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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