The recognition of broadly conserved microorganism components known as pathogen-associated molecular patterns is an essential step in initiating the innate immune response. In the horseshoe crab, stimulation of hemocytes with lipopolysaccharide (LPS) causes the activation of its innate immune response, and Factor C, a serine protease zymogen, plays an important role in this event. Here, we report that Factor C associates with LPS on the hemocyte surface and directly recognizes Gram-negative bacteria. Structure-function analyses reveal that the LPS binding site is present in the N-terminal cysteine-rich (Cys-rich) region of the molecule and that it contains a tripeptide sequence consisting of an aromatic residue flanked by two basic residues that is conserved in other mammalian LPS-recognizing proteins. Moreover, we have demonstrated that the Cys-rich region specifically binds to LPS on Gram-negative bacteria and that mutations in the tripeptide motif abrogate its association with both LPS and Gram-negative bacteria, underscoring the importance of the tripeptide in LPS interaction. Although the innate immune response to LPS in the horseshoe crab is distinct from that of mammals, it appears to rely on structural features that are conserved among LPS-recognizing proteins from diverse species.
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