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A two-tiered mechanism by which Cdc42 controls the localization and activation of an Arp2/3-activating motor complex in yeast.

Lechler T, Jonsdottir GA, Klee SK, Pellman D, Li R - J. Cell Biol. (2001)

Bottom Line: One branch, which requires formin homologues, mediates the recruitment of the Bee1p complex to the cortical site where the activated Cdc42p resides.The other is mediated by the p21-activated kinases, which activate the motor activity of myosin-I through phosphorylation.Together, these findings provide insights into the essential processes leading to polarization of the actin cytoskeleton.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA.

ABSTRACT
The establishment of cell polarity in budding yeast involves assembly of actin filaments at specified cortical domains. Elucidation of the underlying mechanism requires an understanding of the machinery that controls actin polymerization and how this machinery is in turn controlled by signaling proteins that respond to polarity cues. We showed previously that the yeast orthologue of the Wiskott-Aldrich Syndrome protein, Bee1/Las17p, and the type I myosins are key regulators of cortical actin polymerization. Here, we demonstrate further that these proteins together with Vrp1p form a multivalent Arp2/3-activating complex. During cell polarization, a bifurcated signaling pathway downstream of the Rho-type GTPase Cdc42p recruits and activates this complex, leading to local assembly of actin filaments. One branch, which requires formin homologues, mediates the recruitment of the Bee1p complex to the cortical site where the activated Cdc42p resides. The other is mediated by the p21-activated kinases, which activate the motor activity of myosin-I through phosphorylation. Together, these findings provide insights into the essential processes leading to polarization of the actin cytoskeleton.

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A model depicting a bifurcated pathway downstream of Cdc42p required for polarized actin polymerization. In a formin protein-dependent manner, Cdc42p recruits a Bee1p/Vrp1p/myosin-I–containing complex to the polarization site to activate the Arp2/3 complex. The double headed arrow between myosin-I and Vrp1p/Bee1p represents a dynamic interaction. In parallel, Cdc42 recruits the PAKs, which phosphorylate and activate the motor activity of type I myosins.
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fig7: A model depicting a bifurcated pathway downstream of Cdc42p required for polarized actin polymerization. In a formin protein-dependent manner, Cdc42p recruits a Bee1p/Vrp1p/myosin-I–containing complex to the polarization site to activate the Arp2/3 complex. The double headed arrow between myosin-I and Vrp1p/Bee1p represents a dynamic interaction. In parallel, Cdc42 recruits the PAKs, which phosphorylate and activate the motor activity of type I myosins.

Mentions: Our results indicate that even in a simple system such as yeast, polarized actin assembly does not occur by a linear pathway connecting signaling protein to actin assembly factors. In addition to the formin-dependent recruitment, the Bee1p–Vrp1p–myosin-I complex must also be activated through phosphorylation by PAKs, which is required for the motor activity of these myosins (Maruta and Korn, 1977; Wu et al., 1996) (Fig. 7). This finding is consistent with the result from actin patch reconstitution experiments in permeabilized yeast cells, which suggested an essential role for the motor activity in actin polymerization at cell cortex (Lechler et al., 2000). Motor phosphorylation is likely to occur after myosin-I is recruited to the polarization site, since the kinase responsible for this phosphorylation is recruited to the same site by Cdc42p (Peter et al., 1996). Mechanistically how the motor activity participates in polarized actin polymerization is an intensely interesting question. The motor activity may somehow be required for the activation or targeting of the Arp2/3 complex. Alternatively, it may be required after the nucleation step, for example, during filament elongation. In mammalian type I myosins, the PAK phosphorylation site is replaced by an acidic amino acid, resulting in “constitutively active” motors (Bement and Mooseker, 1995). Therefore, other activation mechanisms may exist to accompany the recruitment of actin nucleation factors in order to ensure tight spatial regulation of actin polymerization.


A two-tiered mechanism by which Cdc42 controls the localization and activation of an Arp2/3-activating motor complex in yeast.

Lechler T, Jonsdottir GA, Klee SK, Pellman D, Li R - J. Cell Biol. (2001)

A model depicting a bifurcated pathway downstream of Cdc42p required for polarized actin polymerization. In a formin protein-dependent manner, Cdc42p recruits a Bee1p/Vrp1p/myosin-I–containing complex to the polarization site to activate the Arp2/3 complex. The double headed arrow between myosin-I and Vrp1p/Bee1p represents a dynamic interaction. In parallel, Cdc42 recruits the PAKs, which phosphorylate and activate the motor activity of type I myosins.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2198833&req=5

fig7: A model depicting a bifurcated pathway downstream of Cdc42p required for polarized actin polymerization. In a formin protein-dependent manner, Cdc42p recruits a Bee1p/Vrp1p/myosin-I–containing complex to the polarization site to activate the Arp2/3 complex. The double headed arrow between myosin-I and Vrp1p/Bee1p represents a dynamic interaction. In parallel, Cdc42 recruits the PAKs, which phosphorylate and activate the motor activity of type I myosins.
Mentions: Our results indicate that even in a simple system such as yeast, polarized actin assembly does not occur by a linear pathway connecting signaling protein to actin assembly factors. In addition to the formin-dependent recruitment, the Bee1p–Vrp1p–myosin-I complex must also be activated through phosphorylation by PAKs, which is required for the motor activity of these myosins (Maruta and Korn, 1977; Wu et al., 1996) (Fig. 7). This finding is consistent with the result from actin patch reconstitution experiments in permeabilized yeast cells, which suggested an essential role for the motor activity in actin polymerization at cell cortex (Lechler et al., 2000). Motor phosphorylation is likely to occur after myosin-I is recruited to the polarization site, since the kinase responsible for this phosphorylation is recruited to the same site by Cdc42p (Peter et al., 1996). Mechanistically how the motor activity participates in polarized actin polymerization is an intensely interesting question. The motor activity may somehow be required for the activation or targeting of the Arp2/3 complex. Alternatively, it may be required after the nucleation step, for example, during filament elongation. In mammalian type I myosins, the PAK phosphorylation site is replaced by an acidic amino acid, resulting in “constitutively active” motors (Bement and Mooseker, 1995). Therefore, other activation mechanisms may exist to accompany the recruitment of actin nucleation factors in order to ensure tight spatial regulation of actin polymerization.

Bottom Line: One branch, which requires formin homologues, mediates the recruitment of the Bee1p complex to the cortical site where the activated Cdc42p resides.The other is mediated by the p21-activated kinases, which activate the motor activity of myosin-I through phosphorylation.Together, these findings provide insights into the essential processes leading to polarization of the actin cytoskeleton.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA.

ABSTRACT
The establishment of cell polarity in budding yeast involves assembly of actin filaments at specified cortical domains. Elucidation of the underlying mechanism requires an understanding of the machinery that controls actin polymerization and how this machinery is in turn controlled by signaling proteins that respond to polarity cues. We showed previously that the yeast orthologue of the Wiskott-Aldrich Syndrome protein, Bee1/Las17p, and the type I myosins are key regulators of cortical actin polymerization. Here, we demonstrate further that these proteins together with Vrp1p form a multivalent Arp2/3-activating complex. During cell polarization, a bifurcated signaling pathway downstream of the Rho-type GTPase Cdc42p recruits and activates this complex, leading to local assembly of actin filaments. One branch, which requires formin homologues, mediates the recruitment of the Bee1p complex to the cortical site where the activated Cdc42p resides. The other is mediated by the p21-activated kinases, which activate the motor activity of myosin-I through phosphorylation. Together, these findings provide insights into the essential processes leading to polarization of the actin cytoskeleton.

Show MeSH
Related in: MedlinePlus