Cotyledonary Storage Proteins in Pisum sativum. V. Further Studies on Molecular Heterogeneity in the Vicilin Series of Holoproteins

Abstract

The cotyledonary storage proteins of mature pea seeds were resolved by polyacrylamide gel electrophoresis in an acid buffer system as discrete bands comprising holoproteins referable to either the legumin or vicilin series. Three major and four less abundant vicilin holoproteins were detected in acid-gel separations after subfractionation of storage-protein extracts on the basis of differential solubility at specific pH values and ionic strengths, by electrophoresis in the alkaline pH range or by gel filtration. Acid-gel patterns were qualitatively consistent over a range of genotypes. Up to 12 polypeptides were found in varying proportions in reduced and dissociated eluates from fixed and stained vicilin bands separated on acid gels. Each vicilin holoprotein showed a distinctive quantitative, and in some cases qualitative, polypeptide composition. Genetically determined size or charge variants of certain polypeptides used as markers were found in each of the major vicilin holoproteins. This observation, and the consistent appearance of the vicilin bands on acid gels after exposure to widely different treatments during subfractionation, indicate that these polypeptides are strictly subunits of the vicilins, rather than fortuitously associated components. Amongst the holoproteins of each band, some flexibility of subunit composition is indicated by evidence that the total of the molecular weights of the subunits exceeds the molecular weight of the corresponding holoprotein, and by the apparent relative abundance of certain subunits. The observed complexity of subunit composition of the vicilins is consistent with previous findings of immunological similarities and differences amongst the holoproteins of this series, and with change during development in the subunit composition of these proteins.