Cytoskeleton. Actin and intermediary filaments

Three filament systems, told apart by diameter.

Actin filaments, the microfilaments, are 7 nm, the thinnest filaments: highly dynamic, very thin, double-stranded, located close to the plasma membrane in the cell cortex. They are linear F-actin polymers of G-actin monomers, and they grow by addition at the (+) or barbed end and shrink at the (−) or pointed end, a balance known as treadmilling.

Intermediate filaments are 8 to 10 nm, between microfilaments and microtubules in size, and they are stable. They come in 6 types: keratin, vimentin, desmin, GFAP, neurofilament and lamins.

Thick myosin filaments are 15 nm.

Actin filaments, also called microfilaments, are a highly dynamic, very thin, double-stranded structure.

• Their diameter is 7 nm, making them the thinnest filaments.
• They are abundantly located close to the plasma membrane, at the periphery, in the cell cortex.
• Their structure is linear F-actin polymers of G-actin monomers.
• They are shorter but more flexible than microtubules.

Actin filaments show dynamic instability, as microtubules do, but the ends have their own names.

• Addition of G-actin subunits happens at the (+), or barbed, end.
• Removal happens at the (−), or pointed, end.
• The balance of polymerization and depolymerization is known as treadmilling.

The image the word gives is exact: subunits are added at one end and lost at the other at the same rate, so the filament stays the same length while material moves through it.

• Transport of organelles, vesicles and granules, in the process of cytoplasmic streaming.
• Maintaining cell structure.
• Muscle contraction: they generate forces on the filaments to support contraction.
• Rapid assembly and disassembly of filaments facilitate cell migration.

Intermediate filaments consist of different proteins in different cell types, which are strung together to polymers.

• Their diameter is 8 to 10 nm, in size between microfilaments and microtubules. That is where the name comes from.
• They are stable, unlike the other two systems, which are dynamic.

They are subcategorised into 6 types by amino acid sequence:

The list is worth learning as a map of tissue types: name the tissue and the filament follows. Lamins are the exception, being defined by location inside the nucleus rather than by a tissue.

The figure on this page gives the diameter of intermediate filaments as 8 to 12 nm, where the text gives 8 to 10 nm. Both ranges are in use; what matters is that intermediate filaments sit between the 7 nm actin filaments and the 25 nm microtubules, which is what the name records.

Thick myosin filaments have a diameter of 15 nm.

• Heavy meromyosin, the heavy chain, is the head plus the proximal 60 nm of the tail.
• Light meromyosin, the light chain, is the distal 90 nm of the tail.
• One filament is made of 2 heavy chains and 2 pairs of light chain.