Starch-debranching enzyme involved in amylopectin biosynthesis in endosperm. Functions by removing excess branches or improper branches that interfere with the formation of double helices of the cluster chains of amylopectin and crystallization of starch (PubMed:10517831, PubMed:15618430, PubMed:16953433, PubMed:21436381). Works as ISA1 homooligomer or together with ISA2 as heterooligomer. The heterooligomer ISA1 and ISA2 possesses higher affinity than the ISA1 homooligomer for various branched polyglucans in vitro, but no marked differences exist in chain preferences for debranching of amylopectin and phytoglycogen between these forms (PubMed:16953433, PubMed:21436381). Hydrolysis of (1->6)-alpha-D-glucosidic branch linkages in glycogen, amylopectin and their beta-limit dextrins. Inhibited by copper chloride, mercury chloride, ammonium molybdate and para-chloromercuribenzoate. Optimum pH is 6.5 at 30 degrees Celsius. Glycan biosynthesis; starch biosynthesis. Forms a homo-pentamer and a hetero-hexamer composed of five ISA1 and one ISA2 (PubMed:16953433). Interacts with FLO6/SIP4 (PubMed:24456533). Highly expressed in developing endosperm. Expressed at low levels in leaves. Replacement of endosperm starch by a water-soluble highly and randomly branched polysaccharide structure called phytoglycogen. Belongs to the glycosyl hydrolase 13 family. Sequencing errors. Truncated N-terminus. Truncated N-terminus.