Biochemical and structural studies of target lectin SapL1 from the emerging opportunistic microfungus Scedosporium apiospermum

Abstract

Scedosporium apiospermum is an emerging opportunistic fungal pathogen responsible for life-threatening infections in immunocompromised patients. This fungus exhibits limited susceptibility to all current antifungals and, due its emerging character, its pathogenicity and virulence factors remain largely unknown. Carbohydrate binding proteins such as lectins are involved in host-pathogen interactions and may constitute valuable therapeutic targets to inhibit microbial adhesion to the host cells by using carbohydrate mimics. However, such lectins are still unidentified in S. apiospermum. Here, we present the first report of the identification and characterization of a lectin from S. apiospermum named SapL1. SapL1 is homologous to the conidial surface lectin FleA from Aspergillus fumigatus, known to be involved in the adhesion to host glycoconjugates present in human lung epithelium. The present report includes a detailed strategy to achieve SapL1 soluble expression in bacteria, its biochemical characterization, an analysis of its specificity and affinity by glycan arrays and isothermal titration calorimetry (ITC), as well as the structural characterization of its binding mode by X–ray crystallography. The information gathered here contributes to the understanding of glycosylated surface recognition by Scedosporium species and is essential for the design and development of antiadhesive glycodrugs targeting SapL1.Author summaryThe rate of infections caused by the airborne microfungus Scedosporium apiospermum has increased in the recent decades, especially in immunocompromised patients. It represents a therapeutic challenge due to its low susceptibility to all current antifungals, and the subsequent high mortality rate in disseminated scedosporiosis. Recently, the development of antiadhesive therapy has emerged as a novel strategy for the treatment and prevention of fungal infections. It aims to avoid the first step of infections, adhesion, by blocking the proteins responsible for attachment of the pathogens to the host-cells. Unfortunately, in the case of S. apiospermum, those proteins remain unknown but now we have identified a lectin (SapL1) encoded in its genome that is possibly involved in this process. In order to achieve a deep understanding of SapL1 specificity and interactions, we produced recombinant SapL1 in bacteria to carry out its biochemical and structural characterization. As predicted by our bioinformatics studies, SapL1 is a fucose binding lectin and we elucidated the interactions responsible for its binding specificity. Together, these informations should help to design efficient SapL1 inhibitors and therefore to generate potential antiadhesive drugs against this pathogen. Additionally, we have shown that SapL1 belongs to a highly conserved family of lectins that are present in other pathogens. Thus, the information presented here might also be useful for a broad spectrum drug development.