The Gal/GalNAc-specific lectin from the plant pathogenic basidiomycete Rhizoctonia solani is a member of the ricin-B family

The lectin isolated from the phytopathogenic basidiomycete Rhizoctonia sola ni (RSA) is a homodimer of two noncovalently associated monomers of 15.5 kD a, RSA is a basic protein (pI > 9) which consists mainly of beta -sheets, A presumed relationship with ricin-B is supported by the sequence similarity between the N-terminus of RSA and the N-terminal subdomain of ricin-B, Hyd rophobic cluster analysis confirms that the N-terminus of both proteins has a comparable folding. RSA exhibits specificity towards Gal/GalNAc whereby the hydroxyls at the C3 ', C4 ', and C6 ' positions of the pyranose ring pl ay a key role in the interaction with simple sugars. The carbohydrate-bindi ng site of RSA apparently accommodates only a single sugar unit, Our result s demonstrate an obvious evolutionary relationship between some fungal and plant lectins, but also provide evidence for the occurrence of a lectin con sisting of subunits corresponding to a single subdomain of ricin-B. (C) 200 1 Academic Press.