Role of Tom5 in maintaining the structural stability of the TOM complex of mitochondria.

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.