Mitochondrial dynamics, protein targeting, yeast molecular genetics. Our laboratory is studying the biogenesis and the dynamics of an essential cellular organelle, the mitochondrion. We are asking how new mitochondria are assembled during the growth of cells (mitochondrial biogenesis), and we are asking how cells control the number, the shape, and the location of their mitochondria (mitochondrial dynamics).

Mitochondrial Biogenesis: Import of Proteins from the Cytosol. One key facet of mitochondrial assembly is the import of newly synthesized mitochondrial proteins from the cytosol. Specifically, most mitochondrial proteins are encoded on nuclear genes and synthesized as precursor proteins in the cytoplasm. These precursors are imported into the organelle via a multi-step pathway that includes binding to surface receptors on the mitochondrial outer membrane, unfolding of the precursor, translocation across both mitochondrial membranes, processing to the mature form, refolding of the imported protein, and assembly into multi-subunit enzyme complexes. Using the tools of yeast molecular genetics, we have identified much of the machinery that mediates the import process. Using biochemical and cell biological techniques, we are determining the molecular mechanisms by which this machinery functions. For example, our work suggests that the mitochondrial inner membrane (IM) carries at least two protein-translocating pores. The mitochondrial IM, however, must remain impermeable to small molecules. Consequently, an important goal is to understand how these translocons are regulated, allowing the transport of specific protein substrates. Furthermore, while many proteins are translocated completely across the mitochondrial IM, others are inserted into the bilayer. We are trying to understand how the import machinery sorts proteins into different mitochondrial locations.

Mitochondrial Dynamics: Morphology, Movement, Division and Fusion. While much is known about how proteins are imported into mitochondria, little is known about how mitochondria divide, how mitochondria fuse, how the organelles are transmitted to daughter cells during cell division, or how mitochondria structure is established and maintained. To address these questions, we have isolated and are analyzing yeast mutants defective in the number, the location and the shape of their mitochondria. We have identified several mutants defective in division. In these mutants, mitochondria have lost their normal tubular structure, and instead form a large network of interconnected tubules. We also isolated mutants defective in mitochondrial fusions, and these strains contain numerous mitochondrial fragments. In another class of mutants we find that normal mitochondrial shape is defective. Instead of the long, tubular organelles seen in wild-type cells, we find that mitochondria in these mutants are large, spherical structures. These mutants also rapidly lose mitochondrial DNA. We have used all of our mutants to identify mitochondrial proteins required for division, fuse, shape and segregation, and are continuing our analyses to determine how these proteins function in mitochondrial dynamics.

Publications:

Cerveny, K.L., Studer, S.L., Jensen, R.E., and Sesaki, H. (2007) Yeast mitochondrial division and distribution require the cortical Num1 protein. Dev. Cell 12: 363-375.
PubMed Reference

Meisinger, C., Pfannschmidt, S., Rissler, M., Milenkovic, D., Becker, T., Stojanovski, D., Youngman, M.J., Jensen, R.E., Chacinska, A., Guiard, B., Pfanner, N., and Wiedemann, N. (2007) The morphology proteins Mdm12/Mmm1 function in the major beta-barrel assembly pathway of mitochondria. EMBO J. 26: 2229-2239.
PubMed Reference

Davis, A.J., Alder, N.N., Jensen, R.E., and Johnson, A.E. (2007) The Tim9p/10p and Tim8p/13p complexes bind to specific sites on Tim23p during mitochondrial protein import. Mol. Biol. Cell 18: 475-486.
PubMed Reference

Sesaki, H., Dunn, C.D., Iijima, M., Shepard, K.A., Yaffe, M.P., Machamer, C.E., and Jensen, R.E. (2006) Ups1p, a conserved intermembrane space protein, regulates mitochondrial shape and alternative topogenesis of Mgm1p. J. Cell Biol. 173: 651-658.
PubMed Reference

Jensen, R.E. and Sesaki, H. (2006) Ahead of the curve: mitochondrial fusion and phospholipase D. Nat. Cell Biol. 8: 1215-1217.
PubMed Reference

