Actin comet tails, endosomes and endosymbionts
Department of Anatomy and Cell Biology, Columbia University, New
York, NY, USA
* These individuals contributed equally to this manuscript
Author for correspondence (e-mail:
lap5{at}columbia.edu)
Accepted 15 January 2003
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Summary |
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Key words: actin, cytoskeleton, endocytosis, mitochondria, organelle movement, pathogen
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The Arp2/3 complex |
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The Arp2/3 complex is regulated by a host of nucleation promoting factors (NPFs; Table· 1) that are quite diverse in both structural organization and the proteins with which they interact in the cell. All NPFs contain a conserved Arp2/3 binding sequence, or CA region, consisting of a short stretch of basic (connector region or C) and acidic (acidic region or A) amino acids that is necessary for the Arp2/3 complex activation. Binding of the CA region of NPFs to the Arp2/3 complex induces a conformation change in the complex that facilitates actin nucleation activity. However, the CA region is not sufficient for activation of the Arp2/3 complex. The NPF must also have either a G-actin binding site (Class I NPFs) or an F-actin binding site (Class II NPFs), adjacent to the CA element. In the case of Class I NPFs, presentation of G-actin by the NPF to Arp2/3 complex may facilitate formation of a nucleus for actin polymerization. The role of the F-actin binding site of Class II NPFs in activation of the Arp2/3 complex remains to be determined.
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Roles of the Arp2/3 complex |
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Decoration of the comet tails with the S1 fragment of myosin revealed that
the barbed ends of the filaments are oriented toward the bacterial surface of
each organism, indicating that polymerization is taking place on or near the
bacteria (Tilney et al., 1990;
Gouin et al., 1999
;
Heinzen et al., 1999
;
Van Kirk et al., 2000
).
However, the rates of movement of these bacteria, their direction of movement,
the morphology of their comet tails, and the organization of F-actin and
actin-binding protein composition in the actin comet tail vary in different
species. L. monocytogenes and S. flexneri have comparable
rates of movement, while R. conorii and R. rickettsii move
at about one-third that rate (Gouin et
al., 1999
; Heinzen et al.,
1999
). In addition, L. monocytogenes often moves in tight
circles, while R. rickettsii tends to track along straight lines,
changing direction only when it comes in contact with other structures in the
cell (Heinzen et al., 1999
).
Finally, while the comet tails of Listeria and Shigella tend
to be shorter and composed of highly F-actin branched structures, the tails of
the spotted fever Rickettsia are longer and contain unbranched
bundles of actin filaments
(Fig.·1). Moreover, the
half-life of the actin filaments in the comet tail of Rickettsia is
three times that of Listeria
(Tilney and Portnoy, 1989
;
Gouin et al., 1999
;
Heinzen et al., 1999
;
Van Kirk et al., 2000
).
The recruitment of different actin-binding proteins likely contributes to
the morphological differences and distinct turnover rates observed in the
actin tails of the three microorganisms. Only VASP and -actinin
localize to the actin tail of R. conorii by immunofluorescence, while
the actin tails of L. monocytogenes and S. flexneri contain
additional proteins. These include the Arp2/3 complex, profilin, gelsolin,
ezrin, coronin, capping protein and ADF/cofilin
(Loisel et al., 1999
;
Smith et al., 1996
;
Theriot et al., 1994
;
Gouin et al., 1999
;
Laine et al., 1997
;
Carlier et al., 1997
;
Rosenblatt et al., 1997
).
The mechanism of actin comet tail formation is best understood in L.
monocytogenes and S. flexneri. Listeria monocytogenes and its
closely related species Listeria ivanovii produce the surface
proteins ActA and iActA, respectively. The ActA protein is the only bacterial
protein that is required for actin-based movement of Listeria
monocytogenes. Mammalian cells transfected with the ActA gene have a
greater amount of cellular F-actin (Pistor
et al., 1994). In addition, non-motile bacteria exhibit
actin-based motility in cell extracts upon expression of ActA
(Smith et al., 1995
;
Kocks et al., 1995
). Most
significantly, ActA-coated polystyrene beads generate directional movement in
cytoplasmic extracts (Cameron et al.,
1999
).
