A 56-KDA PROTEIN IS A NOVEL GRANULE-BOUND STARCH SYNTHASE EXISTING INTHE PERICARPS, ALEURONE LAYERS, AND EMBRYOS OF IMMATURE SEED IN DIPLOID WHEAT (TRITICUM-MONOCOCCUM L.)

A novel 56-kDa granule-bound starch synthase (GBSS; NDPglucose-starch glucosyltransferase, EC 2.4.1.21) responsible for amylose synthesis wa s found in the pericarps, aleurone layers and embryos of immature dipl oid wheat (Triticum monococcum L.). The GBSS and other proteins bound to starch granules of various tissues of immature normal and waxy dipl oid wheat seeds were separated by sodium dodecyl sulfate polyacrylamid e gel electrophoresis (SDS-PAGE) and their activities were examined. I n the waxy mutant, the waxy protein (59.5 kDa, GBSSI) was absent, but amylose and GBSS activity were evident in all tissues except the endos perm. Of the proteins bound to starch granules, only the 56-kDa protei n was associated with the presence of amylose and GBSS activities in t he pericarps, aleurone layers and embryos. Mutations at the waxy locus did not affect the 56-kDa protein in these tissues. Changes in the am ount of 56-kDa protein during the course of seed development, and the distribution of the 56-kDa protein in each tissue of immature seeds we re quite different from those of the waxy, protein. On the other hand, the N-terminal amino acid sequence of the 56-kDa protein had a 40-50% similarity to GBSSI of some other plant species and was antigenically related to the waxy protein. These results strongly suggest that the 56-kDa protein in diploid wheat is a GBSSI class enzyme and, hence, an isoform of the waxy protein. The waxy protein and 56-kDa protein, how ever, are expressed in different seed tissues and at different stages of seed development.