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The process for and regulatory mechanism controlling the synthesis and degradation of the polysaccharide starch are only superficially understood. β-amylases (BAMs) are enzymes that hydrolyze starch into maltose which is further used to drive metabolism and other cellular processes. Most BAMs in plants can function as monomeric enzymes and have hyperbolic kinetics. BAM2 from Arabidopsis thaliana is unusual as it forms a homotetramer, displays sigmoidal kinetics, and is stimulated by the presence of potassium cations (K+). We used circular dichroism spectroscopy, small-angle X-Ray scattering, and molecular dynamics to investigate the effect of K+ on the structure of BAM2 and found that K+ induces the formation of an active conformation of BAM2 thereby increasing its activity.
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This content will become publicly available on June 1, 2026
The Pseudoenzyme β‐Amylase9 From Arabidopsis Activates α‐Amylase3: A Possible Mechanism to Promote Stress‐Induced Starch Degradation
ABSTRACT Starch accumulation in plants provides carbon for nighttime use, for regrowth after periods of dormancy, and for times of stress. Both ɑ‐ and β‐amylases (AMYs and BAMs, respectively) catalyze starch hydrolysis, but their functional roles are unclear. Moreover, the presence of catalytically inactive amylases that show starch excess phenotypes when deleted presents questions on how starch degradation is regulated. Plants lacking one of these catalytically inactive β‐amylases, BAM9, have enhanced starch accumulation when combined with mutations in BAM1 and BAM3, the primary starch degrading BAMs in response to stress and at night, respectively. BAM9 has been reported to be transcriptionally induced by stress although the mechanism for BAM9 function is unclear. From yeast two‐hybrid experiments, we identified the plastid‐localized AMY3 as a potential interaction partner for BAM9. We found that BAM9 interacted with AMY3 in vitro and that BAM9 enhances AMY3 activity about three‐fold. Modeling of the AMY3‐BAM9 complex predicted a previously undescribed alpha–alpha hairpin in AMY3 that could serve as a potential interaction site. Additionally, AMY3 lacking the alpha–alpha hairpin is unaffected by BAM9. Structural analysis of AMY3 showed that it can form a homodimer in solution and that BAM9 appears to replace one of the AMY3 monomers to form a heterodimer. The presence of both BAM9 and AMY3 in many vascular plant lineages, along with model‐based evidence that they heterodimerize, suggests that the interaction is conserved. Collectively these data suggest that BAM9 is a pseudoamylase that activates AMY3 in response to cellular stress, possibly facilitating stress recovery.
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- Award ID(s):
- 2322867
- PAR ID:
- 10630287
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Proteins: Structure, Function, and Bioinformatics
- Volume:
- 93
- Issue:
- 6
- ISSN:
- 0887-3585
- Page Range / eLocation ID:
- 1189 to 1201
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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