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 granules in developing pea (Pisum sativum L.) embryos. Mature SSII gives rise to starch synthase activity when expressed in a strain of Escherichia coli lacking glycogen synthase. In transgenic potatoes (Solanum tuberosum L.) expressing SSII, the protein is both soluble and bound to the starch granules. These results confirm that SSII is a starch synthase and indicate that partitioning between the soluble and granule-bound fraction of storage organs is an intrinsic property of the protein. A 60-kD isoform of starch synthase found both in the soluble and granule-bound fraction of the pea embryos is probably derived by the processing of SSII and is a different gene product from GBSSI, the exclusively granule-bound 59-kD isoform of starch synthase that is similar to starch synthases encoded by the waxy genes of cereals and the amf gene of potatoes. Consistent with this, expression in E. coli of an N-terminally truncated version of SSII gives rise to starch synthase activity.