Role of Tom5 in Maintaining the Structural Stability of the TOM Complex of Mitochondria

Simone Schmitt,Uwe Ahting,Lutz Eichacker,Bernhard Granvogl,Nancy E Go,Frank E Nargang,Walter Neupert,Stephan Nussberger

doi:10.1074/jbc.m413667200

Abstract

Transport of nuclear encoded proteins into mitochondria is mediated by multisubunit translocation machineries in the outer and inner membranes of mitochondria. The TOM complex contains receptor and pore components that facilitate the recognition of preproteins and their transfer through the outer membrane. In addition, the complex contains a set of small proteins. Tom7 and Tom6 have been found in Neurospora and yeast, Tom5 has been found so far only in the latter organism. In the present study, we identified Neurospora Tom5 and analyzed its function in comparison to yeast Tom5, which has been proposed to play a role as a receptor-like component. Neurospora Tom5 crosses the outer membrane with its carboxyl terminus facing the intermembrane space like the other small Tom components. The temperature-sensitive growth phenotype of the yeast TOM5 deletion was rescued by overexpression of Neurospora Tom5. On the other hand, Neurospora cells deficient in tom5 did not exhibit any defect in growth. The structural stability of TOM complexes from cells devoid of Tom5 was significantly altered in yeast but not in Neurospora. The efficiency of protein import in Neurospora mitochondria was not affected by deletion of tom5, whereas in yeast it was reduced as compared with wild type. We conclude that the main role of Tom5, rather than being a receptor, is maintaining the structural integrity of the TOM complex.

Highlights

Transport of nuclear encoded proteins into mitochondria is mediated by multisubunit translocation machineries in the outer and inner membranes of mitochondria
The amino acid sequence of Tom5 exhibits a hydropathy profile with one predicted transmembrane segment near the carboxyl terminus, a feature conserved in the proteins of B. fuckeliana and S. cerevisiae (Fig. 1C)
With the identification of Tom5 in Neurospora we provide further evidence that the TOM complexes of Neurospora and yeast contain an identical set of subunits

Summary

Introduction

Transport of nuclear encoded proteins into mitochondria is mediated by multisubunit translocation machineries in the outer and inner membranes of mitochondria. Yeast mitochondria lacking Tom were reported to have severe defects in the import of proteins destined for the outer membrane [19], intermembrane space [20], inner membrane, and matrix compartment [18]. This was taken to suggest that Tom is crucial for protein import to all mitochondrial subcompartments. Its primary structure revealed a cytosolic domain that carries a net negative charge This feature led to the hypothesis that Tom guides positively charged mitochondrial targeting sequences from the surface receptors to the protein-conducting channel [18]. The structure did not provide a hydrophobic surface that could favorably interact with the hydrophobic face of amphiphilic mitochondrial presequences

Methods

Results

Conclusion