Fig. 6. Schematic representation of mitochondrial simplification upon glucose repression in wild-type and fis1 cells. Numbers indicate characteristic time points within the ongoing remodelling process. At time point zero, the mitochondrial networks exhibit morphologies typical for cells grown in medium containing glycerol. (A) In wild-type cells, the daughter cell is invaded by a single mitochondrial tubule shortly after budding. Subsequently the tubule attaches to the cell cortex within the emerging bud (arrow). During subsequent growth the mitochondrion remains attached at this site while the remaining part of the network is moving freely within the daughter cell. During budding, the connection between the mitochondrial compartments of mother and daughter cell is frequently disrupted. Concomitant with the transport of mitochondrial membranes into the daughter cell the network undergoes constantly fission and fusion. Most fusion events occur between two mitochondrial tips. It is conceivable that mitochondrial interactions with cytoskeletal elements are important for these processes (Boldogh et al., 2001). (B) The mitochondria of fis1 cells grown on glycerol are frequently large fenestrated nets located at one side of the cell cortex. In fis1 mitochondria simplification appears not to be directly correlated to budding. Upon glucose repression, single meshes of the fenestrated network are opened by fission. These fission events lead to an ongoing size reduction of the fenestrated network. Membrane fusions extend the (mostly one or two) tubules emerging from the net. The long tubules are frequently separated from the fenestrated net by fission. They subsequently refuse to the same or to a different site. Upon budding one of these single tubules penetrates into the emerging daughter cell. As observed in the wildtype, the tubule attaches with one site at the cortex of the daughter cell (arrow). The rest of the tubule moves freely within the bud. Simplifying a large fenestrated net to an almost tubular structure most probably requires several cell cycles.

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