Micro-Exon Genes are a widespread class of genes known for their high variability, widespread in the genome of parasitic trematodes such as Schistosoma mansoni. In this study, we present a strategy that allowed us to solve the structures of three alternatively spliced isoforms from the Schistoma mansoni MEG 2.1 family for the first time. All isoforms are hydrophobic, intrinsically disordered, and recalcitrant to be expressed in high yield in heterologous hosts. We resorted to the chemical synthesis of shorter pieces, before reconstructing the entire sequence. Here, we show that isoform 1 partially folds in a-helix in the presence of trifluoroethanol while isoform 2 features two rigid elbows, that maintain the peptide as disordered, preventing any structuring. Finally, isoform 3 is dominated by the signal peptide, which folds into a-helix. We demonstrated that combining biophysical techniques, like circular dichroism and nuclear magnetic resonance at natural abundance, with in silico molecular dynamics simulation for isoform 1 only, was the key to solve the structure of MEG 2.1. Our results provide a crucial piece to the puzzle of this elusive and highly variable class of proteins.

Department of Biochemical Sciences Sapienza University of Rome Rome Italy

Department of Chemistry Faculty of Agrobiology Food and Natural Resources Czech University of Life Sciences Prague Prague Czech Republic

Department of Zoology and Fisheries Center of Infectious Animal Diseases Czech University of Life Sciences Prague Prague Czech Republic

Faculty of Environmental Sciences Center of Infectious Animal Diseases Czech University of Life Sciences Prague Prague Czech Republic

Institute of Organic Chemistry and Biochemistry Czech Academy of Sciences Prague Czechia

Université de Lille CNRS UMR8576 UGSF Unité de Glycobiologie Structurale et Fonctionnelle Lille France

Université de Lyon CNRS UCB Lyon1 Institut des Sciences Analytiques UMR5280 5 rue de la Doua Villeurbanne France

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Revisiting Schistosoma mansoni Micro-Exon Gene (MEG) Protein Family: A Tour into Conserved Motifs and Annotation