96
5 Eukaryotic Cells
Fig. 5.6 (a) Transitions between different cytoskeletal structures, showing actin filaments and
thick bundles. Crosslinker proteins are shown by red dots. (b) and (c) Electron micrographs of an
entangled actin network (b) and a composite network (c) with bundles embedded in an isotropic
background (Bausch and Kroy, 2006)
In a filament at rest, the barbed end is attached to another filament or to the plasma
membrane, and should be detached for a moment to enable another monomer unit
to be inserted, so that the treadmill could keep moving. Stressing the filament, either
by external force or with the help of attached myosin motors (Fig. 5.5b), alleviates
insertion and makes the filament grow faster. An overview of the recycling of an
actin filament, aided by deconstructor, constructor, and capping (limiter) molecules,
is sketched in Fig. 5.5c.
Depending on the specific type of crosslinker proteins, different structures,
sketched in Fig. 5.6, are predicted and observed. Bundles may be embedded in
a continuous isotropic background network. The principal cytoskeletal components,
actin filaments and microtubules, “cross-talk" in a number of ways (Dogterom and
Koenderink, 2019). Dynamic links attach actin bundles to the plus ends of growing microtubules, thereby guiding their mutual alignment. On the other hand, the
actin cortex near the plasma membrane anchors or imposes a physical barrier on
the growth of microtubules, preventing them from hitting the plasma membrane
(Fig. 5.7). Conversely, actin filaments nucleate at the ends of microtubules.
Why should Nature prefer dynamic structures, continuously rebuilding themselves, while remaining stationary in the long run? Treadmilling of actin monomers
costs energy, and so does fluctuating stress due to attachment and detachment of
myosin motors: this is why we get tired even holding a hand horizontally in the air
without doing any work. The benefit is flexibility, readiness to reshape and move
at any moment following external inputs or internal needs. A crude analogy is the
advantage of movable type, Gutenberg’s invention, over woodblock printing. The
cell combines amino acids as letters to the lines of proteins, which can be broken
5 Eukaryotic Cells
Fig. 5.6 (a) Transitions between different cytoskeletal structures, showing actin filaments and
thick bundles. Crosslinker proteins are shown by red dots. (b) and (c) Electron micrographs of an
entangled actin network (b) and a composite network (c) with bundles embedded in an isotropic
background (Bausch and Kroy, 2006)
In a filament at rest, the barbed end is attached to another filament or to the plasma
membrane, and should be detached for a moment to enable another monomer unit
to be inserted, so that the treadmill could keep moving. Stressing the filament, either
by external force or with the help of attached myosin motors (Fig. 5.5b), alleviates
insertion and makes the filament grow faster. An overview of the recycling of an
actin filament, aided by deconstructor, constructor, and capping (limiter) molecules,
is sketched in Fig. 5.5c.
Depending on the specific type of crosslinker proteins, different structures,
sketched in Fig. 5.6, are predicted and observed. Bundles may be embedded in
a continuous isotropic background network. The principal cytoskeletal components,
actin filaments and microtubules, “cross-talk" in a number of ways (Dogterom and
Koenderink, 2019). Dynamic links attach actin bundles to the plus ends of growing microtubules, thereby guiding their mutual alignment. On the other hand, the
actin cortex near the plasma membrane anchors or imposes a physical barrier on
the growth of microtubules, preventing them from hitting the plasma membrane
(Fig. 5.7). Conversely, actin filaments nucleate at the ends of microtubules.
Why should Nature prefer dynamic structures, continuously rebuilding themselves, while remaining stationary in the long run? Treadmilling of actin monomers
costs energy, and so does fluctuating stress due to attachment and detachment of
myosin motors: this is why we get tired even holding a hand horizontally in the air
without doing any work. The benefit is flexibility, readiness to reshape and move
at any moment following external inputs or internal needs. A crude analogy is the
advantage of movable type, Gutenberg’s invention, over woodblock printing. The
cell combines amino acids as letters to the lines of proteins, which can be broken