A recombinant Sp185/333 protein from the purple sea urchin has multitasking binding activities towards certain microbes and PAMPs

•Sp185/333 proteins are immune effector proteins expressed by coelomocytes.•rSp0032, a rSp185/333 protein, binds to Vibrio and Saccharomyces, but not Bacillus.•rSp0032 binds to LPS, beta-1,3-glucan and flagellin, but not peptidoglycan.•rSp0032 binding is specific with very strong affinity.•rSp0032 binds multiple targets and may be very effective in pathogen protection.

The purple sea urchin, Strongylocentrotus purpuratus, possesses a sophisticated innate immune system that responds to microbes effectively by swift expression of the highly diverse Sp185/333 gene family. The Sp185/333 proteins are predicted to have anti-pathogen functions based on inducible gene expression and their significant sequence diversity. Sp185/333 proteins are all predicted to be intrinsically disordered and do not exhibit sequence similarities to other known proteins. To test the anti-pathogen hypothesis, a recombinant Sp185/333 protein, rSp0032, was evaluated and found to exhibit specific binding to marine Vibrio diazotrophicus and to Saccharomyces cerevisiae, but not to two Bacillus species. rSp0032 also binds to LPS, β-1,3-glucan and flagellin but not to peptidoglycan. rSp0032 binding to LPS can be competed by LPS, β-1,3-glucan and flagellin but not by peptidoglycan. We speculate that the predicted intrinsically disordered structure of rSp0032 may adapt to different conformations in binding to a limited number of PAMPs and pathogens. Given that rSp0032 binds to a range of targets, and that up to 260 different Sp185/333 proteins can be expressed per individual sea urchin, this family of immune response proteins may facilitate effective host protection against a broad array of potential pathogens encountered in the marine environment.

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