Identification of multiple isoforms of soluble and granule-bound starch synthase in developing wheat endosperm

Abstract

Abstract.We have investigated the nature and locations of isoforms of starch synthase in the developing endosperm of wheat (Triticum aestivum L.). There are three distinct granule-bound isoforms of 60 kDa (the Waxy gene product), 77 kDa and 100-105 kDa. One of these isoforms, the 77-kDa protein, is also present in the soluble fraction of the endosperm but it contributes only a small proportion of the total soluble activity. Most of the soluble activity is contributed by isoforms which are apparently not also granule-bound. The 60-kDa and 77-kDa isoforms of wheat are antigenically related to isoforms of very similar size in the developing pea embryo, but the other isoforms in the endosperm appear to have no counterparts in the pea embryo. The significance of these results in terms of the diversity of isoforms of starch synthase and their locations is discussed.

The Localization and Expression of the Class II Starch Synthases of Wheat1

“…The proteins within the matrix of the wheat starch granule have been extensively studied (Denyer et al, 1995;Rahman et al, 1995;Yamamori and Endo, 1996;Takaoka et al, 1997). The predominant 60-kD protein is exclusively granule bound and is analogous to the "waxy" GBSS gene in maize (Rahman et al, 1995).…”

“…The predominant 60-kD protein is exclusively granule bound and is analogous to the "waxy" GBSS gene in maize (Rahman et al, 1995). The 75-kD SSI is found in both the granule and the soluble fraction of wheat endosperm (Denyer et al, 1995;Li et al, 1999), and has been assigned to chromosomes 7A, 7B, and 7D (Yamamori and Endo, 1996;Li et al, 1999). The 85-kD band contains a class II branching enzyme and an unidentified polypeptide (Rahman et al, 1995).…”

“…The precise molecular masses assigned to these proteins vary depending on the details of the SDS-PAGE separation system employed. The 100-, 108-, and 115-kD proteins have been designated SGP-B1, SGP-D1, and SGP-A1 (Yamamori and Endo, 1996;Takaoka et al, 1997), respectively, and have been shown to be encoded at loci located on the short arms of chromosome 7B, 7D, and 7A, respectively (Denyer et al, 1995;Yamamori and Endo, 1996). It has been suggested that the SGP-1 proteins are SSs, based on assays of SS activity following renaturation of SGPs separated by SDS-PAGE (Denyer et al, 1995) and on protein sequence data (Takaoka et al, 1997).…”

Increased Amylose Content and Altered Molecular Structures of Wheat Starch by SGP-1 Null Mutation

“…2 5) Three isoforms with different molecular weight (100 115 kDa 2 5) ) of SGP 1 are present in wheat starch granules and they are designated SGP A1, B1, D1, 1) each encoded at loci on the short arms o f chromosome 7A, 7 B and 7 D , respectively . 1,2) SGP B1, and very likely the other two isofomrs also, exist in both starch granule and soluble fractions at early stages of endosperm development but at late stages they are exclusively granule bound. 3,5) Yamamori et al .…”

“…Some of these enzymes are found to be bound to or trapped in starch granules. In wheat (Triticum aestivum L.), at least three proteins, other than waxy protein, have been identified as starch granule bound proteins (starch granule protein, SGP) 1,2) and are designated as SGP 1, 2 and 3. 1) The SGPs are thought to be isoforms of starch synthase II, starch branching enzyme II and starch synthase I, respectively, based on their biochemical properties and nucleotide sequence.…”