Wheat starch granule protein: the isolation and characterisation of a salt-extractable protein from starch granules

A salt‐extractable protein has been isolated and purified from large (A‐type) wheat flour starch granules (Triticum aestivum cv. Maris Huntsman). This protein was the major species extractable with NaCl and because of its ease of extraction it was tentatively concluded that it was associated with the surface of the granules. It had a molecular weight of approximately 30 000 as determined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate (SDS‐PAGE) and an isoelectric point (pI) in excess of 10 as judged by analytical isoelectric focusing in polyacrylamide gel. The amino acid composition of the protein was quite different from those of wheat gluten proteins. A protein with similar properties was present on small (B‐type) Maris Huntsman starch granules and it could also be detected on A‐type granules from another variety of hexaploid breadwheat (T. aestivum cv. Flinor) as well as on A‐type granules from tetraploid durum wheat (T. durum). This protein was not present on starch granules prepared from germinated grains (Maris Huntsman). The protein had neither α‐amylase activity nor inhibitory activity against either wheat or hog pancreatic α‐amylases. Several other proteins could be extracted from A‐type starch granules but only after gelatinisation of the starch granule in the presence of SDS. The electrophoretic behaviour of these proteins was quite different from that of the NaCl‐extracted protein and the rigorous conditions required to separate the latter proteins from the starch granules suggested that they are internal components of the granules.