Abstract
The mitochondrial import machinery and the respiratory chain complexes of the inner membrane are highly interdependent for the efficient import and assembly of nuclear encoded respiratory chain components and for the generation of a proton motive force essential for protein translocation into or across the inner membrane. In plant and non-plant systems functional, physical, and evolutionary associations have been observed between proteins of the respiratory chain and protein import apparatus. Here we identify two novel Tim21-like proteins encoded by At2g40800 and At3g56430 that are imported into the mitochondrial inner membrane. We propose that Tim21-like proteins may associate with respiratory chain Complex I, III, in addition to the TIM17:23 translocase of the inner membrane. These results are discussed further with regards to the regulation of mitochondrial activity and biogenesis.
Highlights
Mitochondria are membrane bound organelles that play essential roles in metabolism, energy production, and biosynthesis of a variety of compounds in almost all eukaryotic cells
Two additional Tim21 domain containing proteins are identified via the Conserved Domain Database (Marchler-Bauer et al, 2011), termed AtTim21-like 1 and AtTim21-like 2 encoded by At2g40800 and At3g56430 respectively (Figure 1A)
Both are predicted to be significantly larger proteins of 414 and 434 aa compared to AtTim21 (269 aa) and ScTim21 (239 aa) and predicted to contain N-terminal mitochondrial targeting signals (Figure 1A)
Summary
Mitochondria are membrane bound organelles that play essential roles in metabolism, energy production, and biosynthesis of a variety of compounds in almost all eukaryotic cells. The core components and mechanisms involved in mitochondrial protein import and assembly were established in the earliest eukaryotes with ancient origins of many components of the protein import apparatus (Albrecht et al, 2010; Delage et al, 2011; Hewitt et al, 2011) This is evidenced by the presence of many core and central components, such as the translocase of the outer membrane (TOM), sorting and assembly machinery (SAM), and the translocases of the inner membrane (TIM), in stramenophiles and hydrosomes (Lithgow and Schneider, 2010; Delage et al, 2011; Hewitt et al, 2011; Heinz and Lithgow, 2013). The selective pressure to maintain protein targeting specificity and efficiency has led to the evolution of unique and specialized features for the regulation of mitochondrial biogenesis in plants (Duncan et al, 2013; Murcha et al, 2014)
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