Interplay Between the N-Terminal Domains of Arabidopsis Starch Synthase 3 Determines the Interaction of the Enzyme With the Starch Granule

The elongation of the linear chains of starch is undertaken by starch synthases. class 3 of starch synthase (SS3) has a specific feature: a long N-terminal region containing starch binding domains (SBDs). In this work, we analyze in vivo the contribution of these domains to the localization pattern of the enzyme. For this purpose, we divided the N-terminal region of Arabidopsis SS3 in three domains: D1, D2, and D3 (each of which contains an SBD and a coiled-coil site). Our analyses indicate that the N-terminal region is sufficient to determine the same localization pattern observed with the full-length protein. D2 binds tightly the polypeptide to the polymer and it is necessary the contribution of D1 and D3 to avoid the polypeptide to be trapped in the growing polymer. The localization pattern of Arabidopsis SS3 appears to be the result of the counterbalanced action of the different domains present in its N-terminal region.

Biosynthesis of waxy starch – a review

Starch comprises nearly linear amylose and branched amylopectin, whilst waxy starches are a special form, containing almost exclusively amylopectin. Modern techniques in plant breeding together with new data from starch biosynthesis research have enabled new food and non-food uses of waxy starches. This paper describes the basic ways of glucose conversion to waxy starch in plants. The recent evidence of ADP-Glc accumulation in cytosol of photosynthetically competent cells proposes a more complex pathway of starch biosynthesis based on a tight interconnection of sucrose and starch metabolic pathways. Also many studies indicate the existence of different pathways for the sucrose-starch conversion process in heterotrophic organs of dicotyledonous and monocotyledonous plants. At least six classes of starch synthases (SS) have been recognised in plants including soluble SS1, SS2, SS3, SS4, SS5, and granule bound SS (GBSS), required for the synthesis of short and long chains of amylopectin, till now. As to amylose (not-present in waxy starches), GBSS is the only starch synthase isoform encoded by the waxy genes situated at independent loci.