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Published online 30 May 2006. doi:10.1083/jcb.200512079
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 173, Number 5, 645-650
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 Regulation of mitochondrial fusion by the F-box protein Mdm30 involves proteasome-independent turnover of Fzo1

Mafalda Escobar-Henriques1, Benedikt Westermann2, and Thomas Langer1

1 Institute of Genetics and Center for Molecular Medicine, University of Cologne, D-50923 Cologne, Germany
2 Institute of Cell Biology, University of Bayreuth, D-95447 Bayreuth, Germany

Correspondence to Thomas Langer: Thomas.Langer{at}uni-koeln.de

Mitochondrial morphology depends on balanced fusion and fission events. A central component of the mitochondrial fusion apparatus is the conserved GTPase Fzo1 in the outer membrane of mitochondria. Mdm30, an F-box protein required for mitochondrial fusion in vegetatively growing cells, affects the cellular Fzo1 concentration in an unknown manner. We demonstrate that mitochondrial fusion requires a tight control of Fzo1 levels, which is ensured by Fzo1 turnover. Mdm30 binds to Fzo1 and, dependent on its F-box, mediates proteolysis of Fzo1. Unexpectedly, degradation occurs along a novel proteolytic pathway not involving ubiquitylation, Skp1–Cdc53–F-box (SCF) E3 ubiquitin ligase complexes, or 26S proteasomes, indicating a novel function of an F-box protein. This contrasts to the ubiquitin- and proteasome-dependent turnover of Fzo1 in {alpha}-factor–arrested yeast cells. Our results therefore reveal not only a critical role of Fzo1 degradation for mitochondrial fusion in vegetatively growing cells but also the existence of two distinct proteolytic pathways for the turnover of mitochondrial outer membrane proteins.

Abbreviations used in this paper: CCCP, carbonylcyanide m-chlorophenylhydrazone; mtGFP, mitochondria-targeted GFP; SCF, Skp1–Cdc53–F-box; UPS, ubiquitin–proteasome system.


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