Identification of molecular mechanisms for functional specialization of the dimorphic chloroplasts in single-cell C4 species
Final Report Abstract
Bienertia sinuspersici belongs to a small group of terrestrial plants with the ability to perform C4 photosynthesis within individual photosynthetic cells. Central to the CO2 enrichment mechanism in Bienertia and other single-cell C4 species is a highly unusual cell morphology, including two biochemical and physiological different chloroplast types. Peripheral chloroplasts specialize in production of the primary CO2 acceptor phosphoenolpyruvate, but lost the ability for final CO2 fixation via the reductive pentose phosphate cycle which instead occurs exclusively in the central chloroplasts. How the two different chloroplast types develop and co-exist within individual photosynthetic cells is currently unknow. Central to the functional specialization of the two chloroplast types is the development of specific protein complements for either regeneration of the primary CO2 acceptor as in the case of the peripheral chloroplasts, or for the reductive pentose phosphate cycle as in the case of the central chloroplasts. The majority of chloroplastic proteins involved in these two processes are nuclear encoded. A mechanism that ensures correct targeting of these proteins to the one or the other chloroplast type must therefore exist. Here we studied in detail the localization characteristics of nuclear encoded proteins, targeted to either the peripheral or central chloroplasts. Correct targeting behavior of peripheral chloroplast specific protein GFP-fusions can be reconstructed in protoplasts prepared from very young but not mature leaves, potentially indicating developmental control of the sorting process. Detailed sequence analysis revealed that the transit peptides are necessary and sufficient for peripheral chloroplast specific targeting. The transit peptides of peripheral chloroplast specific proteins carry information generally required for chloroplast import at their N-terminus, whereas information for peripheral chloroplast specificity is encoded at the C-terminus. Codon swap and base wobble experiments as well as the use of artificial mRNA baits indicate that the amino acid but not the mRNA sequence of the transit peptides are determining peripheral chloroplast specificity. Detailed mutation analysis identified a four amino acid long motif at the C-terminus of peripheral chloroplast specific proteins that is essential for correct targeting. Chimeric fusions between the transit peptides of Bienertia and a closely related C3 species indicate that the four amino acid motif is necessary but not sufficient for peripheral chloroplast specific targeting. In summary, our analysis identified a transit peptide mediated sorting mechanism that functions by blocking of peripheral chloroplast specific proteins from the central chloroplasts.
Publications
- (2014): One decade after the discovery of single-cell C4 species in terrestrial plants: what did we learn about the minimal requirements of C4 photosynthesis? Phot Res. 119(1-2):169-80
Sharpe RM & Offermann S
(See online at https://doi.org/10.1007/s11120-013-9810-9) - (2015): Developmental and Subcellular Organization of Single-Cell C4 Photosynthesis in Bienertia sinuspersici Determined by Large-Scale Proteomics and cDNA Assembly from 454 DNA Sequencing. J Prot Res. 14 (5):2090–2108
Offermann S, Friso G, Doroshenk KA, Sun Q, Sharpe RM, Okita TW, Wimmer D, Edwards GE van Wijk KJ
(See online at https://doi.org/10.1021/pr5011907) - (2016) Development, subcellular positioning and selective protein accumulation in the dimorphic chloroplasts of single-cell C4 species. Curr Opin Plant Biol. 31:76-82
Erlinghaeuser M, Hagenau L, Wimmer D, Offermann S
(See online at https://doi.org/10.1016/j.pbi.2016.03.017) - (2017) Transit peptide elements mediate selective protein targeting to two different types of chloroplasts in the single-cell C4 species Bienertia sinuspersici. Sci Rep. 23 (7):41187
Wimmer D, Bohnhorst P, Shekhar V, Hwang I, Offermann S
(See online at https://doi.org/10.1038/srep41187)
