Isolation and characterization of ferritin from soyabeans (Glycine max)

Ferritin from the soyabean Glycine max was isolated and characterized. The protein has many features in common with ferritin from mammalian systems, including extensive sequence homology, as determined by two-dimensional peptide mapping. No immunocross-reactivity between the plant and animal proteins was detected. The ferritin isolated by MgCl2 precipitation has a single subunit of 28 kDa, whereas the ferritin remaining in the supernatant exhibits marked heterogeneity, with a main subunit of 22 kDa. This form of the protein appears to be the result of specific proteolytic processing that is not affected by serine protease inhibitors, and appears only after the seeds have been soaked long enough to induce germination. The appearance of the 22-kDa form corresponds to the appearance of "crystalline arrays" of ferritin in the amyloplasts of the plant cotyledons and may represent a plant form of hemosiderin. In support of this hypothesis, the 22-kDa protein appears to be incompletely assembled, as determined by sucrose gradient centrifugation and iron uptake studies. Although ferritin is normally quite resistant to proteolysis, the 22-kDa protein is easily generated from the 28-kDa form by treatment with subtilisin, suggesting the presence of a specific, protease-sensitive sequence on the protein's surface, possibly used to mark the phytoferritin for conversion to hemosiderin and construction of ferritin crystalline arrays.