Muscle and Nonmuscle Contractile Systems
grow toward the cell membrane and continue to grow at
the membrane, they will create a projection of membrane
and cytoplasm in the direction of growth, especially if the
filaments are connected by short linking proteins into rigid
bundles. In this way, microvilli extend from the surfaces
of many cell types, including highly specialized structures
such as stereocilia in sensory cells of the ear. Projection
of the acrosomal process through the zona pellucida is
driven in this way, as are the extension of filopodia and
lamellipodia at the leading edges of migrating cells.
The structure and properties of actin filaments can be
regulated by controlling the transformation of G-actin to
F-actin or the length of the F-actin filaments, and by mo-
dulating the aggregation of actin filaments into bundles
or three-dimensional arrays. Proteins that bind actin
monomers reduce polymerization. Gelsoin, villin, and
other proteins affect actin polymerization and actin
filament elongation by capping the growing filament and
blocking elongation. Some accelerate nucleation, perhaps
by binding to and stabilizing dimers and trimers. Many
capping proteins can also sever actin filaments without
depolymerizing them, markedly reducing the viscosity of
F-actin gels. The severing and capping activities of
gelsolin require Ca2+. Although nucleation and severing
increase the number of free ends available for growth,
the net effect of these proteins is a greater number of
short actin filaments and an increased concentration of
monomeric actin.
drugs that inhibit cellular processes
that require actin polymerization and depolymerization
(e.g., phagocytosis, cytokinesis, clot retraction, etc.), also
act by severing and capping actin filaments. Actin fila-
ments can be stabilized by
derived from the
poisonous mushroom
Amanita phalloides.
Assembly of
actin filaments into bundles (as in microvilli) and three-
dimensional networks is accomplished by two groups of
cross-linking proteins (Table 21-6).
Tubulin and microtubules occur in all plant, animal, and
prokaryotic cells and participate in a number of essen-
tial processes. As is the case for actin filaments, micro-
tubules occur in highly organized, relatively permanent
forms such as cilia and flagella, and as transient cytoplas-
mic structures. Other similarities between actin filaments
and microtubules are listed in Table 21-1.
are hair-like cell surface projections, typically
in diameter and 10 yum long, found densely
packed on many types of cells. In eukaryotes, they
move fluid past cells by a characteristic whip-like motion
(Figure 21-15).
Eukcu yoUcflagella
are basically elongated
FIGURE 21-15
Cycle of ciliary stroke shown for three cilia beating in synchrony.
(1) Beginning of the power stroke. The cilia are straight and stiff. (2) Half
completion of the power stroke. (3) Start of recovery stroke. The cilia are
flexible, and by bending reduce frictional drag. (4) Near completion of the
recovery stroke. (5) Start of the next power stroke.
cilia, one or two per cell, that propel the cell through a
fluid medium. Flagellar movement is wavelike and dis-
tinct from ciliary beating, although the microtubular ar-
rangement and mechanism of movement are quite similar
in the two structures. Bacterial flagella are structurally and
functionally different from other flagella.
Figure 21-16 shows a cross-section of a cilium, show-
ing the “9 + 2” arrangement of microtubules found in the
axoneme (core) of cilia and flagella. Nine asymmetrical
doublet microtubules are arranged in the periphery, and
a symmetrical pair of singlet tubules is in the center. At-
tached to the A microtubules of each doublet are dynein
arms extending toward the B microtubule of the adja-
cent doublet. The protein tektin, a highly helical protein
about 48 nm long, runs along the outside of each dou-
blet where the A and B microtubules join. It is regarded
as a structural rather than a regulatory protein despite the
similarity to tropomyosin. Three types of links preserve
the axoneme structure. The central singlets are linked by
structures, called inner bridges, like rungs on a ladder, and
are wrapped in a fibrous structure called the inner sheath.
Adjacent outer doublets are joined by links of nexin (M.W.
150,000-165,000) spaced every
8 6
nm. Outer doublets are
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