The ActA protein shares some homology with WASP family members and
stimulates actin nucleation activity of the Arp2/3 complex
(May et al., 1999;
Welch et al., 1997a
). ActA has
VASP binding activity and colocalizes with VASP on the surface of
Listeria (Chakraborty et al.,
1995
). This suggests that the ActA protein recruits the Arp2/3
complex and stimulates Arp2/3 complex actin nucleating activity, perhaps
synergistically with VASP on the surface of Listeria. By activating
the Arp2/3 complex specifically at one pole these bacteria generate
directional movement via continuous actin assembly and the resultant
formation of an actin comet tail.
In Shigella, IcsA, previously identified as VirG
(Lett et al., 1989), is the
pathogen-encoded protein responsible for Arp2/3 complex-driven movement.
Deletion of the IcsA gene causes a loss of actin comet tail assembly
(Bernardini et al., 1989
).
E. coli expressing IcsA are able to form actin comet tails in
cytoplasmic extract (Kocks et al.,
1995
). In addition, IcsA-coated silica particles form actin comet
tails in cytoplasmic extract (Goldberg,
2001
). Taken together, these data indicate that the IcsA protein
is the only Shigella protein that is necessary to induce actin
polymerization and directed movement.
IcsA binds N-WASP via a series of glycine-rich repeats near the N
terminus of IcsA and recruits N-WASP to the trailing edge of Shigella
nearest the comet tail (Suzuki et al.,
1998). N-WASP-/- Shigella cells do not form
actin comet tails, further supporting the role of N-WASP recruitment by IcsA
in actin comet tail formation (Snapper et
al., 2001
). N-WASP then binds to and activates the Arp2/3 complex,
bringing the active actin nucleator to the bacterial surface. Surprisingly,
the in vitro interaction between IcsA and N-WASP is a more potent
stimulator of Arp2/3 activation than Cdc42
(Egile et al., 1999
).
Much less is known about the mechanism whereby the spotted fever group of
Rickettsia (including R. conorii, R. montana, R. parkeri, R.
akari, R. australis and R. rickettsii) control actin
polymerization. A member of the typhus group, R. typhi, also forms
comet tails, although these are much shorter than in the spotted fever group.
It is unclear whether the Arp2/3 complex is even involved, as constituents of
the Arp2/3 complex have not been detected in the actin tails of R.
conorii (Gouin et al.,
1999). In fact, VASP and
-actinin were the only
actin-binding proteins found in the comet tails. What is known is that
Rickettsial protein synthesis is necessary for actin-based motility,
so it may yet be that an ActA/IcsA-type protein will be found on the surface
of Rickettsia (Heinzen et al.,
1993
).
Endocytosis
Imaging studies on cells not infected with bacterial or viral pathogens
revealed that subunits of the Arp2/3 complex localize to the leading edge of
motile cells and to punctate intracellular structures
(Welch et al., 1997b). This
suggested that the function of the Arp2/3 complex was not restricted to
extension of lamellopodia. Indeed, recent studies implicate the Arp2/3 complex
and actin polymerization during endocytosis.
Endocytosis occurs in five basic steps: membrane invagination, coated pit
formation, coated pit sequestration, detachment of a newly formed vesicle, and
movement of the new endocytic compartment away from the plasma membrane into
the cytosol. A growing body of evidence suggests that the actin cytoskeleton
plays a role in each endocytic step. First, an actin cytoskeleton underlying
the plasma membrane could localize the endocytic machinery to specific regions
in the plasma membrane by providing a diffusion barrier or by anchoring
components directly (Gaidarov et al.,
1999). The actin cytoskeleton could also deform or invaginate the
plasma membrane, providing a membrane curvature, which could facilitate the
coating machinery in pinching off the membrane. Moreover, it has been shown
that a rigid cortical actin cytoskeleton can inhibit membrane traffic
(Trifaró and Vitale,
1993
), and actin fibers are essentially absent in regions
immediately surrounding clathrin-coated pits
(Fujimoto et al., 2000
).
Finally, actin could participate in membrane fission events, responsible for
freeing endocytosed vesicles from the plasma membrane
(Lamaze et al., 1997
).
Recent studies indicate that the Arp2/3 complex may participate in multiple
steps during endocytosis. The Arp2/3 complex has been implicated in membrane
internalization in Dictyostelium and budding yeast. Phagocytosis and
macropinocytosis are actin-dependent mechanisms. Arp2/3 complex subunits have
been localized at the site of particle attachment during phagocytosis, at the
phagocytotic cup, to endosomes at early stages (i.e. uptake), and at late and
post-lysosomal stages after internalization in Dictyostelium
(Insall et al., 2001). In
Saccharomyces cerevisiae, mutations in the Arp2 subunit of the Arp2/3
complex, or in type I myosins, proteins that colocalize with and activate
Arp2/3 complex nucleation in vitro, also show defects in endocytosis.
