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Chapter 15 : Actin-Based Motility in Professional Phagocytes

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Actin-Based Motility in Professional Phagocytes , Page 1 of 2

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Abstract:

Phagocytes are among the most dynamic cells in the body. Within seconds of exposure to a chemotactic peptide, they are able to convert from a rounded symmetric shape to a polarized morphology. Actin polymerization not only drives the motility of all mammalian nonmuscle cells, but also provides the propulsive force for the movement of intracellular bacteria including , , and , as well as the poxvirus vaccinia. Actin is the most abundant protein in the cytoplasm of phagocytes, representing 10 to 20% of the total cytoplasmic protein. The cytoplasm of phagocytes contains high concentrations of unpolymerized actin that exceed by several orders of magnitude the macroscopic critical concentration of purified actin. The factor primarily responsible for preventing actin monomer assembly into filaments is the 5-kDa polypeptide known as thymosin β4 (Tβ4). This small protein binds to a single actin monomer to form a 1:1, or binary, complex. High rates of actin assembly are required to produce the rapid changes in shape observed during phagocyte amoeboid movement. Adherence receptors contain high concentrations of actin filaments and are linked to the actin cytoskeleton by ezrin, radixin, and moesin (the ERM proteins). Proteins that bind to the barbed ends of actin filaments have a profound effect on filament growth. The barbed end has a high affinity for actin monomers and in combination with profilin can readily compete with Tβ4 for sequestered ATP-actin monomers.

Citation: Southwick F. 2009. Actin-Based Motility in Professional Phagocytes , p 235-248. In Russell D, Gordon S (ed), Phagocyte-Pathogen Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555816650.ch15

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Surface Receptors
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Bacterial Proteins
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Bacterial Pathogenesis
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Viral Pathogenesis
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Figures

Image of FIGURE 1
FIGURE 1

Polarized neutrophils. Arrows point to the direction the cells are moving. Note the broad lamellipodia at the front of each neutrophil and the narrow uropod at the back.

Citation: Southwick F. 2009. Actin-Based Motility in Professional Phagocytes , p 235-248. In Russell D, Gordon S (ed), Phagocyte-Pathogen Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555816650.ch15
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Image of FIGURE 2
FIGURE 2

Atomic-level structure of an actin monomer showing the ATP-binding site. Based on the work of . The vertical axis of the monomer (as depicted in this figure) runs parallel to the long axis of the filament. The right-hand side of the molecule is exposed to the outside of the actin filament, whereas the left-hand side is nearest the long axis of the filament. Residues 262 to 274 are thought to reach across this axis and interact with the adjacent actin monomer of the double-stranded helix. As oriented here, the polarity of the filament would correspond to the pointed end at the top and the barbed end at the bottom. (From .)

Citation: Southwick F. 2009. Actin-Based Motility in Professional Phagocytes , p 235-248. In Russell D, Gordon S (ed), Phagocyte-Pathogen Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555816650.ch15
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Image of FIGURE 3
FIGURE 3

Schematic drawing of an actin filament under steady-state conditions. ATP-actin monomers add to the barbed or fast-growing end and are hydrolyzed to ADP-P, followed by the slower dissociation of P to form ADP-actin. ADP-actin monomers dissociate from the pointed or slow-growing end. Under these conditions an actin monomer added to the barbed end will eventually treadmill through the filament and dissociate from the pointed end. (From .)

Citation: Southwick F. 2009. Actin-Based Motility in Professional Phagocytes , p 235-248. In Russell D, Gordon S (ed), Phagocyte-Pathogen Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555816650.ch15
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Image of FIGURE 4
FIGURE 4

Schematic drawing of the tertiary structure of an actin filament. Roman numerals correspond to the domains shown in Fig. 2 . (From .)

Citation: Southwick F. 2009. Actin-Based Motility in Professional Phagocytes , p 235-248. In Russell D, Gordon S (ed), Phagocyte-Pathogen Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555816650.ch15
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Image of FIGURE 5
FIGURE 5

Critical-concentration behavior in actin polymerization. (Left) Plot of the steady-state actin filament concentration as a function of monomer concentration. This macroscopic behavior is often measured by the increase in fluorescence when pyrenyl-actin is incorporated into filaments. (Right) When both filament ends are uncapped, the macroscopic critical concentration lies between the microscopic critical concentrations for actin monomer interactions at the barbed end [or (+) end] and the pointed end [or (−) end]. At steady state, monomers will naturally dissociate from the pointed ends and will associate with the more stable barbed ends. This phenomenon is known as treadmilling. Because the exchange rates are higher at the barbed end, the macroscopic critical concentration is closer to the microscopic critical concentration of the barbed end. (From .)

Citation: Southwick F. 2009. Actin-Based Motility in Professional Phagocytes , p 235-248. In Russell D, Gordon S (ed), Phagocyte-Pathogen Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555816650.ch15
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Image of FIGURE 6
FIGURE 6

(A) Schematic diagram of the action of monomer-sequestering agents such as Tβ4. (B) Schematic diagram of how profilin may facilitate the exchange of ATP for ADP on an actin monomer. When profilin binds to an actin monomer, the central cleft of actin opens, making the nucleotide-binding site more accessible for release and exchange. Because the ATP concentration far exceeds the ADP concentration in living cells, ATP will readily replace ADP from the actin monomer. (C) Simplified kinetic diagram showing how free profilin can take actin monomers from the Tβ4 storage pool and usher them onto the barbed end of an actin filament. For simplicity, the reverse arrows from profilin-ATP to free profilin and to Tβ4-ATP-actin are not shown. Once profilin-ATP binds to the barbed end, profilin rapidly dissociates. (D) Model of how Hsp27 phosphorylation and dephosphorylation could serve to shuttle actin monomers to sites of new actin filament assembly and facilitate actin-based motility. Unphosphorylated Hsp27 with bound actin monomers concentrates at the leading edge of motile cells where, upon chemoattractant stimulation, the p38 MAPK signal transduction cascade induces Hsp27 phosphorylation, releasing actin monomers for new actin filament assembly. Phosphorylated Hsp27 then moves toward the center of the cell, where it is dephosphorylated, again binds actin monomers, and then shuttles back to the leading edge. (From .)

