Introduction to Molecular and Cell Biology, Biol. 220 Lecture 27: Actin
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Three types of cytosolic fibers:

• microfilaments (7-9 nm in diameter)  actin
• intermediate filaments (10 nm in diameter)
• microtubules (24 nm in diameter)

Actin

Actin is the most abundant intracellular protein in eukaryotic cells (10% by weight of total cell protein).

Actin cytoskeleton (Fig. 18.1)

Fig. 18.1  (a) external view of cell and types of podia projections. (b) localization of actin at lamellipodium and as stress fibers.

• used to extend filopodium (finger-like projections)
• used to extend lamellipodium (large protrusions)
• used as stress fibers to anchor parts of cells together

Yeast have a single actin gene, humans have six actin genes, and some plants have 60 actin genes.
Highly conserved but isoforms exist.

• Vertebrate alpha actin present in muscle cells.
• Vertebrate beta and gamma actin present in nonmuscle cells.

Actin exists in two interchangeable forms

• G-actin is the globular monomer and can exist in the ATP or ADP associated state
• F-actin is the filamentous polymer and can exist in the ATP or ADP associated state

Increasing the ionic strength of a solution by the addition of Mg2+, K+, or Na+ causes G-actin to be converted into F-actin.  Conversely, lowering the ionic strength causes depolymerization back to the G-actin form.

F-actin shows polarity

Fig. 18.2  (a) Structure of actin.  (b) Actin filaments.  (c) Stacking of actin monomers in the F-actin showing polarity of filament.

• The - end has the ATP binding cleft exposed (Fig. 18.2).
• The + end has the opposite end of the protein exposed.

F-actin stacks in a helical filament (Fig. 18.2c).

Fig. 18.5  (a) Bundled actin filaments (short crosslinker).  (b) Network of actin filaments (long, flexible crosslinker).

Actin is organized into bundles and networks (Fig. 18.4) through the binding of crosslinking protein (Table 18.1). Table 18.1 

Short crosslinking proteins assemble actin filaments into bundles(Fig. 18.5a).
Long, flexible crosslinking proteins can connect actin filament pairs lying at various angles (Fig. 18.5b).

Membrane-bound networks provide a membrane mesh (Fig. 18.6)

Fig. 18.6  Human erythrocyte showing the hubs and linking spectrin network.

that actin presses against to move and support membrane position (Fig. 18.7).

Fig. 18.7  The organization of the major erythrocyte cytoskeletal proteins and their interactions with integral membrane proteins.

Examples

• Platelet (Fig. 18.9a),
• Muscle cell (Fig. 18.9b),
• Epithelial cell (Fig. 18.9c).
• Fig. 18.9  Cross-linkage of actin filament networks to the plasma membrane in (a) platelets, (b) muscle cells, and (c) epithelial cells.
  
  
  

• Microvilli cell in intestine (Fig. 18.10).
• Fig. 18.10  Intestinal cell showing microvilli and the core actin filaments that help them maintain their shape.
  

Activity

Created 2004 by CA Rinehart• email CA Rinehart

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