Targeted Endoplasmic Reticulum Localization of Storage Protein mRNAs Requires the RNA-binding Protein RBP-L

Abstract

The transport and targeting of glutelin and prolamine mRNAs to distinct subdomains of the cortical ER is a model for mRNA localization in plants. This process requires a number of RNA-binding proteins that recognize and bind to the mRNA's cis-localization (zipcode) elements to form mRNP complexes that transport the RNAs to their destination sites at the cortical-ER. Here, we present evidence that the RNA-binding protein, RBP-L, which specifically binds to glutelin and prolamine zipcode RNA sequences is required for proper mRNA localization in rice endosperm cells. A T-DNA insertion within RBP-L 3'UTR resulted in 10-25% expression level of RBP-L gene compared to wild-type. The reduced expression of RBP-L caused partial mis-localization of glutelin and prolamine RNAs and conferred other general growth defects including dwarfism, late flowering and smaller seeds. Transcriptome analysis showed that RBP-L knockdown greatly affected the expression of prolamine family genes and several classes of transcription factors. Collectively, these results suggest that RBP-L is a key RNA-binding protein involved in mRNA localization in rice endosperm cells and regulates several essential biological processes in rice development through direct binding to the corresponding RNAs or indirect regulation by its effect on transcription factors.