Actin molecules (42 kDa) are abundant and may constitute as much as 20% of the total protein of some nonmuscle cells (Fig. 2.45). Similar to the tubulin in microtubules, actin molecules also assemble spontaneously by polymerization into a linear helical array to form filaments 6 to 8 nm in diameter. They are thinner, shorter, and more ﬂexible than microtubules. Free actin molecules in the cytoplasm are referred to as G-actin (globular actin), in contrast to the polymerized actin of the filament, which is called F-actin (flamentous actin). An actin filament is a polarized structure; its fast-growing end is referred to as the plus (barbed) end, and its slow-growing end is referred to as the minus (pointed) end. The dynamic process of actin polymerization requires the presence of K , Mg2 , and ATP, which is hydrolyzed to ADP after each G-actin molecule is incorporated into the filament (Fig. 2.46). The control and regulation of the polymerization process depends on the local concentration of G-actin and the interaction of actinbinding proteins (ABPs), which can prevent or enhance polymerization.
In addition to controlling the rate of polymerization of actin filaments, ABPs are responsible for the filaments’ organization. For example, a number of proteins can modify or act on actin filaments to give them various specific characteristics:
FIGURE 2.44 • Distribution of kinesinlike motor protein within the mitotic spindle. This confocal immunofuorescent image shows a mammary gland epithelial cell in anaphase of mitosis. Each mitotic spindle pole contains two centrioles (green). A mitosis-specifc kinesinlike molecule called Eg5 (red ) is associated with the subset of the mitotic spindle microtubules that connect the kinetochores (white) to the spindle poles. The motor action of Eg5 is required to separate the sister chromatids (blue) into the daughter cells. This cell was frst immunostained with three primary antibodies against Eg5 (red), centrin (green), and kinetochores (white) and then incubated in three different fuorescently tagged secondary antibodies that recognize the primary antibodies. Chromosomes were stained with a fuorescent molecule that intercalates into the DNA double helix. 3,500. (Courtesy of Dr. Wilma L. Lingle and Ms. Vivian A. Negron.)  Actin-bundling proteins cross-link actin filaments into parallel arrays, creating actin filament bundles. An example of this modification occurs inside the microvillus, where actin filaments are cross-linked by the actin-bundling proteins fascin and fmbrin. This cross-linkage provides support and imparts rigidity to the microvilli.