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Mitochondria are branched, tubular structures surrounded by a double membrane, that continually undergo fission and fusion, both within single tubes and at branch points. In cells of the budding yeast Saccharomyces cerevisiae, the mitochondria form a highly interconnected reticulum distributed beneath the cell cortex. Regulation of this dynamic behavior is important for maintaining organelle structure and inheritance during growth, mating, and sporulation. The large GTPase Dnm1p is important for regulating fission of the outer membrane. Dnmp1 is highly related to dynamin, another large GTPase known to regulate scission of the neck, connecting budding clathrin coated pits with the donor membrane.

A temporal and spatial analysis of data obtained using a combination of 3-dimensional fluorescence imaging with time-lapse acquisition methods (Legesse-Miller et al. [2003] Mol. Biol. Cell. 14, 1953-1963) reveals abundant fluctuations in the thickness of the matrix, most of them unrelated to fission events. It also shows the assembly of Dnm1p fused to EGFP into discrete patches of different shapes, such as rings wrapping around a mitochondrion or clusters located on one side of a mitochondrial tube or branch. These clusters and rings constantly change in their shape and size, sometimes disappearing completely, even though the underlying mitochondrion has not engaged in a fission event. It is clear, at least in yeast cells, the Dnm1p cycles through different stages of spatial organization on the mitochondrion surface, with no strict correlation with scission. This behavior may represent a mechanism for sampling the physical state of the mitochondrial tube, so that, when constriction occurs at a Dnm1p spot, scission can then ensue. Two consecutive time-lapse 3-dimensional renditions of mitochondria undergoing fission from a yeast cell expressing Dnm1p-EGFP and mito-RFP are depicted on the cover. Bar, 2μm

-Tomas Kirchhausen