Purification and characterization of arcelin seed protein from common bean

Arcelin, a seed protein originally discovered in wild bean accessions, was purified, characterized, and compared to phaseolin, the major seed protein of common bean, and to phytohemagglutinin (PHA), the major bean seed lectin. Arcelin and PHA has several characteristics in common. Both were glycoproteins having similar subunit M(r), deglycosylated M(r), and amino acid compositions. The two proteins were related antigenically and they had the same developmental timing of accumulation. Arcelin also had some hemagglutinating activity, a characteristic associated with lectins. However, several features distinguished arcelin from PHA. Arcelin had a more basic isoelectric point than PHA, greater numbers of basic amino acid residues, additional cysteine residues, and one methionine residue, which PHA lacks. Native PHA protein is a tetramer of subunits, and although a small component of native arcelin protein was also tetrameric, most of the arcelin preparation was dimeric. The hemagglutinating activity of arcelin was specific only for some pronase-treated erythrocytes. It did not agglutinate native erythrocytes, nor did it bind to thyroglobulin or fetuin affinity resins as did PHA. Although arcelin has lectin-like properties, we believe the distinctions between arcelin and PHA warrant the designation of arcelin as a unique bean seed protein.