Calcium Competitive Inhibition of Langerin by Thiazolopyrimidinones

C-Type lectins are a large family of carbohydrate-binding proteins. Langerin is a member of this family and is expressed by Langerhans cells, involved in pathogen recognition and innate immune activation, making it a target for small-molecule modulation in immunology and infectious diseases. We previously identified thiazolopyrimidinones as a series of allosteric inhibitors, but the underlying mechanism remained unclear. In this study, ⁴³Ca NMR demonstrated that these fragments induce Ca²⁺ release from the receptor. Our ITC data suggested a competitive relationship between inhibitors and Ca²⁺, which was further validated by ¹⁹F NMR spectroscopy showing inhibition of carbohydrate binding. Surprisingly, the fragment binding site was found to be located beneath the long loop, which supports the dynamic nature of the long loop being highly Ca²⁺ dependent. Our findings provide insight into the novel Ca²⁺-competitive inhibitory mechanism of murine langerin and are the first report on such an inhibitory mechanism for a C-type lectin.