Involved in starch granule initiation in leaf chloroplasts. Binds and delivers suitable maltooligosaccharide substrates to starch synthase 4 (SS4). Interacts with PTST3 and SS4. Contains a C-terminal (456-532) carbohydrate-binding domain (CBM). No visible phenotype under normal growth conditions, but leaf chloroplasts exhibit reduced number of starch granules.