Deciphering potent MPL activation by the fucose-binding lectin thrombocorticin.

Thrombocorticin (ThC) is a marine sponge-derived L-fucose-binding lectin that acts as an agonist of the thrombopoietin (TPO) receptor through glycan-mediated receptor crosslinking. We previously identified the bacterial lectin PA-IIL from Pseudomonas aeruginosa as a structurally related MPL agonist; however, its potency is approximately 70-fold lower than that of ThC. To investigate the structural basis underlying this difference, we performed structure-function analyses using ThC and PA-IIL mutants as well as engineered oligomeric variants. These studies demonstrated that MPL activation could not be explained solely by sugar-binding activity or the apparent oligomeric state. In particular, lectins with similar sugar-binding properties displayed markedly different agonist activities, while alterations in oligomeric organization did not consistently correlate with MPL activation potency. These findings suggest that additional structural features are required for productive receptor activation. To further explore these determinants, we performed genome mining of bacterial proteins whose sequences were similar to those of ThC. Four candidate lectins were heterologously expressed and evaluated for MPL agonist activity. Three exhibited partial agonist activity, reaching approximately 20% of the maximal response induced by ThC. Comparative analyses of amino acid sequences, structural models, and biological activities revealed that neither the ability to bind sugars nor the oligomeric state alone predict MPL agonist potency. Collectively, these findings indicate that productive MPL activation requires structural features beyond the ability to bind sugars and the oligomeric state.