bims-plasge 2019-11-17 papers

Issue of 2019–11–17
two papers selected by
Vera S. Bogdanova, ИЦиГ СО РАН

J Exp Bot. 2019 Nov 15. pii: erz517. [Epub ahead of print]

Origins, function and regulation of the TOC-TIC general protein import machinery of plastids.

The evolution of chloroplasts from the original endosymbiont involved the transfer of thousands of genes from the ancestral bacterial genome to the host nucleus, thereby combining the two genetic systems to facilitate coordination of gene expression and achieve integration of host and organelle functions. A key element of successful endosymbiosis was the evolution of a unique protein import system to selectively and efficiently target nuclear-encoded proteins to their site of function within the chloroplast after synthesis in the cytoplasm. The chloroplast TOC-TIC general protein import system is conserved across the plant kingdom, and is a system of hybrid origin, with core membrane transport components adapted from bacterial protein targeting systems, and additional components adapted from host genes to confer the specificity and directionality of import. In vascular plants, the TOC-TIC system has diversified to mediate the import of specific, functionally related classes of plastid proteins. This functional diversification occurred as the plastid family expanded to fulfill cell- and tissue-specific functions in terrestrial plants. In addition, there is growing evidence that direct regulation of TOC-TIC activities plays an essential role in the dynamic remodeling of the organelle proteome that is required to coordinate plastid biogenesis with developmental and physiological events.

Keywords: chloroplast biogenesis; chloroplast protein import; endosymbiosis; protein quality control; protein targeting; transit peptide

DOI: https://doi.org/10.1093/jxb/erz517

Proc Natl Acad Sci U S A. 2019 Nov 15. pii: 201911251. [Epub ahead of print]

The retrograde signaling protein GUN1 regulates tetrapyrrole biosynthesis.

Takayuki Shimizu, Sylwia M Kacprzak, Nobuyoshi Mochizuki, Akira Nagatani, Satoru Watanabe, Tomohiro Shimada, Kan Tanaka, Yuuki Hayashi, Munehito Arai, Dario Leister, Haruko Okamoto, Matthew J Terry, Tatsuru Masuda.

The biogenesis of the photosynthetic apparatus in developing seedlings requires the assembly of proteins encoded on both nuclear and chloroplast genomes. To coordinate this process there needs to be communication between these organelles, but the retrograde signals by which the chloroplast communicates with the nucleus at this time are still essentially unknown. The Arabidopsis thaliana genomes uncoupled (gun) mutants, that show elevated nuclear gene expression after chloroplast damage, have formed the basis of our understanding of retrograde signaling. Of the 6 reported gun mutations, 5 are in tetrapyrrole biosynthesis proteins and this has led to the development of a model for chloroplast-to-nucleus retrograde signaling in which ferrochelatase 1 (FC1)-dependent heme synthesis generates a positive signal promoting expression of photosynthesis-related genes. However, the molecular consequences of the strongest of the gun mutants, gun1, are poorly understood, preventing the development of a unifying hypothesis for chloroplast-to-nucleus signaling. Here, we show that GUN1 directly binds to heme and other porphyrins, reduces flux through the tetrapyrrole biosynthesis pathway to limit heme and protochlorophyllide synthesis, and can increase the chelatase activity of FC1. These results raise the possibility that the signaling role of GUN1 may be manifested through changes in tetrapyrrole metabolism, supporting a role for tetrapyrroles as mediators of a single biogenic chloroplast-to-nucleus retrograde signaling pathway.

Keywords: chloroplast; gun mutants; heme; retrograde signaling; tetrapyrrole

DOI: https://doi.org/10.1073/pnas.1911251116