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The PCH family protein, Cdc15p, recruits two F-actin nucleation pathways to coordinate cytokinetic actin ring formation in Schizosaccharomyces pombe.

Carnahan RH, Gould KL - J. Cell Biol. (2003)

Bottom Line: Cdc15p binds directly to the Arp2/3 complex activator Myo1p, which likely explains why actin patches and the Arp2/3 complex fail to be medially recruited during mitosis in cdc15 mutants.Cdc15p also binds directly to Cdc12p.We propose a model in which Cdc15p plays a critical role in recruiting and coordinating the pathways essential for the assembly of medially located F-actin filaments and construction of the CAR.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell and Developmental Biology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.

ABSTRACT
Cytokinetic actin ring (CAR) formation in Schizosaccharomyces pombe requires two independent actin nucleation pathways, one dependent on the Arp2/3 complex and another involving the formin Cdc12p. Here we investigate the role of the S. pombe Cdc15 homology family protein, Cdc15p, in CAR assembly and find that it interacts with proteins from both of these nucleation pathways. Cdc15p binds directly to the Arp2/3 complex activator Myo1p, which likely explains why actin patches and the Arp2/3 complex fail to be medially recruited during mitosis in cdc15 mutants. Cdc15p also binds directly to Cdc12p. Cdc15p and Cdc12p not only display mutual dependence for CAR localization, but also exist together in a ring-nucleating structure before CAR formation. The disruption of these interactions in cdc15 cells is likely to be the reason for their complete lack of CARs. We propose a model in which Cdc15p plays a critical role in recruiting and coordinating the pathways essential for the assembly of medially located F-actin filaments and construction of the CAR.

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Model of Cdc15p involvement in ring formation. Hyperphosphorylated Cdc15p resides in patches at cell ends during interphase (indicated in gray spots). At the G2/M transition, Cdc15p becomes hypophosphorylated and appears in a medial spot structure (gray spot) with Cdc12p (darker gray spot) near the nucleus (oblong gray structure). This complex recruits actin (black spot and lines) and, through nucleation by Cdc12p, initiates the assembly of the primary F-actin ring. Cdc15p also recruits the Arp2/3-dependent actin nucleation machinery through its direct association with Myo1p. The Arp2/3 complex promotes the formation of the medial actin cable meshwork seen subsequent to primary ring formation. Finally, these actin fibers are bundled to create the mature CAR.
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fig7: Model of Cdc15p involvement in ring formation. Hyperphosphorylated Cdc15p resides in patches at cell ends during interphase (indicated in gray spots). At the G2/M transition, Cdc15p becomes hypophosphorylated and appears in a medial spot structure (gray spot) with Cdc12p (darker gray spot) near the nucleus (oblong gray structure). This complex recruits actin (black spot and lines) and, through nucleation by Cdc12p, initiates the assembly of the primary F-actin ring. Cdc15p also recruits the Arp2/3-dependent actin nucleation machinery through its direct association with Myo1p. The Arp2/3 complex promotes the formation of the medial actin cable meshwork seen subsequent to primary ring formation. Finally, these actin fibers are bundled to create the mature CAR.

Mentions: Placing our results in the context of previous studies, particularly the careful analysis of CAR formation in Arai and Mabuchi (2002), leads us to propose a model for ring formation that includes Cdc15p (Fig. 7). In interphase, hyperphosphorylated Cdc15p is prevented from associating with Cdc12p. Upon commitment to mitosis, Cdc15p is dephosphorylated and associates with Cdc12p and actin to form the pre-ring medial aster/spot. In metaphase, the primary F-actin ring is formed. This step has been suggested to be dependent on Cdc12p (Chang et al., 1997), a hypothesis consistent with the role of formins in nucleating linear unbranched filaments (Evangelista et al., 2002; Pruyne et al., 2002; Sagot et al., 2002). The presence of Cdc15p in this primary ring leads to medial recruitment of the Arp2/3 complex and the subsequent creation of an actin network encircling the equator of the cell. Finally, in late anaphase, these thinner filaments coalesce into what is recognized as the mature CAR structure. This places Cdc15p at a major convergence point for coordination of events required for initiation and formation of the CAR in S. pombe. In the future, it will be interesting to determine if Cdc15p's function is limited to that of recruitment, or whether it might also have a catalytic role in regulating the activity of the proteins it recruits. Domain architecture and localization to actin-rich regions are conserved across the PCH protein family. Given this structural conservation, it is likely that other family members are similarly involved in the organization of dynamic actin structures in higher eukaryotes.


