Studies of Storage Proteins of Higher Plants: I. Concanavalin A from Three Species of the Genus Canavalia12

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Concanavalin A, the lectin of the Jack bean, Canavalia ensiformis, was extracted and compared with homologous proteins from Canavalia gladiata and Canavalia maritima. All proteins were bound to Sephadex G-100 and eluted from the gel with buffered glucose solution. Quantitative recoveries indicated that large quantities (23 to 28% of dry seed protein) of these lectins are synthesized by all three species. Antibody preparations made against C. ensiformis lectin failed to discriminate among the three proteins; the pattern of the precipitin bands indicated identical antigenic determinants in the Ouchterlony double-diffusion assay. Native and sodium dodecyl sulfate polyacryl-amide gel electrophoresis also failed to distinguish differences in the proteins. The storage protein active in carbohydrate binding is composed, in each case, of identical subunits. However, the amino acid composition of the subunit chains from the three sources is not identical. In particular, the lectins from C. ensiformis and C. gladiata contain two methionine residues per protein subunit, while only one methionine residue is found in the C. martima lectin. Cyanogen bromide cleavage of the purified subunit from C. maritima yieded two fragments with molecular weights estimated at 20,400 and 4,600, respectively. Amino acid analysis of the separated fragments indicated that the methionine residue at position 130 in C. ensiformis is absent in the lectin from C. maritima.

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