Structural Characterization of High Molecular Weight Starch Granule-Bound Proteins in Wheat (<i>Triticum aestivum</i> L.)

Abstract

Starch granule-bound proteins in endosperms of common wheat (Triticum aestivum L.) included at least one major protein with a molecular weight of 61 000 (Wx protein: starch granule-bound starch synthase) and six minor proteins with molecular weights of 115 000, 108 000, 100 000, 92 000, 80 000, and 15 000 (SGP-A1, -D1, -B1, -2, -3, and friabilin, respectively). Peptide mapping of SGP-A1, -D1, -B1, -2, and -3 indicated that the primary structures of SGP-A1, -D1, and -B1 are highly homologous but differ considerably from that of SGP-2. The internal sequences of peptides obtained from SGP-D1, -2, and -3 showed that SGP-D1 and SGP-3 are structurally similar to rice soluble starch synthase, whereas SGP-2 has homology to rice starch branching enzyme 3. These results are in agreement with results of immunological and enzymatic studies suggesting that SGP-A1, -D1, -B1, and -3 are types of soluble starch synthases and that SGP-2 is a starch branching enzyme. Keywords: Starch granule-bound protein; amino acid sequence; wheat (Triticum aestivum L.)