Life science
Metabolic engineering, Plant / Algal manipulation

Elucidation of molecular mechanisms of chloroplast protein
import and its applications
Division of Integrated Protein Functions, Institute for Protein Research
Associate Professor Masato Nakai
Researchmap https://researchmap.jp/read0011595?lang=en

Life science

Abstract
All life on earth depends on organic compounds which are produced by photosynthesis accompanied with carbon/sulfur/nitrogenassimilations. In green lineages, namely, green algae and plants,
a specialized intracellular compartment – organelle - called chloroplast plays a key role on this task. To achieve this, chloroplast
requires more than 2000 kinds of different proteins/enzymes , eachof which are correctly localized in respective subornagellar compartment inside the chloroplast. Most of these chloroplast proteins
are nuclear-encoded and synthesized in the cytosol, the outsideof chloroplast, and therefore must be transported across the double envelope membranes which surround this organelle. In recentyears, our molecular analyses have revealed mechanisms of chloroplast protein import in higher plants as well as in green algae.

transport systems in other chloroplast-containing organisms and,ultimately, will further enrich our knowledge of the evolution of thisprotein transport system as well as the evolution of various photosynthetic eukaryotes. Furthermore, as applicable tools, our studies
on protein transport to chloroplast may help to utilize chloroplasts
in green alga or in green plants as factories for foreign / recombinant proteins or compounds which might be used for the sourcesof drugs, vaccines or other therapeutic materials.

▍ Background & Results
In general, since proteins cannot pass through lipid bilayersfreely, protein translocation across biological membranes requires
supramolecular machineries called translocons. Translocons are
particularly important for eukaryotic cells which harbors various
membraneous subcellular compartments- organelles including
chloroplasts and thus have been established along with the evolution of those organelles. Chloroplasts of extant photosynthetic
eukaryotes are assumed to derive from single endosymbiotic event
between an eukaryotic ancestor and a cyanobacterium-like endosymbiont. How translocons, sophisticated molecular machineries,
arose at the double envelope membranes of chloroplast during
evolution of photosynthetic eukaryotes have attracted much attention over the years. My group has focused on elucidation of themolecular mechanisms of chloroplast protein import mainly usinga model plant, Arabidopsis thaliana. In 2013, we discovered agenuine protein translocon at the chloroplast inner envelope membrane (TIC) and, in 2018, we further identified another completely
novel protein complex (FtsHi/Ycf2 complex) at the inner envelope
membrane which functions in concert with the TIC complex asan ATP-driven protein import motor. By international collaborative
research, very recently in 2020, we determined the molecularcomponents which play an important essential role in chloroplast
protein import in a model green algae, Chlamydomonas reinhardtii.
Our findings clearly indicate that the key molecular mechanisms ofprotein translocation across the chloroplast envelope membranes
had already been established when green algae appeared onearth and has been inherited from green algal ancestor to green
plants.

Chloroplasts of photosynthetic eukaryotes originated from cyanobacterial endosymbiosis.

Early divergences of photosynthetic eukaryotes.

▍ Significance of the research and Future perspective
Our extensive research has revealed that molecular mechanismsof chloroplast protein import are highly and widely conserved
among green lineages including green algae and green plants.
Our future research efforts will clarify our understanding of protein

The use of chloroplast engineering to produce foreign compounds such as pharmaceuticals.

P a t e n t
Ramundo, Silvia; Asakura, Yukari; Nakai, Masato et al. Co-expressed subunits of dual genetic origin define a conserved supercomplex mediating essential protein import into chloroplasts. Proceedings of the National Academy of Sciences. 2020; 117(51): 32739-32749. doi: 10.1073/pnas.2014294117
UR
L http://www.protein.osaka-u.ac.jp/enzymology/nakaiJ.html
K e y w o r d plant, chloroplast, protein, green algae, molecular evolution
Tr e a t i s e