Dunn, C.D., Lee, M.S., Spencer, F.A., and Jensen, R.E. (2006) A genomewide screen for petite-negative yeast strains yields a new subunit of the i-AAA protease complex. Mol. Biol. Cell 17: 213-226.
PubMed Reference

Jensen, R.E. (2005) Control of mitochondrial shape. Curr. Opin. Cell Biol. 17: 384-388.
PubMed Reference

Everard-Gigot, V., Dunn, C.D., Dolan, B.M., Brunner, S., Jensen, R.E., and Stuart, R.A. (2005) Functional analysis of subunit e of the F1Fo-ATP synthase of the yeast Saccharomyces cerevisiae: importance of the N-terminal membrane anchor region. Eukaryot. Cell 4: 346-355.
PubMed Reference

Sesaki, H. and Jensen, R.E. (2004). Ugo1p links the Fzo1p and Mgm1p GTPases for mitochondrial fusion. J. Biol. Chem. in press.

Jensen R.E., Dunn C.D., Youngman M.J., Sesaki H. (2004) Mitochondrial building blocks. Trends Cell Biol. 14: 215-8.
PubMed Reference

Johnson, A.E. and Jensen, R.E. (2004) Barreling through the membrane. Nat Struct Mol Biol 11:113-114.
PubMed Reference

Youngman, M.J., Aiken Hobbs, A.E., Burgess S.M., Srinivasam, M. and Jensen, R.E. (2004) Mmm2p, a mitochondrial outer membrane protein required for yeast mitochondrial shape and maintenance of mtDNA nucleoids. J Cell Biol 164:677-688.
PubMed Reference

Sesaki H., Southard S.M., Hobbs A.E., and Jensen R.E. (2003) Cells lackingPcp1p/Ugo2p, a rhomboid-like protease required for Mgm1p processing, lose mtDNA and mitochondrial structure in a Dnm1p-dependent manner, but remain competent for mitochondrial fusion. Biochem Biophys Res Commun 308:276-283.
PubMed Reference

Cerveny, K.L. and Jensen, R.E. (2003) The WD-repeats of Net2p interact with Dnm1p and Fis1p to regulate division of mitochondria. Mol Biol Cell 14:4126-39.
PubMed Reference

Dunn, C.D. and Jensen, R.E. (2003) Suppression of a defect in mitochondrial protein import identifies cytosolic proteins required for viability of yeast cells lacking mitochondrial DNA.
Genetics 165:35-45.
PubMed Reference

Sesaki, H., Southard, S.M., Yaffe, M.P. and Jensen, R.E. (2003) Mgm1p, a Dynamin-related GTPase, is essential for fusion of the mitochondrial outer membrane. Mol Biol Cell 14:2342-56.
PubMed Reference

Jensen, R.E. and Dunn, C. (2002) Protein import into and across the mitochondrial inner membrane: role of the TIM23 and TIM22 translocons. Bioc Biophys Acta 1592:25-34.
PubMed Reference

Kovermann, P., Truscott, K.N., Guiard, B., Rehling, P.B., Sepuri, N.B., Müller, H., Jensen, R.E., Wagner, R. and Pfanner, N. (2002) Tim22, the essential core of the mitochondrial protein insertion complex, forms a voltage-activated and signal-gated channel.
Mol Cell 9:363-373.
PubMed Reference

Jensen, R.E. and Johnson, A.E. (2001) Opening the door to mitochondrial protein import. Nature Strut Biol 8:1008-1010.
PubMed Reference

Sesaki, H. and Jensen, R.E. (2001) UGO1 encodes an outer membrane protein required for mitochondrial fusion.
J Cell Biol 152:1-13.
PubMed Reference

Cerveny, K.L. and Jensen, R.E. (2001) Division of mitochondria requires a novel Dnm1p-interacting protein, Net2p. Mol Biol Cell 12:309-321.
PubMed Reference

Aiken Hobbs, A.E., McCaffery, J.M. and Jensen, R.E. (2001) Mmm1p, a mitochondrial outer membrane protein, is connected to mitochondrial DNA nucleoids and required for mtDNA stability. J Cell Biol 152:401-410