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Myosin and the PAR proteins polarize microfilament-dependent forces that shape and position mitotic spindles in Caenorhabditis elegans.

Severson AF, Bowerman B - J. Cell Biol. (2003)

Bottom Line: Unlike MFs, dynein, and dynactin, myosin II is not required for the production of these forces.Instead, myosin influences embryonic polarity by limiting PAR-3 to the anterior cortex.This in turn produces asymmetry in the forces applied to MTs at each pole and allows PAR-2 to accumulate in the posterior cortex of a one-cell zygote and maintain asymmetry.

View Article: PubMed Central - PubMed

Affiliation: Institute of Molecular Biology, University of Oregon, Eugene, OR 97403-1229, USA.

ABSTRACT
In Caenorhabditis elegans, the partitioning proteins (PARs), microfilaments (MFs), dynein, dynactin, and a nonmuscle myosin II all localize to the cortex of early embryonic cells. Both the PARs and the actomyosin cytoskeleton are required to polarize the anterior-posterior (a-p) body axis in one-cell zygotes, but it remains unknown how MFs influence embryonic polarity. Here we show that MFs are required for the cortical localization of PAR-2 and PAR-3. Furthermore, we show that PAR polarity regulates MF-dependent cortical forces applied to astral microtubules (MTs). These forces, which appear to be mediated by dynein and dynactin, produce changes in the shape and orientation of mitotic spindles. Unlike MFs, dynein, and dynactin, myosin II is not required for the production of these forces. Instead, myosin influences embryonic polarity by limiting PAR-3 to the anterior cortex. This in turn produces asymmetry in the forces applied to MTs at each pole and allows PAR-2 to accumulate in the posterior cortex of a one-cell zygote and maintain asymmetry.

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Dynein–dynactin function and intact MTs are required for centrosome flattening. (a) In wild-type embryos expressing tubulin–GFP and histone–GFP fusion proteins, the posterior centrosome becomes flattened in telophase. (b–d) Partial depletion of the dynactin component DNC-1 (b) or of the dynein heavy chain DHC-1 (c) disrupts centrosome flattening as does exposure to low doses of the MT-depolymerizing drug nocodazole (d).
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fig4: Dynein–dynactin function and intact MTs are required for centrosome flattening. (a) In wild-type embryos expressing tubulin–GFP and histone–GFP fusion proteins, the posterior centrosome becomes flattened in telophase. (b–d) Partial depletion of the dynactin component DNC-1 (b) or of the dynein heavy chain DHC-1 (c) disrupts centrosome flattening as does exposure to low doses of the MT-depolymerizing drug nocodazole (d).

Mentions: The results described above suggest that MFs either recruit or activate a cortical motor protein that pulls on astral MTs to influence the shape and position of mitotic spindles. Both the dynactin complex and the minus end–directed MT motor dynein localize to the cortex of early embryos, and spindle rotation fails in two-cell stage embryos in which the dynein–dynactin complex has been partially depleted by RNA interference (Skop and White, 1998; Gönczy et al., 1999). Further reducing dynein–dynactin function disrupts pronuclear migration and the assembly and orientation of the first mitotic spindle (Gönczy et al., 1999). To determine whether dynein and dynactin are required for centrosome flattening in one-cell stage embryos, we partially depleted either the dynein heavy chain DHC-1 or a C. elegans orthologue of the dynactin component p150glued DNC-1 (see Materials and methods). The posterior centrosome remained spherical in all DNC-1–depleted embryos in which the first mitotic spindle rotated to lie along the a-p axis (Fig. 4 b; n = 9). Similarly, we observed spherical centrosomes in some DHC-1–depleted embryos (Fig. 4 c; 4 out of 20 embryos; 4 embryos exhibited defects in chromosome segregation and in centrosome flattening, whereas 16 embryos appeared wild type during the first mitotic division). Exposure of embryos to low doses of nocodazole that shorten but do not eliminate MTs also disrupted centrosome flattening (Fig. 4 d; five out of seven embryos). We conclude that both dynein function and contact between astral MTs and the cortex are required for centrosome flattening.


