Growth, Differentiation and Sexuality

cumulation of mitochondria in the bud, and (4) two
MYO2 alleles that showed genetic interactions with
YPT11 also affected mitochondrial inheritance.
These findings raise the possibility that
Myo2p and Ypt11p may drive mitochondrial
movement during inheritance. However, recent
studies indicate that neither protein is required for
mitochondrial motility in budding yeast (Boldogh
et al. 2004). That is, deletion of YPT11, reduction
of the length of the Myo2p lever arm (myo2–Δ6IQ),
or deletion of MYO4, the other type V myosin of
yeast, have no effect on mitochondrial morphology,
co-localization of mitochondria with actin
cables, or the velocity of bud-directed movement
of mitochondria. By contrast, retention of mitochondria
in the bud, a process that results in the
immobilization and accumulation of mitochondria
in the bud tip, was compromised in YPT11 and
MYO2 mutants. Since Myo2p and Ypt11p localize
to the bud tip, they may serve as capture devices
that contribute to the retention of mitochondria in
the bud tip. Alternatively, Myo2p and/or Ypt11p
may mediate the transport of retention factors or
mRNA that encode retention factors to the bud tip.
An alternative, motor-independent mechanism
for mitochondrial movement in budding
yeast has been identified (Boldogh et al. 2001a).
This mechanism is similar to that used by bacterial
and viral pathogens for their movement through
the cytoplasm of infected cells. Propulsion of these
pathogens occurs by Arp2/3 complex-mediated
nucleation of F-actin. This evolutionarily conserved,
seven-subunit complex is regulated by
a variety of nucleation-promoting factors such
as WASp. In its activated state, Arp2p and Arp3p
subunits of the complex serve as platforms for
the nucleation of F-actin. In addition, the Arp2/3
complex can bind to the pointed, slow-growing
end of F-actin, allowing for the addition of actin
monomers at the barbed, fast-growing end. Finally,
Arp2/3 complex can bind to the lateral surface of
F-actin, creating and stabilizing filament branches
(reviewed by Pollard and Beltzner 2002). These activities
of the Arp2/3 complex lead to the assembly
of a higher-order dendritic array at the membrane
surface, eventually leading to a branched network
of actin filaments in vivo and in vitro (Svitkina
and Borisy 1999; Volkmann et al. 2001).
In budding yeast, the Arp2/3 complex is associated
with endosomes (actin patches) and vacuoles
(Moreau et al. 1996; Winter et al. 1997; Insall et al.
2001; Eitzen et al. 2002; Chang et al. 2003). Several
findings also support a role for the Arp2/3
Organelle Inheritance in Fungi 27
complex in driving mitochondrial movement during
inheritance in budding yeast (Boldogh et al.
2001a). First, Arp2/3 complex subunits co-localize
with mitochondria in intact cells and are recovered
with mitochondria after subcellular fractionation.
Second, Arp2/3 complex activity is detected on mitochondria
in living yeast and in isolated yeast
mitochondria. Third, mitochondrial movement requires
constant actin assembly and disassembly,
and is impaired by an agent that perturbs actin dynamics.
Fourth, mutations in Arp2/3 complex subunits
inhibit mitochondrial movements but have
no effect on the co-localization of mitochondria
with actin cables. These findings indicate that the
Arp2/3 complex is associated with yeast mitochondria,
and that mitochondria use Arp2/3-complexdriven
actin assembly to drive their movement during
inheritance.
One feature that distinguishes mitochondrial
movement from other Arp2/3 complex-driven
movements is track dependence. In contrast to
endosomes and bacterial pathogens that undergo
non-linear movement, yeast mitochondria
use actin cables as tracks for linear anterograde
and retrograde movement during cell
division. Molecules that contribute to the association
of mitochondria with actin cables were
revealed by visual screens for mutations that
interfere with normal mitochondrial distribution
and morphology (MDM) and mitochondrial
morphology maintenance (MMM; McConnell et al.
1990; Burgess et al. 1994; Sogo and Yaffe 1994;
Hermann et al. 1997). Three of the genes identified
(MMM1, MDM10, and MDM12) encode integral
membrane proteins that form a complex in the mitochondrial
outer membrane (Boldogh et al. 2003).
Two lines of evidence support a role for the
Mmm1p/Mdm10p/Mdm12p complex in the association
of mitochondria to actin cables during movement
and inheritance. First, deletion of any subunit
in the complex results in the loss of all anterograde
and retrograde mitochondrial movement,
and produces a pattern of mitochondrial movement
similar to that observed upon destabilization
of actin cables (Boldogh et al. 2001a, 2003; Fehrenbacher
et al. 2004). Second, subunits of this complex
are required for reversible, ATP-sensitive binding
of mitochondria to F-actin in vitro (Boldogh
et al. 2001a). These observations support a role
for Mmm1p/Mdm10p/Mdm12p in mediating interactions
between mitochondria and actin cables
through cyclic, reversible, ATP-sensitive binding of
mitochondria to F-actin within actin cables.