Involved in the trimming of pre-amylopectin chains. Accelerates the crystallization of nascent amylopectin molecules during starch synthesis. ISA1 and ISA2 work exclusively together as a multimeric holoenzyme. ISA1-ISA2 removes preferentially branches that are very close to other branches. Hydrolysis of (1->6)-alpha-D-glucosidic branch linkages in glycogen, amylopectin and their beta-limit dextrins. Glycan biosynthesis; starch biosynthesis. Associates with ISA2 to form the heteromultimeric complex Iso1 required for amylopectin synthesis. Strong reduction of the starch level in leaves, but 50-fold increase of water-soluble polysaccharides. No alteration of the amylase-to-amylopectin ratio. In the absence of ISA2, ISA1 may be unstable. Belongs to the glycosyl hydrolase 13 family.