Master of non-enzymatic functions: ISOAMYLASE complex structure reveals additional metabolic roles

Plants store glucose polymers in semicrystalline starch granules. Starch debranching enzymes, including isoamylases (ISAs), are involved in maintaining starch’s crystalline organization. In maize, sugary1 (su1) mutants with impaired ISA1 enzymatic activity accumulate soluble polyglucan chains (phytoglycogen). Unlike ISA1, ISA2 is enzymatically inactive. However, it remains selected for in plants, and is structurally conserved including an additional β-sandwich domain not present in ISA1. Hennen-Bierwagen and colleagues investigated the non-enzymatic functions of the maize ISA1-ISA2 complex, using non-catalytic ISA1 alleles that accumulate varying starch/phytoglycogen ratios. Removing ISA2 exacerbated the ISA1 phenotypes, demonstrating it has non-enzymatic functionality. AlphaFold3 models revealed 8 glucan binding sites in the ISA1-ISA2 heterotetramer, with the point mutations clustering in the border lobe, affecting polyglucan binding. The non-enzymatic functions of the ISA complex may include recruitment of binding-partners to these glucans, or shielding them from other proteins. Bringing these glucan chains into proximity with one another may promote helix formation, which gives starch its crystallinity. Crucially, this paper highlights the importance of considering the non-enzymatic functions of proteins, and how structural modelling can shed light on these roles. (Summary by Ciara O’Brien @ciara-obrien.bsky.social) Plant Cell  10.1093/plcell/koaf220