Atomic crystal structure and sugar specificity of a β-trefoil lectin domain from the ectomycorrhizal basidiomycete Laccaria bicolor

Lectins from fruiting bodies are a diverse group of sugar-binding proteins from mushrooms that face the bio-logically relevant challenge of discriminating self-from non-self carbohydrate structures, therefore providing a basis for an innate defence system. Such a system entails both detection and destruction of invaders and/or feeders, and in contrast to more complex organisms with immense immune systems, these two functions nor-mally rely on multitasking lectins, namely, lectins with different functional modules. Here, we present a novel fungal lectin, LBL, from the basidiomycete Laccaria bicolor. Using a diverse set of biophysical techniques, we unveil the fine details of the sugar-binding specificity of the N-terminal beta-trefoil of LBL (LBL152), whose structure has been determined at the highest resolution so far reported for such a fold. LBL152 binds complex poly-N- Acetyllactosamine polysaccharides and also robust LBL152 binding to Caenorhabditis elegans and Drosophila melanogaster cellular extracts was detected in microarray assays, with a seeming preference for the fruit fly adult and pupa stages over the larva stage. Prediction of the structure of the C-terminal part of LBL with AlphaFold reveals a tandem repeat of two structurally almost identical domains of around 110 amino acids each, despite sharing low sequence conservation.

Automated summary

This article introduces a new fungal lectin LBL, which can recognize and bind to various complex polysaccharide structures, and exhibit toxicity to invaders and parasites. The research found that LBL has a high affinity for insect cellular extracts, especially for fruit flies in the adult and pupa stages.