It takes a complex to degrade starch

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Jian Liu, Xuecui Wang, Zeyuan Guan, Junjie Yan and colleagues reveal how the glucan phosphatase LSF1 acts as a scaffold to bring together the necessary enzymes for starch degradation.

https://doi.org/10.1093/plcell/koad259

Background:

Starch is a glucan-polymer that accumulates in chloroplasts during the day as a product of photosynthesis and is mobilized at night to continuously provide sugars to sustain plant growth and development. Efficient starch degradation involves multiple enzymes such as glucan phosphatase and b-amylase. LIKE SEX FOUR 1 (LSF1) is a unique multi-domain inactive glucan phosphatase, with the lsf1 mutant exhibiting a starch-excess phenotype. LSF1 is thought to interact with other proteins to form a protein complex to regulate starch degradation. However, the composition of this protein complex and how its assembly is regulated remain largely unknown.

Question:

How does LSF1 coordinate with other proteins to form a starch degradation complex? What are the molecular functions of LSF1 in starch degradation?

Findings:

We discovered that LSF1 interacts with plastid localized NAD-dependent malate dehydrogenase (MDH) to recruit b-amylase (BAM1) and form the BAM1–LSF1–MDH complex. We also demonstrate that the starch hydrolysis activity of BAM1 drastically increases in the presence of the LSF1–MDH complex. A comprehensive analysis of cryo-electron microscopy structure, crosslinking coupled with mass spectrometry, and molecular docking revealed the molecular mechanism by which the LSF1–MDH complex improves the starch degradation activity of BAM1. Specifically, the proximity of the starch-binding domains provided by the dual-specificity phosphatase (DSP) and the carbohydrate-binding module (CBM) of LSF1 to BAM1 facilitates the access of BAM1 to the polyglucans of starch, helping in their hydrolysis by BAM1. This study uncovers the scaffold function of the LSF1–MDH complex in starch degradation.

Next steps:

Engineering LSF1 to be an enzymatically active glucan phosphatase may allow the BAM1–LSF1–MDH complex to successively catalyze starch dephosphorylation and hydrolysis. Further studies are however needed to explore the effects of active LSF1 on plant growth and development.

Reference:

Jian Liu, Xuecui Wang, Zeyuan Guan, Menglong Wu, Xinyue Wang, Rong Fan, Fei Zhang, Junjun Yan, Yanjun Liu, Delin Zhang, Ping Yin, and Junjie Yan (2023). The LSF1-MDH complex functions as a scaffold to recruit b-amylase to promote starch degradation. https://doi.org/10.1093/plcell/koad259