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Vol. 17, Issue 9, 3756-3767, September 2006

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The Novel F-Box Protein Mfb1p Regulates Mitochondrial Connectivity and Exhibits Asymmetric Localization in Yeast

Noriko Kondo-Okamoto^*, Kentaro Ohkuni, Katsumi Kitagawa, J. Michael McCaffery, Janet M. Shaw^*, and Koji Okamoto^*

*Department of Biochemistry, University of Utah School of Medicine, Salt Lake City, UT 84112; Department of Molecular Pharmacology, St. Jude Children’s Research Hospital, Memphis, TN 38105; and Integrated Imaging Center, Department of Biology, Johns Hopkins University, Baltimore, MD 21218

Submitted February 17, 2006; Revised June 7, 2006; Accepted June 8, 2006
Monitoring Editor: Sandra Schmid

Although it is clear that mitochondrial morphogenesis is a complex process involving multiple proteins in eukaryotic cells, little is known about regulatory molecules that modulate mitochondrial network formation. Here, we report the identification of a new yeast mitochondrial morphology gene called MFB1 (YDR219C). MFB1 encodes an F-box protein family member, many of which function in Skp1-Cdc53/Cullin-F-box protein (SCF) ubiquitin ligase complexes. F-box proteins also act in non-SCF complexes whose functions are not well understood. Although cells lacking Mfb1p contain abnormally short mitochondrial tubules, Mfb1p is not essential for known pathways that determine mitochondrial morphology and dynamics. Mfb1p is peripherally associated with the mitochondrial surface. Coimmunoprecipitation assays reveal that Mfb1p interacts with Skp1p in an F-box–dependent manner. However, Mfb1p does not coimmunoprecipitate with Cdc53p. The F-box motif is not essential for Mfb1p-mediated mitochondrial network formation. These observations suggest that Mfb1p acts in a complex lacking Cdc53p required for mitochondrial morphogenesis. During budding, Mfb1p asymmetrically localizes to mother cell mitochondria. By contrast, Skp1p accumulates in the daughter cell cytoplasm. Mfb1p mother cell-specific asymmetry depends on the F-box motif, suggesting that Skp1p down-regulates Mfb1p mitochondrial association in buds. We propose that Mfb1p operates in a novel pathway regulating mitochondrial tubular connectivity.

This article was published online ahead of print in MBC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E06-02-0145) on June 21, 2006.

Address correspondence to: Koji Okamoto (koji{at}biochem.utah.edu) or Janet M. Shaw (shaw{at}biochem.utah.edu)

Abbreviations used: 3-PGK, 3-phosphoglycerate kinase; DIC, differential interference contrast; mito-GFP, mitochondria-targeted GFP; mito-RFP, mitochondria-targeted DsRed; PK, proteinase K; PMS, postmitochondrial supernatant; TMS, transmembrane segment.

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