The barley starch granule proteome—internalized granule polypeptides of the mature endosperm

We describe the initial characterization of the starch granule proteome of barley, containing the proteins firmly associated with the granule. Starch granule preparations from mature barley seeds were subjected to SDS washes and enzymatic shaving with thermolysin prior to protein extraction in boiling SDS buffer. Proteins were examined by two- and one-dimensional electrophoresis. A typical granule proteome contained around 150 proteins spots. A total of 74 spots were identified by mass fingerprinting and sequencing using mass spectrometry with integrated NCBI database searches. Four of the central enzymes in starch synthesis, i.e. granule bound starch synthase I (GBSSI), SSI, SSII and SBEIIb, together with hordeins B and D, serpin Z4, pyruvate orthophosphate dikinase (PPDK) and an enzyme involved in thiamine biosynthesis, were identified. Unexpectedly, of the identified spots on the two-dimensional gel, 49 were found to belong to GBSSI, representing variations in both relative molecular mass and isoelectric point. The GBSSI fragmentation, which was evident from day 10 after pollination, was consistent between preparations and cultivars and is inferred to result from in vivo proteolysis. Based on the enzymatic shaving, we conclude that the starch synthesis enzymes, including the GBSSI fragments, are integral components of the granule, whereas the other granule-bound proteins are attached to the surface.