Favin versus concanavalin A: Circularly permuted amino acid sequences

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Abstract
The tentative amino acid sequence was determined of the .beta. chain (MW 20,000) of the lectin favin (Vicia faba). The .alpha. chain (MW 5600) of this lectin has been shown to be homologous to a region in the middle of the concanavalin A (Con A) sequence (residues 70-119). The .beta. chain is evidently homologous to 2 discrete segments of Con A. The homology begins at residue 120 of Con A, extends to the COOH terminus (residue 237) and continues without interruption through the NH2-terminal 69 residues of Con A. Together, the .alpha. and .beta. chains of favin account for a polypeptide chain equivalent in size to that of Con A. The comparison of the 2 proteins thus reveals a circular permutation of extensive homologous sequences. The favin molecule contains residues identical to many of the residues postulated to be involved in sugar binding by Con A, and contains all of the direct metal ligands as well as residues homologous to most of the residues that form the .beta.-pleated sheets of Con A. These homologies suggest that the 3-dimensional structures of the 2 lectins are likely to be very similar. Favin appears to be even more closely related in primary structure and sugar specificity to the lectins from pea and lentil, raising the possibility that all of these lectins may have structures that resemble Con A. Some of these similarities may also extend to the lectins from soybean, peanut and red kidney bean, which have different sugar specificities but share sequence homologies with the favin .beta. chain.