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J. Biol. Chem., Vol. 281, Issue 25, 17312-17320, June 23, 2006

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Dimeric Dnm1-G385D Interacts with Mdv1 on Mitochondria and Can Be Stimulated to Assemble into Fission Complexes Containing Mdv1 and Fis1^*

Debjani Bhar, Mary Anne Karren, Markus Babst, and Janet M. Shaw¹

From the Biochemistry Department, University of Utah School of Medicine, Salt Lake City, Utah 84112-5650 and Biology Department, University of Utah, Salt Lake City, Utah 84112-0840

Interactions between yeast Dnm1p, Mdv1p, and Fis1p are required to form fission complexes that catalyze division of the mitochondrial compartment. During the formation of mitochondrial fission complexes, the Dnm1p GTPase self-assembles into large multimeric complexes on the outer mitochondrial membrane that are visualized as punctate structures by fluorescent labeling. Although it is clear that Fis1p·Mdv1p complexes on mitochondria are required for the initial recruitment of Dnm1p, it is not clear whether Dnm1p puncta assemble before or after this recruitment step. Here we show that the minimum oligomeric form of cytoplasmic Dnm1p is a dimer. The middle domain mutant protein Dnm1^G385Dp forms dimers in vivo but fails to assemble into punctate structures. However, this dimeric mutant stably interacts with Mdv1p on the outer mitochondrial membrane, demonstrating that assembly of stable Dnm1p multimers is not required for Dnm1p-Mdv1p association or for mitochondrial recruitment of Dnm1p. Dnm1^G385Dp is reported to be a terminal dimer in vitro. We describe conditions that allow assembly of Dnm1^G385Dp into functional fission complexes on mitochondria in vivo. Using these conditions, we demonstrate that multimerization of Dnm1p is required to promote reorganization of Mdv1p from a uniform mitochondrial localization into punctate fission complexes. Our studies also reveal that Fis1p is present in these assembled fission complexes. Based on our results, we propose that Dnm1p dimers are initially recruited to the membrane via interaction with Mdv1p·Fis1p complexes. These dimers then assemble into multimers that subsequently promote the reorganization of Mdv1p into punctate fission complexes.

Received for publication, December 20, 2005 , and in revised form, February 27, 2006.

^* This work was funded by National Institutes of Health Grant GM53466 (to J. M. S.) and American Heart Association Grant 0530210N (to M. B.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Biochemistry, University of Utah School of Medicine, Salt Lake City, UT 84112-5650. Tel.: 801-585-6205; Fax: 801-581-7959; E-mail: shaw{at}biochem.utah.edu.

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