Abstract
Mitochondria and chloroplasts are two distinct organelles essential for plant viability. They evolved from prokaryotic endosymbionts and share a common ancestor with extant Gram-negative bacteria (Gray et al., 1999; Gould et al., 2008). Successful conversion of the free-living prokaryotes to the cytoplasmic organelles via endosymbiosis required conservation and adaptation of the outer membranes to the dramatic change of surroundings. In prokaryotes, the outer membrane serves as a physical barrier that protects cells from the extracellular environment and allows import of necessary nutrients, and also directly participates in interaction with other organisms (Nikaido, 2003). As part of the semi-autonomous organelles, by contrast, the outer membranes of mitochondria and chloroplasts have gained ability to participate in intracellular communication and organelle biogenesis, i.e., import and export of various ions and metabolites, import of nuclear-encoded proteins, various metabolic processes including the biosynthesis of membrane lipids, and division and movement of the organelles that require physical interaction with cytoplasmic components (Breuers et al., 2011; Inoue, 2011; Duncan et al., 2013). Our understanding of the organelle outer membranes have been advanced greatly in the last decade or so, and the last eight years have seen about a three-fold increase in the number of proteins identified or predicted to be in the chloroplast outer envelope of Arabidopsis thaliana (Arabidopsis) [total 117 proteins listed in Table Table1;1; compare 34 proteins in Inoue (2007)]. This Research Topic is intended to provide snapshots of recent research on the organelle outer membranes. It collects seven original research, three review and two method articles, which can be divided into four groups according to the subjects – (1) outer membrane protein targeting, (2) functions, targeting and evolution of protein import components, (3) lipid metabolism, and (4) method development. Table 1 One hundred and seventeen proteins identified or predicted to be in the outer membrane of the Arabidopsis chloroplast envelope.a
Highlights
Mitochondria and chloroplasts are two distinct organelles essential for plant viability
As part of the semi-autonomous organelles, by contrast, the outer membranes of mitochondria and chloroplasts have gained ability to participate in intracellular communication and organelle biogenesis, i.e., import and export of various ions and metabolites, import of nuclear-encoded proteins, various metabolic processes including the biosynthesis of membrane lipids, and division and movement of the organelles that require physical interaction with cytoplasmic components (Breuers et al, 2011; Inoue, 2011; Duncan et al, 2013)
Our understanding of the organelle outer membranes have been advanced greatly in the last decade or so, and the last eight years have seen about a three-fold increase in the number of proteins identified or predicted to be in the chloroplast outer envelope of Arabidopsis thaliana (Arabidopsis) [total 117 proteins listed in Table 1; compare 34 proteins in Inoue (2007)]
Summary
All proteins identified so far in the organelle outer membranes are encoded in the nucleus (e.g., Table 1), and most of them use internal signals for targeting. 16 of the mitochondria outer membrane proteins identified by the previous work (Duncan et al, 2013) and Marty et al (2014) are found in the chloroplast outer envelope membrane (Table 1). This may suggest the presence of targeting mechanisms and functions shared between the outer membranes of the two organelles. Day et al (2014) report phylogenetic relationships and in vitro targeting of the Toc homologs including the truncated forms of OEP80/Toc75-V, which are known as P39 (Hsueh et al, 2014) and P36 (Nicolaisen et al, 2015) (Table 1)
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