The PCH family protein, Cdc15p, recruits two F-actin nucleation pathways to coordinate cytokinetic actin ring formation in Schizosaccharomyces pombe.

Carnahan RH, Gould KL - J. Cell Biol. (2003)

Model of Cdc15p involvement in ring formation. Hyperphosphorylated Cdc15p resides in patches at cell ends during interphase (indicated in gray spots). At the G2/M transition, Cdc15p becomes hypophosphorylated and appears in a medial spot structure (gray spot) with Cdc12p (darker gray spot) near the nucleus (oblong gray structure). This complex recruits actin (black spot and lines) and, through nucleation by Cdc12p, initiates the assembly of the primary F-actin ring. Cdc15p also recruits the Arp2/3-dependent actin nucleation machinery through its direct association with Myo1p. The Arp2/3 complex promotes the formation of the medial actin cable meshwork seen subsequent to primary ring formation. Finally, these actin fibers are bundled to create the mature CAR.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2172828&req=5

fig7: Model of Cdc15p involvement in ring formation. Hyperphosphorylated Cdc15p resides in patches at cell ends during interphase (indicated in gray spots). At the G2/M transition, Cdc15p becomes hypophosphorylated and appears in a medial spot structure (gray spot) with Cdc12p (darker gray spot) near the nucleus (oblong gray structure). This complex recruits actin (black spot and lines) and, through nucleation by Cdc12p, initiates the assembly of the primary F-actin ring. Cdc15p also recruits the Arp2/3-dependent actin nucleation machinery through its direct association with Myo1p. The Arp2/3 complex promotes the formation of the medial actin cable meshwork seen subsequent to primary ring formation. Finally, these actin fibers are bundled to create the mature CAR.
Mentions: Placing our results in the context of previous studies, particularly the careful analysis of CAR formation in Arai and Mabuchi (2002), leads us to propose a model for ring formation that includes Cdc15p (Fig. 7). In interphase, hyperphosphorylated Cdc15p is prevented from associating with Cdc12p. Upon commitment to mitosis, Cdc15p is dephosphorylated and associates with Cdc12p and actin to form the pre-ring medial aster/spot. In metaphase, the primary F-actin ring is formed. This step has been suggested to be dependent on Cdc12p (Chang et al., 1997), a hypothesis consistent with the role of formins in nucleating linear unbranched filaments (Evangelista et al., 2002; Pruyne et al., 2002; Sagot et al., 2002). The presence of Cdc15p in this primary ring leads to medial recruitment of the Arp2/3 complex and the subsequent creation of an actin network encircling the equator of the cell. Finally, in late anaphase, these thinner filaments coalesce into what is recognized as the mature CAR structure. This places Cdc15p at a major convergence point for coordination of events required for initiation and formation of the CAR in S. pombe. In the future, it will be interesting to determine if Cdc15p's function is limited to that of recruitment, or whether it might also have a catalytic role in regulating the activity of the proteins it recruits. Domain architecture and localization to actin-rich regions are conserved across the PCH protein family. Given this structural conservation, it is likely that other family members are similarly involved in the organization of dynamic actin structures in higher eukaryotes.

Bottom Line: Cdc15p binds directly to the Arp2/3 complex activator Myo1p, which likely explains why actin patches and the Arp2/3 complex fail to be medially recruited during mitosis in cdc15 mutants.Cdc15p also binds directly to Cdc12p.We propose a model in which Cdc15p plays a critical role in recruiting and coordinating the pathways essential for the assembly of medially located F-actin filaments and construction of the CAR.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell and Developmental Biology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.

ABSTRACT
Cytokinetic actin ring (CAR) formation in Schizosaccharomyces pombe requires two independent actin nucleation pathways, one dependent on the Arp2/3 complex and another involving the formin Cdc12p. Here we investigate the role of the S. pombe Cdc15 homology family protein, Cdc15p, in CAR assembly and find that it interacts with proteins from both of these nucleation pathways. Cdc15p binds directly to the Arp2/3 complex activator Myo1p, which likely explains why actin patches and the Arp2/3 complex fail to be medially recruited during mitosis in cdc15 mutants. Cdc15p also binds directly to Cdc12p. Cdc15p and Cdc12p not only display mutual dependence for CAR localization, but also exist together in a ring-nucleating structure before CAR formation. The disruption of these interactions in cdc15 cells is likely to be the reason for their complete lack of CARs. We propose a model in which Cdc15p plays a critical role in recruiting and coordinating the pathways essential for the assembly of medially located F-actin filaments and construction of the CAR.

Show MeSH
Related in: MedlinePlus