Immunological homologies between wheat gluten and starch granule proteins

Abstract

Structural homologies between wheat (Triticum aestivum L.) gluten proteins and proteins present in well-washed starch granules were examined with a panel of mouse monoclonal and mouse and rabbit polyclonal antibodies, using immunoblotting, enzyme-linked immunosorbent assay (ELISA) and immunocytochemical methods. Many antibodies raised against gluten protein fractions cross-reacted with starch granule proteins (SGP), but often weakly. Antibodies with similar gliadin and glutenin subunit specificities had similar SGP specificities: (1) Antibodies to high-mobility (α-, β- or γ-) gliadins cross-reacted weakly with low molecular weight SGP (Mr, 8000, 19000 and 30000) on immunoblots, and very weakly in indirect ELISAs. Some of these antibodies labelled both protein bodies and the periphery of starch granules in sections of immature grain, consistent with low molecular weight SGP, deemed to be ‘surface’ SGP on the basis of extractability studies, indeed being present on the granule surface. (2) Monoclonal antibodies that bound γ- or ω-gliadins and glutenin subunits bound to higher molecular weight SGP, especially a protein of Mr 77000, at concentrations only slightly above those which labelled gluten proteins. As the interior of the starch granule section was labelled, these proteins are likely to be ‘integral’ to the granule. (3) Antibodies binding broadly to all major gluten protein classes also bound most high and low molecular weight SGPs. Some starch proteins of Mr 15000, which have been associated with endosperm softness, appeared to be immunologically distinct.