Starch Granule-Associated Proteins and Polypeptides: A Review

This paper reviews current information regarding the (non-gluten) proteins which are naturally located in and on starch granules, and which are termed starch granule associated proteins (SGAPs). At least ten major SGAPs can be extracted from most starches and have molecular weights in the range of approximately 5 to 149 kDa. A substantial number of these proteins are located at the starch granule surface, where their presence in association with that of other minor granule components (such as lipids), appears to significantly influence the overall properties of both starch granules and starchy products. The different extraction conditions which can be exploited to selectively remove SGAPs are detailed. Although some SGAPs may serve as supplementary storage proteins, the majority of SGAPs are believed to be starch biosynthetic enzymes, a proportion of which remain trapped within the growing granule structure. The identity and quantities of the biosynthetic enzymes varies with botanical source, genetic variety and even with time. Current knowledge regarding the identities, compositions, locations, properties and functions of several of the more common SGAPs (e.g. the ∼15 kDa friabilin/puroindoline proteins, the ∼30 kDa protein, and the ∼60 kDa starch granule-bound starch synthase (SGBSS) protein) are reviewed in detail.