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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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Nonmuscle Myosin II is required to establish a normal PAR boundary, but is not required for centrosome flattening. (a and e) PAR-2 usually accumulates in a small posterior patch in nmy-2(RNAi) and mlc-4(RNAi) mutant embryos. (b and f) Both centrosomes are round in nmy-2(RNAi) and mlc-4(RNAi) embryos. (d and h) Both centrosomes flatten in nmy-2(RNAi); par-3(RNAi) and mlc-4(RNAi); par-3(RNAi) embryos. (c and g) PAR-2 extends around the anterior of nmy-2(RNAi); par-3(RNAi) and mlc-4(RNAi); par-3(RNAi) embryos.
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fig3: Nonmuscle Myosin II is required to establish a normal PAR boundary, but is not required for centrosome flattening. (a and e) PAR-2 usually accumulates in a small posterior patch in nmy-2(RNAi) and mlc-4(RNAi) mutant embryos. (b and f) Both centrosomes are round in nmy-2(RNAi) and mlc-4(RNAi) embryos. (d and h) Both centrosomes flatten in nmy-2(RNAi); par-3(RNAi) and mlc-4(RNAi); par-3(RNAi) embryos. (c and g) PAR-2 extends around the anterior of nmy-2(RNAi); par-3(RNAi) and mlc-4(RNAi); par-3(RNAi) embryos.

Mentions: We next examined how myosin II influences the MF-dependent forces that flatten spindle poles. Depletion of the nonmuscle myosin II heavy chain (NMY)-2 or of the myosin II regulatory light chain (MLC)-4 (Guo and Kemphues, 1996; Shelton et al., 1999) results in embryonic polarity defects similar to those in LatA-treated embryos: the first mitotic spindle remains centrally positioned and both spindle poles remain spherical (Fig. 3, b and f) . However, one difference is that PAR-3 accumulates around both the anterior and posterior cortex in embryos depleted of either myosin II subunit (Guo and Kemphues, 1996; Shelton et al., 1999), whereas PAR-3 is not present at the cortex in LatA-treated embryos (see above). In contrast to PAR-3, PAR-2 was usually present in a reduced cortical patch in mutant embryos depleted of NMY-2 or MLC-4, (Fig. 3, a and e; five out of eight nmy-2 and six out of eight mlc-4 embryos) or was undetectable at the cortex (three out of eight nmy-2 and two out of eight mlc-4 mutants) (Shelton et al., 1999). Thus, unlike MFs, neither NMY-2 nor MLC-4 are required for PAR-3 to associate with the cortex, but they are required for the polarized distribution of cortical PAR-3 and for the posterior cortical localization of PAR-2.


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)

Nonmuscle Myosin II is required to establish a normal PAR boundary, but is not required for centrosome flattening. (a and e) PAR-2 usually accumulates in a small posterior patch in nmy-2(RNAi) and mlc-4(RNAi) mutant embryos. (b and f) Both centrosomes are round in nmy-2(RNAi) and mlc-4(RNAi) embryos. (d and h) Both centrosomes flatten in nmy-2(RNAi); par-3(RNAi) and mlc-4(RNAi); par-3(RNAi) embryos. (c and g) PAR-2 extends around the anterior of nmy-2(RNAi); par-3(RNAi) and mlc-4(RNAi); par-3(RNAi) embryos.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2172887&req=5

fig3: Nonmuscle Myosin II is required to establish a normal PAR boundary, but is not required for centrosome flattening. (a and e) PAR-2 usually accumulates in a small posterior patch in nmy-2(RNAi) and mlc-4(RNAi) mutant embryos. (b and f) Both centrosomes are round in nmy-2(RNAi) and mlc-4(RNAi) embryos. (d and h) Both centrosomes flatten in nmy-2(RNAi); par-3(RNAi) and mlc-4(RNAi); par-3(RNAi) embryos. (c and g) PAR-2 extends around the anterior of nmy-2(RNAi); par-3(RNAi) and mlc-4(RNAi); par-3(RNAi) embryos.
Mentions: We next examined how myosin II influences the MF-dependent forces that flatten spindle poles. Depletion of the nonmuscle myosin II heavy chain (NMY)-2 or of the myosin II regulatory light chain (MLC)-4 (Guo and Kemphues, 1996; Shelton et al., 1999) results in embryonic polarity defects similar to those in LatA-treated embryos: the first mitotic spindle remains centrally positioned and both spindle poles remain spherical (Fig. 3, b and f) . However, one difference is that PAR-3 accumulates around both the anterior and posterior cortex in embryos depleted of either myosin II subunit (Guo and Kemphues, 1996; Shelton et al., 1999), whereas PAR-3 is not present at the cortex in LatA-treated embryos (see above). In contrast to PAR-3, PAR-2 was usually present in a reduced cortical patch in mutant embryos depleted of NMY-2 or MLC-4, (Fig. 3, a and e; five out of eight nmy-2 and six out of eight mlc-4 embryos) or was undetectable at the cortex (three out of eight nmy-2 and two out of eight mlc-4 mutants) (Shelton et al., 1999). Thus, unlike MFs, neither NMY-2 nor MLC-4 are required for PAR-3 to associate with the cortex, but they are required for the polarized distribution of cortical PAR-3 and for the posterior cortical localization of PAR-2.

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