Probing genetic variation and glycoform distribution in lectins of the Erythrina genus by mass spectrometry

Six leguminous lectins from the seeds of plants of the Erythrina genus, namely E. caffra (ECafL), E. cristagalli (ECL), E. flabelliformis (EFL), E. lysistemon (ELysL), E. rubrinerva (ERL), and E. vespertilio (EVL), were examined to establish their sequence homology and to determine the structure and sites of attachment of their glycans. Tryptic digests of these lectins were analyzed by capillary electrophoresis coupled to electrospray mass spectrometry (CE-ESMS). Assignments were made by comparing the molecular masses of the observed tryptic peptides with those of Erythrina corallodendron lectin (ECorL), the sequence of which had been established previously. Glycan structure and genetic variations in the amino acid sequence were probed by tandem mass spectrometry. Small differences were found between the sequences of the various lectins examined and all of them exhibited C-terminal processing resulting in proteins with a C-terminal Asn residue. The major glycan of these glycoproteins was shown to be the heptasaccharide Man(3)XylFucGlcNAc(2), consistent with previous investigations on ECL and ECorL. A minor glycan heterogeneity was observed for most lectins examined except for that of ECafL and ECorL where an extra hexose residue was observed on the reducing GlcNAc residue of the heptasaccharide.