Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

CYTOKINESIS
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remaining interpolar microtubules
from central spindle
(A)
contractile ring of actin and
myosin filaments in cleavage furrow
(B)
(C)
0.5 µm 10 µm
nucleate the assembly of parallel arrays of linear, unbranched actin filaments (discussed
in Chapter 16). After anaphase, the overlapping arrays of actin and myosin
II filaments contract to generate the force that divides the cytoplasm in two. Once
contraction begins, the ring exerts a force large enough to bend a fine glass needle
that is inserted in its path. As the ring constricts, it maintains the same thickness,
suggesting that its total volume and the number of filaments it contains decrease
steadily. Moreover, unlike actin in muscle, the actin filaments in the ring are
highly dynamic, and their arrangement changes continually during cytokinesis.
The contractile ring is finally dispensed with altogether when cleavage ends,
as the plasma membrane of the cleavage furrow narrows to form the midbody.
The midbody persists as a tether between the two daughter cells and contains
MBoC6 m17.50/17.42
the remains of the central spindle, a large protein structure derived from the antiparallel
interpolar microtubules of the spindle midzone, packed tightly together
within a dense matrix material (Figure 17–43). After the daughter cells separate
completely, some of the components of the residual midbody often remain on the
inside of the plasma membrane of each cell, where they may serve as a mark on
the cortex that helps to orient the spindle in the subsequent cell division.
Figure 17–42 The contractile ring.
(A) A drawing of the cleavage furrow in a
dividing cell. (B) An electron micrograph of
the ingrowing edge of a cleavage furrow
of a dividing animal cell. (C) Fluorescence
micrographs of a dividing slime mold
amoeba stained for actin (red) and myosin
II (green). Whereas all of the visible myosin
II has redistributed to the contractile ring,
only some of the actin has done so; the
rest remains in the cortex of the nascent
daughter cells. (B, from H.W. Beams and
R.G. Kessel, Am. Sci. 64:279–290, 1976.
With permission from Sigma Xi; C, courtesy
of Yoshio Fukui.)
Local Activation of RhoA Triggers Assembly and Contraction
of the Contractile Ring
RhoA, a small GTPase of the Ras superfamily (see Table 15–5), controls the assembly
and function of the contractile ring at the site of cleavage. RhoA is activated at
the cell cortex at the future division site, where it promotes actin filament formation,
myosin II assembly, and ring contraction. It stimulates actin filament formation
by activating formins, and it promotes myosin II assembly and contractions
by activating multiple protein kinases, including the Rho-activated kinase Rock
(Figure 17–44). These kinases phosphorylate the regulatory myosin light chain,
a subunit of myosin II, thereby stimulating bipolar myosin II filament formation
and motor activity.
RhoA is thought to be activated by a guanine nucleotide exchange factor (Rho-
GEF), which is found at the cell cortex at the future division site and stimulates
the release of GDP and binding of GTP to RhoA (see Figure 17–44). We know little
about how the RhoGEF is localized or activated at the division site, although the
microtubules of the anaphase spindle seem to be involved, as we discuss next.
The Microtubules of the Mitotic Spindle Determine the Plane of
Animal Cell Division
The central problem in cytokinesis is how to ensure that division occurs at the
right time and in the right place. Cytokinesis must occur only after the two sets of
chromosomes are fully segregated from each other, and the site of division must