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(Lodish et al., 2000, Section 18.2, 18.3, 18.5)

Actin polymerization occurs in three steps (Fig. 18.11) (Movie).

Lag period in which G-actin aggregates.
Elongation period with rapid addition of actin monomers to both of its ends.
Steady state where monomers exchange with subunits at filament ends but there is no net change in total filament mass.

After ATP-G-actin is incorporated the ATP is slowly hydrolyzed.

The + end of the filament grows 5-10 times faster than the - end (Fig. 18.13a).

Actin filaments can be caused to grow in one direction by capping the opposite end (Fig 18.13.b).

When the G-actin concentration is less than the critical concentration (Fig. 18.12) then depolymerization occurs, if the G-actin concentration is above the critical concentration then polymerization will occur (Fig. 18.13c).

Toxins, such as the fungal alkaloid cytochalasin D and the sponge toxin latrunculin, block the addition of subunits to F-actin.

The conentration of G-actin available for polymerization is controlled by the amount of thymosin beta 4, which binds 1:1 with actin and prevents polymerization.

Profilin binds to the + side of G-actin and facilitates binding at the + end of growing filaments.

Profilin also binds to PIP2 at membranes and is released by cell-cell signalling.
Release of profilin results in polymerization of actin near membranes (Fig. 18.15).

Gelsolin and cofilin operate as "serving proteins" that break long filaments into shorter filaments. This is important in reducing the gel-like viscosity of the cytosol where actin filaments are long and converts the cytosol into a more liquid and fluid state that is able to move in the cells direction of movement. These proteins are also bound by PIP2 at membranes where they remain inactive until released.

Capping proteins bind to the ends of actin filaments and prevent further polymerization at that end. Capped actin filaments are needed in places there the organization of the cytoskeleton us unchanging (Fig 18.13.b).

CapZ binds the + end of actin filaments and is inhibited when bound to PIP2.
Tropomodulin binds the - end of actin filaments and capping activity is enhanced by tropomyosin.

Myosin acts as a motor that walks along actin filaments. There are several types of myosin but three major types are found in cells (Fig. 18.20).

Myosin I and V are involved in cytoskeleton-membrane interactions (Fig. 18.21a) and transport of membrane vesicles to and from golgi.
Myosin II powers muscle contraction (Fig. 18.21b) and cytokinesis (division of cell, Fig. 18.37a).

Myosin uses ATP as an energy source to move along the actin filament (Fig. 18.25) (Movie).

Myosin and actin filaments are used to power cytoplasmic streaming in Nitella cells (Fig. 18.40).

Cell movement (Fig. 18.42).
The interaction of polymerizing actin filaments and movement of myosin 1 is associated with the movement of the leading edge of cell movement (Fig. 18.43a) (Movie).
The interaction of cofilin with the actin filament at the trailing edge of the cell helps convert the actin gel to a soluble form. Myosin II is also found at the trailing edge (Fig. 18.43a) and operates in cortical contraction to pull the membrane free of the adhesion points.

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References:
Lodish, H., Berk, A., Zipursky, S.L., Matsudaira, P., Baltimore, D., Darnell, J., 2000, Molecular Cell Biology, 4th Ed., W.H. Freeman and Company, NY, New York. ISBN 0-7167-3136-3.