Transit peptide elements mediate selective protein targeting to two different types of chloroplasts in the single-cell C4 species Bienertia sinuspersici

Abstract

Bienertia sinuspersici is a terrestrial plant that performs C4 photosynthesis within individual cells through operating a carbon concentrating mechanism between different subcellular domains including two types of chloroplasts. It is currently unknown how differentiation of two highly specialized chloroplasts within the same cell occurs as no similar cases have been reported. Here we show that this differentiation in photosynthetic cells of B. sinuspersici is enabled by a transit peptide (TP) mediated selective protein targeting mechanism. Mutations in the TPs cause loss of selectivity but not general loss of chloroplast import, indicating the mechanism operates by specifically blocking protein accumulation in one chloroplast type. Hybrid studies indicate that this selectivity is transferable to transit peptides of plants which perform C4 by cooperative function of chloroplasts between two photosynthetic cells. Codon swap experiments as well as introducing an artificial bait mRNA show that RNA affects are not crucial for the sorting process. In summary, our analysis shows how the mechanism of subcellular targeting to form two types of chloroplast within the same cell can be achieved. This information is not only crucial for understanding single-cell C4 photosynthesis; it provides new insights in control of subcellular protein targeting in cell biology.

Methods of analysis of chloroplast genomes of C3, Kranz type C4 and Single Cell C4 photosynthetic members of Chenopodiaceae

“…The other type of chloroplast has the Calvin-Benson cycle with Rubisco fixing CO 2 that is generated by decarboxylation of C 4 acids (utilizing plastid-targeted NADPmalic enzyme in some C 4 species). Currently all enzymes required in chloroplasts to support the C 4 cycle and Calvin-Benson cycle are considered to be nuclear encoded except the gene for the large subunit of Rubisco which is in the chloroplast genome, while the small subunit gene is in the nucleus [39,[76][77][78][79]. In the dual-cell Kranz type C 4 plants, cell specific control of transcription of nuclear genes may contribute to development of dimorphic chloroplasts.…”

Electron Microscopy Views of Dimorphic Chloroplasts in C4 Plants

“…Theories for explaining how proteins from nuclear encoded genes are selectively targeted include selective protein degradation, mRNA targeting, and selective import (Offermann et al, 2015;Erlinghaeuser et al, 2016). Wimmer et al provided evidence that transit peptides of B. sinuspersici chloroplast proteins have two elements, one for general chloroplast import and the other for selective targeting (Wimmer et al, 2017). Plastidtargeted pyruvate orthophosphate-dikinases (PPDKs) in C4 plants regenerate phosphoenolpyruvate, the CO2 acceptor in C4 cycle.…”

“…Wimmer et al. provided evidence that transit peptides of B. sinuspersici chloroplast proteins have two elements, one for general chloroplast import and the other for selective targeting ( Wimmer et al., 2017 ). Plastid-targeted pyruvate orthophosphate-dikinases (PPDKs) in C4 plants regenerate phosphoenolpyruvate, the CO2 acceptor in C4 cycle.…”

Comparative Chloroplast Genomics of C3, Kranz type C4 and Single Cell C4 photosynthetic members of Chenopodiaceae

“…The eight chloroplast genomes studied, include the C3 species S. maritima and seven forms of Currently, all enzymes required in chloroplasts to support the C4 cycle and Calvin-Benson cycle are considered to be nuclear-encoded except the gene for the large subunit of Rubisco, which is in the chloroplast genome, while the small subunit gene is in the nucleus [75][76][77][78][79]. In the dual-cell Kranz type…”