Introduction to Molecular and Cell Biology, Biol. 220 Lecture 28: Actin Assembly
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Actin polymerization occurs in three steps (Fig. 18.11) (Movie).

Fig. 18.11  (a) Polymerization of actin.  (b) Different rates of polymerization occur with and without nucleating particles.

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).

Fig. 18.12  The critical concentration Cc- is illustrated as the actin concentration at which filaments begin to form.

Fig. 18.13a  Demonstration that polymerization occurs faster at the + end of the F-actin filament.

Fig. 18.13b  Capping proteins attached to the end of F-actin prevents further polymerization or depolymerization at that end.

Fig. 18.13c  Actin monomers can be treadmilled from one end of the growing actin to the other at a monomer concentration between the  critical concentration of the - and + ends.

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).
• Fig. 18.15  Action of Profilin on acceleration of ATP-G-actin polymerization.
  
  
  

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 where the organization of the cytoskeleton us unchanging (Fig 18.13.b).

Fig. 18.13b  Capping proteins attached to the end of F-actin prevents further polymerization or depolymerization at that end.

• 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

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).

Fig. 18.20  Different types of myosin motors.

• Myosin I and V are involved in cytoskeleton-membrane interactions (Fig. 18.21a) and transport of membrane vesicles to and from golgi.
• Fig. 18.21a  Membrane bound myosins.
  

• Myosin II powers muscle contraction (Fig. 18.21b) and cytokinesis (division of cell, Fig. 18.37a).

Fig. 18.21b  The structure of myosin II in bundles used in muscles.

Fig. 18.37a  The localization and contraction of myosin II and actin filaments during cytokinesis.

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

Fig. 18.25  Energy dependent movement of myosin motor along actin filament.

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

Fig. 18.40  Myosin motors bound to organelles move along actin tracks to create cytoplasmic streaming in Nitella cells.

Cell movement (Fig. 18.42).

Fig. 18.42  Action of myosin and actin in cell movement.

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).

Fig. 18.43  Localization of myosin I at the leading edge of moving cell.  The localization of myosin II at the lagging edge ofa moving cell.

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.

Created 2004 by CA Rinehart• email CA Rinehart

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