Actin Cytoskeleton

(Lodish et al., 2000, Section 18.1 )

Three types of cytosolic fibers:

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

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

Actin cytoskeleton (Fig. 18.1)

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 GTP or GDP associated state
F-actin is the filamentous polymer and can exist in the GTP or GDP 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

The - end has the GTP 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).
Actin is organized into bundles and networks (Fig. 18.4) through the binding of crosslinking protein (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 anbles (Fig. 18.5b).

Membrane-bound networks provide a membrane mesh (Fig. 18.6) that actin presses against to move and support membrane position (Fig. 18.7).

Platelet (Fig. 18.9a),
Muscle cell (Fig. 18.9b),
Epithelial cell (Fig. 18.9c).
Microvilli cell in intestine (Fig. 18.10).

Activity

Quiz

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.