THE MAJOR PROTEINS OF WHEAT ENDOSPERM STARCH GRANULES

Wheat starch contains two classes of associated proteins: proteins whi ch are embedded within the granule and loosely associated surface prot eins. The characterisation of the major proteins that are embedded in the granule are described. Gel electrophoresis on the basis of size re solved these proteins into five bands of molecular weights 60, 75, 85, 100 and 105 kDa. These polypeptides were demonstrated to be within th e granule by their resistance to proteinase K digestion when granules were ungelatinised. The N-terminal sequences of these polypeptides are reported. The most prominent polypeptide is the 60 kDa granule-bound starch synthase. The N-terminal sequence obtained from the 75 kDa poly peptide shows homology to rice soluble starch synthase. The 85 kDa ban d was resolved into at least two types of polypeptides, one of which r eacted with polyclonal antiserum to the maize branching enzyme IIb. Th e 100 and 105 kDa polypeptides were located only in the granule and ar e related, on the basis of N-terminal sequence similarity and cross-re activity to monoclonal antibodies. SDS-PAGE and monoclonal antibody cr oss-reactivity experiments suggest that the 100 and 105 kDa polypeptid es are absent from starch granules from all other species examined, in cluding other cereals. It is speculated that all the major granule pro teins are involved in starch biosynthesis.