Two γ-zeins induce the unfolded protein response.

Area of Science:

• Plant Biology
• Molecular Biology
• Biochemistry

Background:

• Seed development involves massive storage protein synthesis, stressing the endoplasmic reticulum (ER) and activating the unfolded protein response (UPR).
• The specific contribution of different storage proteins to UPR activation remains unclear.

Purpose of the Study:

• To investigate how distinct storage proteins, specifically maize γ-zeins and bean PHASEOLIN, differentially modulate the plant UPR.
• To understand the protein-specific effects on UPR branches and downstream gene expression.

Main Methods:

• Transgenic Arabidopsis plants expressing maize 16-kDa γ-zein, 27-kDa γ-zein, or bean PHASEOLIN were analyzed.
• Quantitative PCR and protein blot analyses were used to assess UPR activation and the expression of UPR-controlled genes (e.g., BIP, GRP94, ERDJ3A).
• The activation of specific UPR branches (bZIP60, bZIP28) and autophagy was examined.

Main Results:

• 16-kDa γ-zein significantly activated the INOSITOL REQUIRING ENZYME1/BASIC LEUCINE ZIPPER 60 (bZIP60) UPR branch, leading to increased expression of folding helper genes.
• 27-kDa γ-zein showed a weaker induction of UPR genes, primarily affecting BIP3 and ERDJ3A transcription.
• Soluble proteins like PHASEOLIN and a mutated 27-kDa γ-zein did not significantly induce UPR, indicating the importance of protein properties and localization.

Conclusions:

• Maize 16-kDa γ-zein is a potent inducer of the plant UPR, while 27-kDa γ-zein has a subtler effect, suggesting protein-specific modulation of UPR.
• The differential UPR induction by storage proteins may have influenced their evolutionary adaptation regarding tissue and subcellular localization.
• Understanding these mechanisms provides insights into plant stress responses and protein evolution.