Involved in starch degradation and also probably in the trimming of pre-amylopectin chains during starch synthesis. Hydrolysis of (1->6)-alpha-D-glucosidic linkages in alpha- and beta-limit dextrins of amylopectin and glycogen, and in amylopectin and pullulan. Glycan biosynthesis; starch biosynthesis. Glycan degradation; starch degradation. No effect on the starch level in leaves and slight increase of water-soluble polysaccharides. No alteration of the amylase-to-amylopectin ratio. ISA3 is able to fully compensate for the loss of PU1. Double mutant shows that PU1 and ISA3 have redundant function for starch degradation. The involvement of PU1 in amylopectin synthesis is infered from the phenotype of double mutant in PU1 and ISA2. Belongs to the glycosyl hydrolase 13 family.