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. Glycan biosynthesis; starch biosynthesis. Associates with ISA1 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. Belongs to the glycosyl hydrolase 13 family. Amino acids thought to be required for catalysis are not conserved in ISA2, suggesting that it may not be an active debranching enzyme and acts via its interaction with ISA1.