Polypeptide ligation occurs during post-translational modification of concanavalin A.

Lectins are proteins with multivalent carbohydrate-binding sites, which confer the ability to agglutinate. The seeds of legumes are particularly rich in lectins, for example, concanavalin A (Con A) comprises up to 15% of the protein in the cotyledons of jack bean (Canavalia ensiformis) seeds. The amino acid sequences of Con A and several other legume lectins have been partially or fully determined, and comparison of these sequences from different species reveals a circular homology (Fig. 1A); rearrangements within the genome have been suggested to explain this. We report here that the circular homology displayed by Con A is due to a post-translational transposition and ligation within the initial polypeptide. This type of modification has not been reported previously for eukaryotes, although it has been suggested to occur in bacteriophage lambda.