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Mitochondrial and plastid evolution in eukaryotes: an outsiders' perspective

An Erratum to this article was published on 28 July 2009

Abstract

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The eukaryotic organelles mitochondrion and plastid originated from eubacterial endosymbionts. Here we propose that, in both cases, prokaryote-to-organelle conversion was driven by the internalization of host-encoded factors progressing from the outer membrane of the endosymbionts towards the intermembrane space, inner membrane and finally the organelle interior. This was made possible by an outside-to-inside establishment in the endosymbionts of host-controlled protein-sorting components, which enabled the gradual integration of organelle functions into the nuclear genome. Such a convergent trajectory for mitochondrion and plastid establishment suggests a novel paradigm for organelle evolution that affects theories of eukaryogenesis.

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Figure 1: Putative early stages in mitochondrial and plastid organellogenesis.
Figure 2: Later events in mitochondrial and plastid organellogenesis.

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Acknowledgements

D.B. acknowledges support from the National Science Foundation and the National Institutes of Health. The authors also thank W. Lanier (Iowa) for a critical reading of the manuscript.

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Correspondence to Debashish Bhattacharya.

Supplementary information

Supplementary Table 1

Function and phylogenetic affiliation of molecular components involved in protein sorting in the mitochondrion (Saccharomyces cerevisiae) and plastid (Pisum sativum). (PDF 367 kb)

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Glossary

β-Barrel proteins

A class of membrane proteins composed of antiparallel β-strands that form barrel-type pores. Porins are typical β-barrel pores in the outer membrane of Gram-negative bacteria.

Chaperones

These are molecular components that fold, unfold, stabilize or escort the transit of protein substrates. In addition, chaperones such as the 70 kDa heat shock protein (Hsp70) and Hsp93 hydrolyse ATP to provide the energy for protein import across organelle membranes.

Endosymbiotic gene transfer

The migration and fixation of endosymbiont genes in the nuclear genome of the host.

Muller's ratchet

Describes the progressive irreversible accumulation of deleterious mutations in asexual populations. Muller's ratchet explains the genomic and physiological degeneration that is usually observed among obligatory endosymbionts.

Membrane anchor signal

This is a topogenic signal used to anchor a membrane protein in the lipid bilayer. The core of a membrane anchor signal is usually the first hydrophobic α-helix that is C-terminal to the presequence.

Presequence

This is a transient topogenic signal appended to the N-terminus of a sorted protein that is cleaved from the mature protein.

Presequence translocase-associated motor

(PAM). This is a module of the Tim23 complex associated with Hsp70 that provides the energy for translocation of proteins across the mitochondrial inner membrane. In yeast, PAM is composed of the structural platform Tim44, the co-chaperones Pam14, Pam16 and Pam18, and the nucleotide exchange factor Mge1.

Single transmembrane domain

(STMD). α-Helical STMDs are found in membrane proteins, and fold into a hydrophobic, helical structure spanning the lipid bilayer.

Sorting and assembly machinery

(SAM). The Sam complex inserts and assembles β-barrel proteins in the OM of mitochondria. In yeast, it is comprised of the protein pore Sam50 and the peripheral subunits Sam35 and Sam37.

Sorting substrate

This is a protein that is relocated by the catalytic action of a translocase or an insertase. A sorting substrate is also referred to as a precursor or a preprotein.

Tic

(Translocon at the inner envelope membrane of chloroplasts). In higher plants, the Tic complex is composed of the protein-conducting channel Tic110, the putative protein pores Tic20 and Tic21, the intermembrane space protein Tic22, the chaperone Hsp93 and its co-chaperone Toc40. Tic32, Tic55 and Tic62 are regulatory subunits.

Tim22 insertase

An insertase is a molecular machine usually consisting of a receptor and a protein pore that recognizes specific protein substrates and catalyses their insertion into the lipid bilayer of a membrane. The Tim22 insertase is specialized in the insertion of the mitochondrial carriers into the inner membrane. In yeast, it is comprised of the Tim22 protein pore and the subunits Tim54 and Tim18.

Tim23

(Translocase of the mitochondrial inner membrane). In yeast, the Tim23 and Tim17 subunits constitute the protein-conducting pore of the Tim23 complex that, in combination with Tim50 and Tim21, acts as an insertase for single transmembrane domain proteins. The addition of the PAM module confers a translocase function for the Tim23 complex.

Toc

(Translocon at the outer envelope membrane of chloroplasts). In higher plants, the Toc complex is composed of the protein-conducting pore Toc75, the receptors Toc34 and Toc159, and the accessory subunits Toc12 and Toc64.

Tom

(Translocase of the mitochondrial outer membrane). In yeast, the Tom complex is formed by the Tom40 protein-conducting pore, the receptors Tom22, Tom20 and Tom70, and the structural subunits Tom5, Tom6 and Tom7

Topogenic signal

This is a segment of amino acids in a sorted protein that provides information about its final location and conformation (topology) in a cellular compartment.

Translocon

A molecular machine usually consisting of a receptor and a protein pore that recognizes specific protein substrates and catalyses their movement across a membrane. 'Translocase' is a general term to describe an enzyme that facilitates the movement of a molecule, usually across a membrane.

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Gross, J., Bhattacharya, D. Mitochondrial and plastid evolution in eukaryotes: an outsiders' perspective. Nat Rev Genet 10, 495–505 (2009). https://doi.org/10.1038/nrg2610

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