EVIDENCE THAT A 77-KILODALTON PROTEIN FROM THE STARCH OF PEA EMBRYOS IS AN ISOFORM OF STARCH SYNTHASE THAT IS BOTH SOLUBLE AND GRANULE BOUND

In this paper we provide further evidence about the nature of a 77-kD starch synthase (SSII) that is both soluble and bound to the starch gr anules in developing pea (Pisum sativum L.) embryos. Mature SSII gives rise to starch synthase activity when expressed in a strain of Escher ichia coli lacking glycogen synthase. In transgenic potatoes (Solanum tuberosum L.) expressing SSII, the protein is both soluble and bound t o the starch granules. These results confirm that SSII is a starch syn thase and indicate that partitioning between the soluble and granule-b ound fraction of storage organs is an intrinsic property of the protei n. A 60-kD isoform of starch synthase found both in the soluble and gr anule-bound fraction of the pea embryos is probably derived by the pro cessing of SSII and is a different gene product from GBSSI, the exclus ively granule-bound 59-kD isoform of starch synthase that is similar t o starch synthases encoded by the waxy genes of cereals and the amf ge ne of potatoes. Consistent with this, expression in E. coli of an N-te rminally truncated version of SSII gives rise to starch synthase activ ity.