Myosin and the PAR proteins polarize microfilament-dependent forces that shape and position mitotic spindles in Caenorhabditis elegans.

Severson AF, Bowerman B - J. Cell Biol. (2003)

Dynein–dynactin function and intact MTs are required for centrosome flattening. (a) In wild-type embryos expressing tubulin–GFP and histone–GFP fusion proteins, the posterior centrosome becomes flattened in telophase. (b–d) Partial depletion of the dynactin component DNC-1 (b) or of the dynein heavy chain DHC-1 (c) disrupts centrosome flattening as does exposure to low doses of the MT-depolymerizing drug nocodazole (d).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2172887&req=5

fig4: Dynein–dynactin function and intact MTs are required for centrosome flattening. (a) In wild-type embryos expressing tubulin–GFP and histone–GFP fusion proteins, the posterior centrosome becomes flattened in telophase. (b–d) Partial depletion of the dynactin component DNC-1 (b) or of the dynein heavy chain DHC-1 (c) disrupts centrosome flattening as does exposure to low doses of the MT-depolymerizing drug nocodazole (d).
Mentions: The results described above suggest that MFs either recruit or activate a cortical motor protein that pulls on astral MTs to influence the shape and position of mitotic spindles. Both the dynactin complex and the minus end–directed MT motor dynein localize to the cortex of early embryos, and spindle rotation fails in two-cell stage embryos in which the dynein–dynactin complex has been partially depleted by RNA interference (Skop and White, 1998; Gönczy et al., 1999). Further reducing dynein–dynactin function disrupts pronuclear migration and the assembly and orientation of the first mitotic spindle (Gönczy et al., 1999). To determine whether dynein and dynactin are required for centrosome flattening in one-cell stage embryos, we partially depleted either the dynein heavy chain DHC-1 or a C. elegans orthologue of the dynactin component p150glued DNC-1 (see Materials and methods). The posterior centrosome remained spherical in all DNC-1–depleted embryos in which the first mitotic spindle rotated to lie along the a-p axis (Fig. 4 b; n = 9). Similarly, we observed spherical centrosomes in some DHC-1–depleted embryos (Fig. 4 c; 4 out of 20 embryos; 4 embryos exhibited defects in chromosome segregation and in centrosome flattening, whereas 16 embryos appeared wild type during the first mitotic division). Exposure of embryos to low doses of nocodazole that shorten but do not eliminate MTs also disrupted centrosome flattening (Fig. 4 d; five out of seven embryos). We conclude that both dynein function and contact between astral MTs and the cortex are required for centrosome flattening.

Bottom Line: Unlike MFs, dynein, and dynactin, myosin II is not required for the production of these forces.Instead, myosin influences embryonic polarity by limiting PAR-3 to the anterior cortex.This in turn produces asymmetry in the forces applied to MTs at each pole and allows PAR-2 to accumulate in the posterior cortex of a one-cell zygote and maintain asymmetry.

View Article: PubMed Central - PubMed

Affiliation: Institute of Molecular Biology, University of Oregon, Eugene, OR 97403-1229, USA.

ABSTRACT
In Caenorhabditis elegans, the partitioning proteins (PARs), microfilaments (MFs), dynein, dynactin, and a nonmuscle myosin II all localize to the cortex of early embryonic cells. Both the PARs and the actomyosin cytoskeleton are required to polarize the anterior-posterior (a-p) body axis in one-cell zygotes, but it remains unknown how MFs influence embryonic polarity. Here we show that MFs are required for the cortical localization of PAR-2 and PAR-3. Furthermore, we show that PAR polarity regulates MF-dependent cortical forces applied to astral microtubules (MTs). These forces, which appear to be mediated by dynein and dynactin, produce changes in the shape and orientation of mitotic spindles. Unlike MFs, dynein, and dynactin, myosin II is not required for the production of these forces. Instead, myosin influences embryonic polarity by limiting PAR-3 to the anterior cortex. This in turn produces asymmetry in the forces applied to MTs at each pole and allows PAR-2 to accumulate in the posterior cortex of a one-cell zygote and maintain asymmetry.

Show MeSH
Related in: MedlinePlus