PPR-DYW Protein EMP17 Is Required for Mitochondrial RNA Editing, Complex III Biogenesis, and Seed Development in Maize

Xin-Yuan Liu,Zi-Qin Huang,Xiaomin Wang,Yan-Yan Li,Zhi-Qun Gu,Feng Sun,Aqib Sayyed,Bao-Cai Tan,Yan-Zhuo Yang,Yong Wang

doi:10.3389/fpls.2021.693272

Xin-Yuan Liu, Zi-Qin Huang + Show 8 more

Open Access

https://doi.org/10.3389/fpls.2021.693272

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Abstract

The conversion of cytidines to uridines (C-to-U) at specific sites in mitochondrial and plastid transcripts is a post-transcriptional processing event that is important to the expression of organellar genes. Pentatricopeptide repeat (PPR) proteins are involved in this process. In this study, we report the function of a previously uncharacterized PPR-DYW protein, Empty pericarp17 (EMP17), in the C-to-U editing and kernel development in maize. EMP17 is targeted to mitochondria. The loss-function of EMP17 arrests maize kernel development, abolishes the editing at ccmFC-799 and nad2-677 sites, and reduces the editing at ccmFC-906 and -966 sites. The absence of editing causes amino acid residue changes in CcmFC-267 (Ser to Pro) and Nad2-226 (Phe to Ser), respectively. As CcmFC functions in cytochrome c (Cytc) maturation, the amount of Cytc and Cytc1 protein is drastically reduced in emp17, suggesting that the CcmFC-267 (Ser to Pro) change impairs the CcmFC function. As a result, the assembly of complex III is strikingly decreased in emp17. In contrast, the assembly of complex I appears less affected, suggesting that the Nad2-226 (Phe to Ser) change may have less impact on Nad2 function. Together, these results indicate that EMP17 is required for the C-to-U editing at several sites in mitochondrial transcripts, complex III biogenesis, and seed development in maize.

Highlights

The mitochondrion is a semi-autonomous organelle that provides energy and metabolites for cell activity
We provide further evidence that the Ser-toPro change at CcmFC-267 impairs the CcmFC activity, blocks the maturation of cytochrome c (Cytc) and Cytochrome c1 (Cytc1), and disrupts the assembly of complex III, which attributes to the arrest of seed development in the emp17 maize mutant
Empty pericarp17 (EMP17) consists of 645 amino acid residues, and possesses 8 Pentatricopeptide repeat (PPR) motifs, an E1 and E2 domain, and a DYW domain with the conserved cytidine deaminase (CDA)-like signature residues (HxE(x)nCxxC) (Figure 1A and Supplementary Figure 1). Quantitative real-time PCR (qRT-PCR) analysis showed that Emp17 was ubiquitously expressed in major tissues in maize, with a relatively high expression in stem and leaf (Supplementary Figure 2)

Summary

INTRODUCTION

The mitochondrion is a semi-autonomous organelle that provides energy and metabolites for cell activity. As ubiquitous RNA binding factors, PPR proteins were found to take part in almost all of the post-transcriptional processing in mitochondria and plastids (Barkan and Small, 2014). A deficiency of CcmFN protein in the emp and ppr mutants results in a deficiency of Cytc and Cytc, leading to impaired assembly of complex III and arrested seed development in maize (Sun F. et al, 2015; Liu et al, 2020). We provide further evidence that the Ser-toPro change at CcmFC-267 impairs the CcmFC activity, blocks the maturation of Cytc and Cytc, and disrupts the assembly of complex III, which attributes to the arrest of seed development in the emp maize mutant

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DATA AVAILABILITY STATEMENT