Figure 14–61 Biogenesis of the respiratory-chain proteins in human mitochondria. Most of the protein components of the mitochondrial respiratory chain are encoded by nuclear DNa, with only a small number encoded by mitochondrial DNa (mtDNa). transcription of mtDNa produces 13 mrNas, all of which encode subunits of the oxidative phosphorylation system, and the 24 rNas (22 transfer rNas and 2 ribosomal rNas) needed for translation of these mrNas on the mitochondrial ribosomes (brown).
the mrNas produced by transcription of nuclear genes are translated on cytoplasmic ribosomes (green), which are distinct from the mitochondrial ribosomes. the nuclear-encoded mitochondrial proteins (dark green) are imported into mitochondria through two protein translocases called tOM and tIM, and constitute the vast majority of the approximately 1000 different protein species present in mammalian mitochondria. the nuclear-encoded mitochondrial proteins in humans include the majority of the oxidative phosphorylation system subunits, all proteins needed for expression and maintenance of mtDNa, and all proteins of the mitochondrial ribosomes.
the mtDNa-encoded subunits (orange) assemble together with the nuclear subunits to form a functional oxidative phosphorylation system. (adapted from
N.G. Larsson, Annu. Rev. Biochem. 79:683–706, 2010.)
Figure 14–62 The mitochondrial reticulum is dynamic. (a) In yeast cells, mitochondria form a continuous reticulum on the cytoplasmic side of the plasma membrane (stereo pair). (B) a balance between fission and fusion determines the arrangement of the mitochondria in different cells. (C) time-lapse fluorescent microscopy shows the dynamic behavior of the mitochondrial network in a yeast cell. In addition to shape changes, fission and fusion constantly remodel the network (red arrows). these pictures were taken at 3-minute intervals. (a and C, from J. Nunnari et al., Mol. Biol. Cell 8:1233–1242, 1997. With permission from the american Society for Cell Biology.) fluorescent protein (GFP), or cells can be incubated with a fluorescent dye that is specifically taken up by mitochondria because of their membrane potential. Such images demonstrate that the mitochondria in living cells are dynamic— frequently dividing by fission, fusing, and changing shape (Figure 14–62 and Movie 14.12). The fission of mitochondria may be necessary so that small parts of the network can pinch off and reach remote regions of the cell—for example in the thin, extended axon and dendrites of a neuron.
The fission and fusion of mitochondria are topologically complex processes that must ensure the integrity of the separate mitochondrial compartments defined by the inner and outer membranes. These processes control the number and shape of mitochondria, which can vary dramatically in different cell types, ranging from multiple spherical or wormlike organelles to a highly branched, net-shaped single organelle called a reticulum. Each depends on its own special set of proteins. The mitochondrial fission machine works by assembling dynamin-related GTPases (discussed in Chapter 13) into helical oligomers that cause local constrictions in tubular mitochondria. GTP hydrolysis then generates the mechanical force that severs the inner and outer mitochondrial membranes in one step (Figure 14–63). Mitochondrial fusion requires two separate machineries, one each for the outer and the inner membrane (Figure 14–64). In addition to GTP hydrolysis for force generation, both mechanisms also depend on the mitochondrial proton-motive force for reasons that are still unknown.
animal Mitochondria Contain the Simplest Genetic Systems Known