Size and Charge Heterogeneity in Subunits of the Major Seed Storage Protein, Cucurbitin. Identification of a Polyprotein Precursor in Immature Seeds of the Water Melon Citrullus lanatus

Oil-rich seeds of the water melon (Citrullus lanatus (Thunb. ) Mansf. ) also contain substantial amounts (30% w/w) of protein. The major storage globulin, called ‘ cucurbitin’, exists as a hexamer of non-covalently associated subunits of Mr 51000–54 000 which can be further dissociated in the presence of disulphide-reducing agents to yield two groups of polypeptides of apparent Mr 33 000–37 000 and 20 000–25 000 respectively. One- and two-dimensional polyacrylamide gel electrophoretic separations on the basis of size and/or charge confirm that each polypeptide group is more heterogeneous than previously suspected. A putative polyprotein precursor containing sequences related to each of the polypeptides of mature cucurbitin can be isolated and characterized from immature seeds. Cucurbitin is an important constituent of the diet in many Asian and African countries, a fact which prompted us to characterize more fully the molecular structure of this valuable storage protein as a prelude to future cloning and genetic manipulation experiments.