Laboratory of Organelle Biology
Institute for Protein Research

Associate Professor

Masato NAKAI

nakai

URL: http://www.protein.osaka-u.ac.jp/enzymology/
The basic unit of the animal andplant body is the cell. Within thesecells are intracellular organelles
surrounded by biologicalmembranes called organelles,
such as the nucleus, mitochondria,
peroxisomes, and chloroplasts,
which are responsible for avariety of metabolic processes.
How did eukaryotic cells withcomplex organelles arise from thefirst bacterial prokaryotic cells onearth, which were surrounded bya simple membrane structure?
This is the result of a longevolutionary process in which
bacteria symbiotic in the host cell,from which eukaryotic cells
originated, became organelles.
Our research focuses onchloroplasts of plants and algae,
focusing on the protein transportsystems that have been
established in association withorganelle formation.
The Mystery of Chloroplast
Evolution, which Started from
Intracellular Symbiosis
Chloroplasts are the site of photosynthesis
and support many forms of life on Earth.
Chloroplasts originated about a billion years
ago when photosynthetic prokaryotes such as
cyanobacteria formed intracellular symbioses
with eukaryotes, which have nuclei andmitochondria. Many of the endosymbiont
genes have since been transferred to thenuclear genome of the host, and more than
2,000 chloroplast proteins, including newly
added ones, are now encoded in the nuclear
genome. Since the synthesis of these proteinstakes place outside the chloroplast (in thecytosol), a system that specifically transports
only chloroplast proteins had to beestablished in the membrane surrounding thechloroplast (Fig. 1). We succeeded inpurifying the TIC translocon, a proteintransport system of the chloroplast inner
membrane, as a supramolecular complex
with a molecular weight of 1 million for the
first time in the world, and identified all of
its components (Science, 2013).

This finding also suggests thatchanges in the chloroplast
protein delivery system may
have contributed to theevolution of green algae andland plants. We will explore
the mysteries of chloroplastevolution, such as why a hugemembrane permeabilizer witha molecular weight of 1 million
was necessary and how it wasestablished (PNAS, 2020).

Figure 1. chloroplasts by endosymbiosis of cyanobacteria.

Elaborate mechanism by which cells
send only chloroplast proteins tochloroplasts
Transport of macromolecules such asproteins across biological membranes requires
sophisticated molecular devices - translocons that allow proteins to pass through themembrane while maintaining the membrane
barrier. Life has produced several different
types of translocons during the course ofevolution. Because of the different membrane
systems and evolutionary backgrounds inwhich they emerged, their components andtransport mechanisms are quite different. The
above-mentioned translocon TIC in thechloroplast inner envelope, the recently
identified novel ATP-dependent transport
motor complex with a molecular weight of 2
million that works in association with TIC(Plant Cell, 2018), and the translocon TOC in
the outer envelope with a molecular weight of
10,000,100000, are all examples of the type oftranslocons that are being generated by thesemegacomplexes. We will reveal the elaboratemechanism of chloroplast protein transport
through functional coordination, including
plant genetic engineering (Fig. 2) andstructural biology (Fig. 3). We are close toelucidating the principles of buildingFigure 2. albino trait exhibited by an Arabidopsis
mutant defective in the protein membrane
permeabilizer of the chloroplast envelope.

Figure 3.
Megacomplexes
involved in chloroplast
protein transport

Let's enjoy research together withhigh aspirations, aiming forbreakthroughs against the world!

References
A distinct class of GTP-binding proteins mediates chloroplast protein
import in Rhodophyta. PNAS 119:e2208277119 (2022)
Tic12, a 12-kDa essential component of the translocon at the innerenvelope membrane of chloroplasts in Arabidopsis. Plant
Cell 34:4569-82 (2022)
Coexpressed subunits of dual genetic origin difine a conserved
supercompex mediating essential protein import intochloroplasts PNAS 117:32739-49 (2020)
A Ycf2-FtsHi heteromeric AAA-ATPase complex is required forchloroplast protein import. Plant Cell 30:2677-703(2018)
Uncovering the Protein Translocon at the Chloroplast Inner
Envelope Membrane. Science 339:571-4(2013)

Institute for Protein Research, Osaka University
3-2 Yamadaoka, Suita, Osaka 565-0871, Japan
TEL: +81-6-6879-8612
FAX : +81-6-6879-8613
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