4.5 The Cytoskeleton
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Recognize the structure (including size, polarity) and describe the functions of microfilaments, intermediate filaments, and microtubules
- Evaluate the role of ATP and GTP in cytoskeleton polymerization and depolymerization
- Explain the how motor proteins interact with different components of the cytoskeleton to promote movement (of cells, within cells)
- Compare and contrast the structure and function of cilia and flagella
- Give examples of methods that can be used to visualize microfilaments, intermediate filaments, and microtubules with microscopy
- Give examples of how the abundance or structure of each cytoskeletal component varies among different cell types
As briefly discussed in Chapter 4.2, the cytoskeleton is a protein framework that has multiple roles, including structural support, locomotion, cell division, and bulk transport. The cytoskeleton is composed of three main components: microfilaments, intermediate filaments, and microtubules. Each component is fibrous (long and “stringy”) and made up of specific proteins. Throughout this chapter section we will discuss the structure and functions of these cytoskeletal components in more detail. We will also look at a two specialized cellular structures that utilize microtubules for cellular locomotion: flagella and cilia.
Microfilaments
Of the three types of protein fibers in the cytoskeleton, microfilaments are the narrowest. They are 7 nm in diameter and between 100 nm and 2 µm in length. Microfilaments are composed of multiple copies of a protein called actin. When actin is first synthesized, it is a globular protein, and thus referred to as G-actin. Multiple G-actin subunits bind to each other, creating a filamentous form of actin known as F-actin (Figure 4.17). ATP is required for G-actin to polymerize (assemble) into F-actin. Two strands of F-actin then coil around each other creating the long and narrow microfilament. The microfilaments have a plus (+) and a minus (-) end to denote some polarity to the model (i.e. the two free ends are different), but the ends of the filaments are not electrically charged.
Many microfilaments can come together to form a mesh framework. This formation gives them strong tensile force, which is imperative for their function. The mesh framework is particularly abundant underneath the cell membrane. Due to its tensile strength, it can support the shape of the cell membrane, especially in cells that do not have a cell wall. Because actin is so thin, it can support very delicate shapes such as the microvilli (cell membrane folds) of cells lining animal (including human) intestines (Figure 4.18).
Microfilaments can quickly depolymerize (disassemble) and repolymerize (reassemble), allowing them to “move”. This is done by adding or removing G-actin monomers, which are loose in the cytosol, to the + or – end of the filament, through ATP hydrolysis. Whenever