Citation: Southwick F. 2009. Actin-Based Motility in Professional Phagocytes , p 235-248. In Russell D, Gordon S (ed), Phagocyte-Pathogen Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555816650.ch15
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Image of FIGURE 7
FIGURE 7

(A) Schematic diagram of the modular organization of VASP. The EVH1 domain binds to FP sequences found in ActA, vinculin, and zyxin. The EVH2 domain contains both a G-actin-binding site (GAB) adjacent to the last GP profilin-binding site in the proline-rich domain, and an F-actin-binding site (FAB). This region also contains a coiled-coil domain responsible for forming VASP tetramers. (B) Schematic diagram showing how VASP may usher an actin monomer from profilin to the barbed end of an actin filament tethered by the FAB. Profilin binds to the GP polyproline sequences on VASP. The final GP sequence is adjacent to the actin-monomer-binding site (GAB), allowing both the profilin and actin molecule to be bound simultaneously. The monomer can then be transferred to the actin filament end, and free profilin is released to bind a new ATP-actin monomer (arrows). (Images adapted from .)

Citation: Southwick F. 2009. Actin-Based Motility in Professional Phagocytes , p 235-248. In Russell D, Gordon S (ed), Phagocyte-Pathogen Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555816650.ch15
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Image of FIGURE 8
FIGURE 8

(A) Schematic view of Arp2/3 complex nucleated actin filament assembly. (B) Schematic view of formin-mediated actin filament assembly. (From .)

Citation: Southwick F. 2009. Actin-Based Motility in Professional Phagocytes , p 235-248. In Russell D, Gordon S (ed), Phagocyte-Pathogen Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555816650.ch15
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Image of FIGURE 9
FIGURE 9

(A) Mechanism of a barbed-end capping protein binding to the barbed (or plus) end of an actin filament. Bound capping proteins prevent both association and dissociation of monomers; under such conditions, only the pointed end can interact with the actin monomer pool. (B) Effects of a barbed-end capping protein on the critical concentration and rate of depolymerization of actin. (Left) Graph of steady-state actin filament concentration as a function of actin monomer concentration. As shown in panel A, capping blocks all exchange at the barbed end. The free pointed end has a lower affinity for actin monomers, and this lower affinity is reflected as an increase in the critical concentration (see Fig. 5 for comparison). (Right) Plot of the decrease in filamentous actin versus time after diluting actin filaments to below their critical concentration in the absence and presence of a barbed-end capping protein. Capping of the barbed end retards the depolymerization because dissociation of actin monomers occurs only at the pointed end. (From .)

Citation: Southwick F. 2009. Actin-Based Motility in Professional Phagocytes , p 235-248. In Russell D, Gordon S (ed), Phagocyte-Pathogen Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555816650.ch15
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Image of FIGURE 10
FIGURE 10

(A) Model for the actin filament-severing proteins. The severing protein first binds along the side of the actin filament, next interposes itself between neighboring actin subunits within the filament, and then remains tightly bound to the barbed end of one of the severed filaments. (From .) (B) Schematic diagram of actin filament cycling before and after addition of ADF/cofilin or gelsolin-Ca. At steady state (left), actin monomers come on the filament at the barbed end as ATP-actin monomers. As they enter the filament, the ATP is hydrolyzed, forming an intermediate ADP + P and then ADP-actin. Once the ADP-actin monomer dissociates from the pointed end, ATP is exchanged for ADP, and the actin monomer can again add to the barbed end. This process is called treadmilling, and the rate of treadmilling depends on the number of free filament ends. Doubling the number of free ends of the same concentration of filamentous actin would be expected to double the rate of treadmilling. In the slow-cycling filament, significant amounts of ADP-actin exist in the filament; therefore, ADF/cofilin can bind and sever. Each time ADF severs, it doubles the filament ends. Gelsolin-Ca has very high affinity for filaments and can bind and sever regions of the filament containing ADP-or ATP-actin. Because gelsolin also binds and caps the barbed ends, severing and capping doubles the free pointed ends but does not increase the free barbed ends. However, when chemotactic signal transduction pathways increase the concentration of phosphatidylinositol bisphosphate, gelsolin will dissociate from the barbed end and the number of free barbed ends will double. When actin filaments rapidly cycle, there is reduced time for ATP hydrolysis and the filament would be expected to have a lower content of ADP-actin. This condition would be expected to reduce the ability of ADF/cofilin to enhance treadmilling, but would not impair gelsolin.

Citation: Southwick F. 2009. Actin-Based Motility in Professional Phagocytes , p 235-248. In Russell D, Gordon S (ed), Phagocyte-Pathogen Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555816650.ch15
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References

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Tables

Generic image for table
TABLE 1

Actin regulatory proteins

Citation: Southwick F. 2009. Actin-Based Motility in Professional Phagocytes , p 235-248. In Russell D, Gordon S (ed), Phagocyte-Pathogen Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555816650.ch15

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