The immunomodulatory IL-13 binding protein of Trichuris muris selectively binds signalling lipids in unusual binding sites

JOHNS, MALCOLM E.; LAURA KAMENETZKY; LUCAS L MALDONADO

Hydra

Congreso; Parasitic Helminths: New Perspectives in Biology and Infection, Hydra, Greece, 2020.; 2020

The predominant secreted protein of Trichuris muris, p43 (Tm43), is non-immunogenic in infection, and has IL-13 and glycan-binding activity that may underpin chronic intestinal infection. Tm43 is now found to be orthologous to the major pseudocoelomic P44 protein (Dr44) of the giant kidney worm, Dioctophyme renale, that binds fatty acids and probably transports hydrophobic compounds (at least) within the parasite. Using a competitive fluorescence-based approach, we show that Tm43 also binds fatty acids and a range of other ligands, such as retinol and pharmacologically-active lipids or their precursors, some of which reveal binding sites of differing selectivity. For instance, while arachidonic acid (itself an inflammatory mediator and precursor to prostaglandins and leukotrienes) binds to Tm43, the structurally similar neurotransmitter anandamide appears to bind to only a subset of arachidonic acid-binding sites. The lipid-binding characteristics are subtly different between Tm42 and Dr44. Comparing the amino acid sequences of Tm43 and Dr44 orthologues among Clade I nematodes including Trichinella spiralis reveals sections that are highly conserved, interrupted by sections that are highly divergent. Mapping the divergent regions onto the x-ray crystal structure of Tm43 reveals potential differences in surface-connected cavities within the protein and areas on the protein?s surface that are potentially involved in external interactions. Tm43 is the result of an ancient duplication and the internal cavities in its two domains have diverged in shape and surface charge distributions, which might explain the ligand binding selectivity observed. In addition to Tm43?s IL-13-binding activity, its binding of lipids may be involved in compromising host immunity to the parasite by delivering lipids that modulate local immune or inflammatory activities, or by sequestering host signalling lipids involved in rejection.