IDENTIFICATION OF MULTIPLE ISOFORMS OF SOLUBLE AND GRANULE-BOUND STARCH SYNTHASE IN DEVELOPING WHEAT ENDOSPERM

We have investigated the nature and locations of isoforms of starch sy nthase in the developing endosperm of wheat (Triticum aestivum L.). Th ere 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 p rotein, is also present in the soluble fraction of the endosperm but i t contributes only a small proportion of the total soluble activity. M ost of the soluble activity is contributed by isoforms which are appar ently not also granule-bound. The 60-kDa and 77-kDa isoforms of wheat are antigenically related to isoforms of very similar size in the deve loping pea embryo, but the other isoforms in the endosperm appear to h ave no counterparts in the pea embryo. The significance of these resul ts in terms of the diversity of isoforms of starch synthase and their locations is discussed.