Llama nanobody aids structural characterisation of Cathepsin D1 enzyme from Schistosoma mansoni

What is it about?

We have determined the first crystal structure of Cathepsin D1 zymogen from the parasite, Schistosoma mansoni (abbreviated as SmCD1). The S. mansoni worm causes Schistomiasis, also known as Bilhazia, a tropical disease transmitted by freshwater snails. Like other aspartyl proteases, SmCD1 is produced as an inactive pro-enzyme or zymogen so that it can be stored by the worm and only becomes catalytically active on exposure to the acidic pH in the gut of the parasite. Activation involves cleavage of a pro-peptide sequence at the amino terminus of the enzyme which in turn exposes the active site. By immunising a llama with the SmCD1 zymogen, expressed recombinantly in mammalian cells, we produced a single domain antibody (nanobody) that bound with high affinity to the enzyme. We determined the structure of the enzyme in complex with the nanobody revealing the conformation of the pro-peptide bound state. Comparison of SmCD1 to human Cathepsin D (hCD) and human Cathepsin E (hCE) showed that their structures are largely conserved, with the active site residues positioned in equivalent locations. Despite this structural similarity, the anti-SmCD1 nanobody did not bind to the human homologues, recognising a sequence that is conserved in schistosome parasites but not the human enzymes.

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Photo by Chris on Unsplash

Photo by Chris on Unsplash

Why is it important?

Schistosomiasis is one of the most prevalent global helminth infections leading to acute and chronic disease, affecting 200 million people and threatening over 800 million in 54 endemic countries. Current treatment relies on mass administration of a single 45 year-old drug – praziquantel - which has significant limitations as a therapeutic due to increasing resistance. In a complex life cycle that involves a blood borne stage, the Schistosome worm produces proteases, including SmCD1, that digest haemoglobin to provide the amino acids that are required for survival of the parasite. SmCD1 is therefore an essential enzyme for the parasite and a potential target for the development of new anti-schistosomal inhibitors. The nanobody that we have produced to SmCD1 provides a schistosome-specific tool and has identified a binding epitope on the enzyme for future structure-guided drug design of novel therapeutics for schistosomiasis.