Figure 3.46 (a) A microfilament is composed of a pair of actin filaments. (b) Each actin filament is a string of
polymerized actin monomers. (c) The dynamic nature of actin, due to its polymerization and depolymerization and its
association with myosin, allows microfilaments to be involved in a variety of cellular processes, including ameboid
movement, cytoplasmic streaming, contractile ring formation during cell division, and muscle contraction in animals.
Intermediate filaments are a diverse group of cytoskeletal filaments that act as cables within the cell. They are termed
“intermediate” because their 10-nm diameter is thicker than that of actin but thinner than that of microtubules.[26]
They are composed of several strands of polymerized subunits that, in turn, are made up of a wide variety of
monomers. Intermediate filaments tend to be more permanent in the cell and maintain the position of the nucleus.
They also form the nuclear lamina (lining or layer) just inside the nuclear envelope. Additionally, intermediate
filaments play a role in anchoring cells together in animal tissues. The intermediate filament protein desmin is found
in desmosomes, the protein structures that join muscle cells together and help them resist external physical forces.
The intermediate filament protein keratin is a structural protein found in hair, skin, and nails (Figure 3.47).
- E. Fuchs, D.W. Cleveland. “A Structural Scaffolding of Intermediate Filaments in Health and Disease.”Science279 no. 5350
(1998):514–519.
Chapter 3 | The Cell 123