Involved in targeting GBSS1 to the starch granule (PubMed:25710501). Was originally thought to be a carbohydrate-binding scaffold protein, but it has been shown that it is mainly found as a soluble protein and that interaction with GBSS1 is a pre-requisite for subsequent starch granule binding (PubMed:19038037, PubMed:25710501). Dissociation from starch as a function of pH, Mg(2+) concentration or redox state is not observed (PubMed:19038037). Interacts primarily with amylopectin and is required for amylose synthesis (PubMed:25710501). Interacts with GBSS1. The vast majority of PTST is soluble and only a small amount is granule-bound. No localization to the starch granules in the absence of GBSS1. Circadian-regulation. Up-regulated during the day (at protein level). Contains a C-terminal (199-277) carbohydrate-binding domain (CBM). 100-fold reduction of starch-bound GBSS1 protein and production of amylose-free starch.