Plant Mitochondrial Inner Membrane Protein Insertion

Abstract

During the biogenesis of the mitochondrial inner membrane, most nuclear-encoded inner membrane proteins are laterally released into the membrane by the TIM23 and the TIM22 machinery during their import into mitochondria. A subset of nuclear-encoded mitochondrial inner membrane proteins and all the mitochondrial-encoded inner membrane proteins use the Oxa machinery-which is evolutionarily conserved from the endosymbiotic bacterial ancestor of mitochondria-for membrane insertion. Compared to the mitochondria from other eukaryotes, plant mitochondria have several unique features, such as a larger genome and a branched electron transport pathway, and are also involved in additional cellular functions such as photorespiration and stress perception. This review focuses on the unique aspects of plant mitochondrial inner membrane protein insertion machinery, which differs from that in yeast and humans, and includes a case study on the biogenesis of Cox2 in yeast, humans, two plant species, and an algal species to highlight lineage-specific similarities and differences. Interestingly, unlike mitochondria of other eukaryotes but similar to bacteria and chloroplasts, plant mitochondria appear to use the Tat machinery for membrane insertion of the Rieske Fe/S protein.

Keywords: Oxa; membrane insertion; plant mitochondria; twin-arginine translocation.

Figure 1

Mitochondrial protein transport and inner membrane insertion pathways. The MIM proteins can follow several routes for membrane insertion. There are the two translocase complexes in the inner membrane, TIM22 and TIM23, which laterally insert nuclear-encoded proteins into the MIM. Proteins with internal targeting signals are inserted into MIM by TIM22, while those with cleavable N-terminal targeting signals are inserted by TIM23. The conservatively sorted proteins are first targeted to the matrix by TIM23 and then inserted to the MIM in a manner reminiscent of bacterial inner membrane protein insertion. These conservatively sorted and mitochondrial-encoded proteins require Oxa1 or Cox18 (Oxa2) for membrane insertion. The newest member for conservative sorting is the Bcs1 protein for membrane insertion of the Rieske Fe/S protein. MOM = mitochondrial outer membrane, IMS = intermembrane space, MIM = mitochondrial inner membrane, MPP = mitochondrial processing peptidase, Oxa = oxidase assembly factor, TIM = translocase of the inner membrane.

Figure 2

Phylogenetic and structural analysis of the YidC/Oxa1/Alb3 family and the mitochondrial-encoded membrane proteins of Arabidopsis thaliana. (A) A phylogenetic tree was generated by Clustal omega based on the full-length protein sequences for the indicated species of the YidC/Oxa1/Alb3 family [60]. The numbers next to each node represent the measure of support for the node. The conserved five TMH secondary structures are found in all members (except archaea) of the protein family. Differences at the C-terminal ends are indicated by different colors: pink = ribosome-binding, blue = TPR domain, green = CP43-interacting. (B) Transmembrane topologies of the 20 putative MIM proteins encoded in the mitochondrial genome of Arabidopsis thaliana. The proteins are displayed in an N- to C-terminus orientation going from left to right.

Figure 3

Cox2 biogenesis in yeast, mammals, Arabidopsis, Glycine, and Polytomella. Our current understanding of how Cox2 is inserted into the MIM of yeast (Saccharomyces cerevisiae) and mammals (Homo sapiens) is as depicted. The four steps of Cox2 assembly are N-tail export with TMH1 membrane insertion, N-tail processing, C-tail translocation with TMH2 membrane insertion and maturation by copper insertion. The last arrow represents the assembly step into complex IV. Based on this knowledge and some limited available studies with plant mitochondria, we have hypothesized the Cox2 insertion pathways for two plant species, Arabidopsis thaliana, Glycine max, and an algal species, Polytomella sp. In yeast, mammals, and Arabidopsis, Cox2 is synthesized by the mitochondrial ribosome whereas in Glycine and Polytomella, it is cytosolically synthesized and imported into the mitochondria. For full details, see the corresponding sections in the text.