β-Trefoil structure enables interactions between lectins and protease inhibitors that regulate their biological functions.

Fungal ricin B-like lectins and protease inhibitors, mycocypins and mycospins, are important mediators in fungal defence against antagonists and all possess the β-trefoil fold. We demonstrate here that fungal β-trefoil proteins interact with each other, in addition to their apparent targets, and that these interactions modulate their biological activity. Such regulation of carbohydrate binding or inhibitory activity is observed for the first time in β-trefoil proteins and could constitute a mechanism for regulating their physiological functions. It could also have implications in molecular recognition of other combinations of β-trefoil proteins in other species.