Characterization of Pea Vicilin. 1. Denoting Convicilin as the α-Subunit of the Pisum Vicilin Family

Vicilin, a major globulin protein of pea that has been described as “extremely heterogeneous in terms of its polypeptide composition”, was extracted from pea flour under alkaline conditions and subsequently fractionated by salt under acid conditions. This procedure induced the separation of vicilin into two fractions, which, after purification, were called vicilin 1° and vicilin 2°. Vicilin 2° was seen on SDS-PAGE to contain the third globulin protein of pea, convicilin (a band at ∼70 kDa). Vicilin fractions were thus characterized using gel electrophoresis, differential scanning calorimetry, circular dichroism, and pH-dependent solubility in order to determine whether the convicilin should in fact be considered as a third separate globulin protein of pea. On the basis of the results obtained it was concluded that this distinct polypeptide of the Pisum vicilin gene family should be further denoted as a subunit of the salt extractable protein vicilin. The definition of vicilin heterogeneity should therefore be extended to acknowledge the possible oligomeric inclusion of the 70 kDa polypeptide that is here denoted as the α-subunit. Keywords: Pisum; storage proteins; purification; vicilin; convicilin; subunit composition; heterogeneity