Characterisation and Subunit Structures of the Vicilin Storage Proteins of Pea (Pisum sativum L.)

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Abstract
Investigation of the vicilin fraction of the storage proteins of pea (P. sativum L.) showed that its major components are a number of protein species of MW 170,000. Convicilin (MW 280,000, composed of 71,000-MW subunits) is a separable component of this fraction. The vicilin proteins are composed principally of .apprxeq. 50,000-M, polypeptides, but also contain a number of smaller polypeptides. The subunit polypeptide composition of vicilin changes during seed development quantitatively and qualitatively. Vicilin subunits have been shown to be synthesised as polypeptides of MW .apprxeq. 50,000 by means of pulse-labeling experiments in vivo, and synthesis of vicilin in vitro directed by mRNA, polysomes and microsomes extracted from pea cotyledons in cell-free translation systems. Polypeptides then undergo 2 distinct types of proteolytic modification: co-translational removal of a small polypeptide (MW < 1000); nicking of polypeptide chains in assembled vicilin molecules, which occurs more than 4 h after their initial synthesis. The basic structure of the vicilin molecule is thus a multimer, possibly a trimer, of .apprxeq. 50,000-MW subunits. The heterogeneity of the initially synthesised 50,000-MW subunits accounts not only for the several different 50,000-MW polypeptides found in vicilin, but also for the range of minor polypeptides, since the nicking points will differ among subunits. It also accounts for the observed partial separation of vicilin into different molecular species, since different subunit combinations will give rise to molecules with different properties. Vicilin is also glycosylated and this is a source of further variation.