In addition, arp2-1 mutants were synthetically lethal with
end3-1, a known endocytosis mutant
(Moreau et al., 1997
). The
interaction of Arp2/3 complex nucleation-promoting factors and endocytic
proteins suggests a model in which actin polymerization might provide the
force behind plasma membrane invagination or the `pinching off' of endocytic
vesicles.
Taunton et al. (2000)
provided evidence that Arp2/3 complex participates in another step in
endocytosis, namely in endosome movement. First, they observed formation of
actin comet tails on endosomes and movement of these vesicles in
Xenopus extract and eggs. Consistent with this, they found that Rho
GTPase inhibitors blocked the observed formation of actin comet tails, and
that actin comet tails were restored upon treatment with Cdc42, a protein that
stimulates WASP- and Arp2/3 complex-mediated actin assembly. Finally, the
group showed that N-WASP colocalizes with the membrane-proximal ends of comet
tails in vitro, and that recruitment of N-WASP to endosomes is
blocked by the Rho GTPase inhibitor ToxB but not by an agent that inhibits
actin polymerization
(Figs·2,3).
Taken together, these data suggest a mechanism for endosome movement whereby
Cdc42 drives recruitment of an Arp2/3 complex activator to the endosome.
Arp2/3 complex-mediated actin nucleation then drives actin comet tail
formation and endosome movement.
|
|
The link between the Arp2/3 complex and endocytosis is supported by
findings in many cell systems. First, actin comet tails have been observed on
endosomes or lysosomes in a number of other cell systems including HeLa cell
extracts, cultured mast cells and NIH 3T3 cells
(Merrifield et al., 1999;
Kaksonen et al., 2000
).
Second, cortactin, Abp1p and Pan1p have been implicated in endocytosis
(Duncan et al., 2001
;
Goode et al., 2001
;
Urono et al., 2001
;
Weaver et al., 2001
). All of
these NPFs fall outside of the SCAR/WASP (Class I NPF) family of proteins, and
appear to activate Arp2/3 complex by a mechanism that is different from that
of the SCAR/WASP proteins. Finally, recent studies using GFP fused to the
pheromone receptor, Ste2p, as a marker for endosomes, provide preliminary
evidence that endosomal motility in budding yeast depends on the Arp2/3
complex activator, Las17p/Bee1p, and F-actin polymerization (Chang and Blumer,
2002).
Mitochondrial movement and inheritance in budding yeast
Mitochondria are essential organelles that cannot be synthesized de
novo. Therefore, these organelles must be transferred from mother to
developing daughter cell during cell division. In budding yeast, mitochondria
exist as a branched network at the cell cortex that undergoes remodeling by
fission, fusion and movement (Stevens,
1977; Nunnari et al.,
1997
; Simon et al.,
1997
). Time-lapse imaging of mitochondria in living yeast revealed
a series of cell-cycle-dependent motility events that contribute to
segregation of the organelle between mother cells and buds. There are two key
events in this mitochondrial inheritance process: (1) vectorial movement of
the organelle from mother cell to bud and (2) retention of mitochondria at
specific positions within both mother and bud
(Yang et al., 1999
).
Mounting evidence suggests a role for the actin cytoskeleton in
mitochondrial motility in S. cerevisiae. First, destabilization of
the actin cytoskeleton by mutations in the actin-encoding ACT1 gene
resulted in defects in mitochondrial morphology and transfer of mitochondria
from mother cells to buds (Drubin et al.,
1993; Lazzarino et al.,
1994
). Second, destabilization of the actin cytoskeleton by
treatment with Latrunculin-A (an actin-monomer sequestering agent) blocks
mitochondrial movement (Simon et al.,
1995
). Third, mitochondria colocalize with actin cables, bundles
of actin filaments that align along the mother-bud axis during polarized
growth (Drubin et al., 1993
;
Lazzarino et al., 1994
).
