The Amino‐Acid Sequence of the α Subunit of the Mitogenic Lectin from Vicia sativa

The complete amino acid sequence of the alpha chain of the mitogenic lectin from Vicia sativa has been determined. The polypeptide was digested by trypsin and chymotrypsin. The fragments were isolated and most of them were sequenced by automatic solid-phase Edman degradation. A total sequence of 52 amino acid residues was obtained which is homologous to the alpha subunits of the lectins from Pisum sativum, Lens culinaris and Vicia faba. The central part (residues 13-32) in particular is completely conserved in all four proteins. In contrast to the three other known sequences, the V. sativa alpha subunit has an additional serine at the N terminus. The differences of the related amino acid sequences of these lectins and concanavalin A, the lectin from Canavalia ensiformis, have been compared using the relative substitution frequency found in homologous proteins by McLachlan. The degree of homology decreases in the order: V. sativa greater than P. sativum greater than V. faba greater than L. culinaris greater than C, ensiformis. Prediction of the secondary structure according to Chou and Fasman reveals that the lectin alpha chain is similar to concanavalin A in the first half of the molecule whereas the C-terminal part apparently tends to form an alpha helix.