Mitochondrial translation initiation machinery: conservation and diversification

Abstract

The highly streamlined mitochondrial genome encodes almost exclusively a handful of transmembrane components of the respiratory chain complex. In order to ensure the correct assembly of the respiratory chain, the products of these genes must be produced in the correct stoichiometry and inserted into the membrane, posing a unique challenge to the mitochondrial translational system. In this review we describe the proteins orchestrating mitochondrial translation initiation: bacterial-like general initiation factors mIF2 and mIF3, as well as mitochondria-specific components - mRNA-specific translational activators and mRNA-nonspecific accessory initiation factors. We consider how the fast rate of evolution in these organelles has not only created a system that is divergent from that of its bacterial ancestors, but has led to a huge diversity in lineage specific mechanistic features of mitochondrial translation initiation among eukaryotes.

Keywords: IF2; IF3; Mitochondria; Ribosome; Translational activators.

Fig. 1

Domain organization of IF2 and mIF2. Location and sequence alignment of the mIF2 insertion region is shown for a set of representative species. The yellow highlighting shows the taxonomic limits of the conserved insertion region. See Ref. for a larger alignment.

Fig. 2

Human mIF3 rescues an S. cerevisiae strain lacking the genomic copy of AIM23, whereas E. coli IF3 has a weak, but detectable complementation activity. (A) mIF3/IF3 consensus sequences calculated at the 60% level using the Python script Consensus Finder . See Ref. for a larger alignment and three-dimensional location of conserved sites. Domain organization is indicated on the ruler above the alignment. (B) Restoration of mitochondrial functionality was assessed by growth of yeast strains on non-fermentable media YPGly requiring mitochondrial respiration. The genomic copy of AIM23 was knocked out with a gentamicin cassette resulting in AIM23Δ strain, which was complemented with plasmids expressing mIF3 from S. cerevisiae (WT), mIF3 from H. sapiens (AIM23Δ_pHs), or IF3 from E. coli fused with AIM23 mitochondrial import signal (AIM23Δ_pEc) under the control of S. cerevisiae 5′ and 3′ flanking regions. Yeast suspensions were spotted on the plate in ten-fold serial dilutions (OD₆₀₀ is indicated above the spots) and incubated at 30 °C for 48 and 72 h.