Essential Cell Biology 5th edition

Actin Filaments
595
keeps the actin monomers in cells from polymerizing totally into filaments?
The answer is that cells contain small proteins, such as thymosin
and profilin, that bind to actin monomers in the cytosol, preventing them
from adding to the ends of actin filaments. By keeping actin monomers
in reserve until they are required, these proteins play a crucial role in
regulating actin polymerization. When actin filaments are needed, other
actin-binding proteins such as formins and actin-related proteins (ARPs)
promote actin polymerization.
There are a great many actin-binding proteins in cells. Most of these
bind to assembled actin filaments and control their behavior (Figure
17–32). Actin-bundling proteins, for example, hold actin filaments
together in parallel bundles in microvilli; others cross-link actin filaments
together in a gel-like meshwork within the cell cortex—the specialized
layer of actin-filament-rich cytoplasm just beneath the plasma membrane.
Filament-severing proteins fragment actin filaments into shorter
lengths and thus can convert an actin gel to a more fluid state. Actin
filaments can also associate with myosin motor proteins to form contractile
bundles, as in muscle cells. And they often form tracks along which
myosin motor proteins transport organelles, a function that is especially
conspicuous in plant cells.
In the remainder of this chapter, we consider some characteristic structures
that actin filaments can form, and we discuss how different types
Figure 17–32 Actin-binding proteins
control the behavior of actin filaments
in vertebrate cells. Actin is shown in red,
and the actin-binding proteins are shown in
green.
actin monomers
monomersequestering
protein
nucleating protein
(e.g., formin, ARP complex)
severing protein
actin filaments
bundling protein
(in filopodia)
myosin motor protein
cross-linking
protein (in cell cortex)
side-binding
protein
(e.g., tropomyosin)
capping (plus-end-blocking) protein