Structural and functional divergence of duplicated mosquito Toll and Spz compared to Drosophila

Abstract

Abstract Mosquitoes are vectors of many infectious diseases that constitute a risk to human health around the globe. Arthropod-borne viruses (arboviruses) such as Dengue and Zika viruses, are mainly transmitted by Aedes mosquito species, while plasmodium is spread by members of the Anopheles genus. The vectorial capacity of mosquito is based on a complex balance between the mosquito’s immune control and evasion mechanisms by the pathogen. The Toll signalling pathway is a key component of insect innate immunity that underwent duplication in mosquitoes compared to Drosophila melanogaster (Dm). Here we produced duplicated Spz isoforms and ectodomains of Toll1A and Toll5A in order to study binding specificity and signalling in Aedes aegypti (Aa). Surprisingly, Toll1A, the closest structural homolog to DmToll, did not bind to DmSpz, AaSpz1B or AaSpz1C, while Toll5A specifically bound to Spz1C. In Aag2 cells, however, DmSpz triggered gambicin production upon proteolytic processing, in contrast to mosquito Spz isoforms, which lead to a distinct set of antimicrobial peptides. Computational pairing of Toll and Spz failed to predict isoform and species-specificity, probably due to conformational changes upon ligand binding as observed by SAXS analysis of Toll5A-Spz1C complex. This study sheds light on the molecular basis of Toll and Spz neofunctionalization in the mosquito.