Introduction to Molecular and Cell Biology, Biol. 220 Lecture 31: Intermediate Fillaments
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Intermediate filaments are 10 nm in diameter, and are intermediate in size between microfilaments (7 nm) and microtubules (24 nm).

Intermediate filament monomers are not globular proteins, nor do they bind nucleotides, but are alpha-helical rods that assemble into rope-like structures (Fig. 19.51).

Fig. 19.51  Intermediate Filament Structure

Cytoplasmic intermediate filaments are present only in cells that display multicellular organization.

Intermediate filaments distribute tensile forces across cells in a tissue (Fig. 19.56).

Fig. 19.56  Cell support through intermediate filaments attached to cell junctions.

Intermediate filaments are not involved in cell movement and do not have associated motor proteins.

Unlike microfilaments and microtubules, intermediate filaments (IFs) assemble from a large number of different IF proteins.

There are six general types of IF proteins (Table 19.3).

Table 19.3

• Lamins, which provide strength to the nuclear membrane, are expressed in all eukaryotic cells, where other types are expressed in specific tissues.

Assembly, Disassembly, and Organization

• Intermediate filaments are more stable and less dynamic in their assembly / disassembly than other filaments.
• Phosphorylation of the head domain depolymerizes intermediate filaments in vivo.
• Assembly of intermediate filaments probably involves both lateral and end-to-end additions (Fig. 19.51).
• The organization of intermediate filaments into networks and bundles is mediated by various intermediate filaments associated proteins (IFAPs).
• IFAPs also crosslink intermediate filaments to the plasma and nuclear membranes, microtubules, and microfilaments (Fig. 19.54).
  Fig. 19.54  Intermediate Filament Associated Proteins (IFAPs, green) connect Intermediate Filaments (blue) to microcubules (red).
  
  

Major degenerative diseases of the skin, muscle, and neurons are caused by disruption of the intermediate filament cytoskeleton or its connections to other cell structures.

• Muscle cells, desmin (Fig. 19.57).
  Fig. 19.57  Desmin filaments in muscle.
  
  
• Neuron cells, axon diameter determined by several neurofilaments.
• Skin disease caused by keratins mutations (Fig. 19.58).

Fig. 19.58  Skin tissue with normal and mutant keratin.