEP1*R1, displays flexibility in the N-terminal fragment comprising domains MG1-MG6. Amino acid differences between TEP1*R1 and TEP1*S1 are localized to the TED-MG8 domain interface that protects the thioester bond from hydrolysis and structural changes are apparent at this interface. As a consequence cleaved TEP1*S1 (TEP1*S1cut) is significantly more susceptible to hydrolysis of its intramolecular thioester bond than TEP1*R1cut. TEP1*S1cut is stabilized in solution by the heterodimeric LRIM1/APL1C complex, which preserves the thioester bond within TEP1*S1cut. These results suggest a mechanism by which selective pressure on the TEP1 gene results in functional variation that may influence the vector competence of A. gambiae towards Plasmodium infection.

Highlights

Thioester-containing proteins (TEPs) are a major component of the innate immune response of insects to invasion by bacteria and protozoa [1,2]
The mosquitoes’ innate immune system is a significant factor that may influence the level of malaria infection; in particular the thioester-containing protein 1 (TEP1) targets malaria parasites for destruction during their initial invasion of the body cavity
Differences between the structures are localized around the active site and thioester bond, and correlate with a difference in stability of this bond within the two proteins and their interaction with a heterodimer of two other immune genes, LRIM1 and APL1C

Summary

Introduction

Thioester-containing proteins (TEPs) are a major component of the innate immune response of insects to invasion by bacteria and protozoa [1,2]. Anopheles gambiae thioester-containing protein 1 (TEP1) is a complement-like protein that plays a central role in the opsonization of gram-negative bacteria in the hemolymph [3]. Deciphering the molecular basis of the TEP1-mediated immune response is relevant to understanding the determinants of vector competence and a potential source of novel vector-based malaria control strategies. Based upon the known mechanism of complement factors [8], activation of the thioester in TEP1 is presumed to involve a large conformational change causing dissociation of the TED-MG8 interface in the direct proximity of a pathogen, whereupon the thioester may react with nucleophilic groups on, and covalently attach TEP1 to, the surface of the pathogen

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