Consistent with this, disruption of actin cables by deletion of the
tropomyosin-encoding gene (TPM1) results in a loss of polarized
mitochondrial movement into the bud (Simon
et al., 1997
). These observations support the model that
mitochondria use actin cables as tracks for movement from the mother cell to
the bud.
Although type V myosin proteins have been implicated as the motor molecules
that drive movement of cargo along actin cables in yeast, recent studies
suggest a role for the Arp2/3 complex and actin polymerization in
mitochondrial movement and inheritance in yeast
(Boldogh et al., 2001). First,
mitochondrial movement requires constant actin assembly and disassembly, and
is impaired by an agent that perturbs actin dynamics. Second, Arp2/3 complex
subunits colocalize with mitochondria in intact cells
(Fig.·4) and are tightly
associated with the surface of isolated yeast mitochondria. Third, Arp2/3
complex-dependent actin nucleation activity is observed in isolated yeast
mitochondria and associated with mitochondria in living yeast. Finally,
mutations in Arp2/3 complex subunits inhibit mitochondrial movement, but have
no obvious effect on colocalization of mitochondria with actin cables. These
observations indicate that the Arp2/3 complex is associated with yeast
mitochondria and that the Arp2/3-complex-driven actin assembly provides the
driving force for mitochondrial movement.
|
Although yeast mitochondria and L. monocytogenes require Arp2/3
complex and actin dynamics for movement, there are fundamental differences in
their mechanism of movement. First, yeast mitochondria and L.
monocytogenes display different patterns of movement. Arp2/3
complex-mediated movement of L. monocytogenes and endosomes has no
obvious direction or track dependence. In contrast, yeast mitochondria
colocalize with actin cables, and use actin cables as tracks for linear,
polarized movement from the mother cell to the bud. Second, there are no
obvious actin comet tails on yeast mitochondria. However, since mitochondria
are associated with actin cables, which are linear, F-actin containing
structures, comet tails on mitochondria may not be distinguishable from actin
cables. This interpretation is supported by the finding that destabilization
of actin cables results in generation of Arp2/3 complex-dependent `clouds' of
F-actin on mitochondria that resemble intermediates in actin comet tail
assembly observed on L. monocytogenes in vitro
(Boldogh et al., 2001).
What mediates association of mitochondria to actin cables in yeast?
Previous studies revealed that yeast mitochondria contain an actin binding
activity that may be distinct from the Arp2/3 complex. Using a sedimentation
assay to study direct interactions between mitochondria and F-actin, Boldogh
et al. (1998) showed that a
protein(s) on the mitochondrial surface binds to F-actin. This actin-binding
activity is ATP-sensitive, reversible, saturable, and requires two integral
mitochondrial outer membrane proteins (Mmm1p and Mdm10p). Collectively, these
proteins are referred to as the mitochondrial actin
binding particle (mABP;
Boldogh et al., 1998
).
Three lines of evidence suggest that mABP is distinct from the Arp2/3
complex. First, although the Arp2/3 complex has F-actin binding activity,
binding of the Arp2/3 complex to the lateral surface of F-actin occurs in the
presence of ATP (Mullins et al.,
1998). In contrast, F-actin binding by mABP is ATP-sensitive
(Lazzarino et al., 1994
).
Thus, F-actin binding by mABP and the Arp2/3 complex show different
biochemical properties. Second, mABP activity is observed in mitochondria
isolated from yeast carrying mutations in the Arp2/3 complex (I. Boldogh and
L. Pon, unpublished observations). Thus, mABP-mediated binding of mitochondria
to F-actin does not require the Arp2/3 complex. Third, mitochondria colocalize
with actin cables in yeast carrying mutations in Arp2/3 complex
(Boldogh et al., 2001
). Thus,
association of mitochondria with actin cables does not require the Arp2/3
complex.
The model that emerges from these studies invokes mABP as the mediator for binding of mitochondria to actin cables. Since mABP-mediated binding of mitochondria to F-actin is reversible, mitochondria could bind to actin cables and move along them in the presence of an applied force. The available evidence supports a role for actin polymerization as the force generator for mitochondrial movement. Taken together, mABP mediates reversible binding of mitochondria and their associated Arp2/3 complex to actin cables. Arp2/3 complex-driven actin nucleation would then initiate actin comet tail formation at the interface between mitochondria and actin cables. This would generate forces that drive mitochondrial movement along actin cables from mother cells to developing daughter cells.
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Future perspectives |
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Acknowledgments |
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