Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

THE GENETIC SYSTEMS OF MITOCHONDRIA AND CHLOROPLASTS
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FISSION
FUSION
(A)
(B)
5 µm
(C)
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.)
MBoC6 m14.55/14.62
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
Comparisons of DNA sequences in different organisms reveal that, in vertebrates
(including ourselves), the mutation rate during evolution has been roughly 100
times greater in the mitochondrial genome than in the nuclear genome. This
difference is likely to be due to lower fidelity of mitochondrial DNA replication,
Figure 14–63 A model for mitochondrial division. Dynamin-1 (yellow)
exists as dimers in the cytosol, which form larger oligomeric structures in a
process that requires GTP hydrolysis. Dynamin assemblies interact with the
outer mitochondrial membrane through special adaptor proteins, forming a
spiral of GTP-dynamin around the mitochondrion that causes a constriction.
A concerted GTP-hydrolysis event in the dynamin subunits is then thought to
produce the conformational changes that result in fission. (Adapted from S.
Hoppins, L. Lackner and J. Nunnari, Annu. Rev. Biochem. 76:751–780, 2007.)
dynamin-1
GTP
mitochondrion
targeting
assembly-driven constriction
P i
hydrolysis-driven constriction
fission