Cytoskeleton:
1. Definition: network of fibrillar proteins organized into filaments or tubules.
2. Function of the cytoskeleton:
Gives structure to a cell
Provides mechanical force allowing the cell to change shape and to move
Moves organelles inside cells
Provides mechanical force for cell division
3. Types of cytoskeletal filaments:
Microfilaments (7nm diameter actin protein)
Microtubules (25nm diameter tubulin proteins)
Intermediate Filaments (10nm diameter IF proteins)
MICROFILAMENTS:
Extremely fine strands of the protein actin. Each actin filament consists of two strings of bead-
like subunits twisted together like a rope. The globular subunits (G-actin) are stabilized by
calcium ions and associated with ATP molecules to provide energy for contraction.
Actin filaments are best demonstrated histologically in skeletal muscle cells where they form a
stable arrangement with another type of filamentous protein called myosin. Contraction occurs
when both of the protein filaments slide relative to one another due to the rearrangement of the
intercellular bonds, fueled by ATP.
Actin filaments have structural polarity with plus end and minus end. Actin binds ATP or
ADP (growth is faster on ‘barbed end’ (plus end) where the ATP—actin binds are more
favorably.
Actin filaments assemble by nucleation (new actin filaments are formed)
Formation of G-Actin trimer is energetically unfavorable and so is rate limiting for
nucleation of actin filaments
, Hydrolysis of ATP-actin to ADP-actin decreases stability of actin filament
ATP hydrolysis is not driving polymerization but acts as a timer for filament stability and
disassembly.
The cells contains small proteins, such as thymosin and profilin, that bind to actin monomers in the
cytosol, preventing them from adding to the ends of actin filaments. By keeping actin monomers in
reserve until they are required, these proteins play a crucial role in regulating actin polymerization.
When actin filaments are needed, other actin-binding proteins such as formins and actin-related
proteins (ARPs) promote actin polymerization.
There are a great many actin-binding proteins in cells. Most of these bind to assembled actin filaments
rather than to actin monomers and control the behavior of the intact filaments (Figure 17–32 Actin-
binding proteins control the behavior of actin filaments in vertebrate cells. Actin is shown in
red, and the actin-binding proteins are shown in green.). Actin-bundling proteins, for example,
hold actin filaments together in parallel bundles in microvilli; others cross-link actin filaments together
in a gel-like meshwork within the cell cortex—the specialized layer of actin-filament-rich cytoplasm
just beneath the plasma membrane. Filament-severing proteins fragment actin filaments into shorter
lengths and thus can convert an actin gel to a more fluid state. Actin filaments can also associate with
myosin motor proteins to form contractile bundles, as in muscle cells. And they often form tracks
along which myosin motor proteins transport organelles, a function that is especially conspicuous in
plant cells.
1. Definition: network of fibrillar proteins organized into filaments or tubules.
2. Function of the cytoskeleton:
Gives structure to a cell
Provides mechanical force allowing the cell to change shape and to move
Moves organelles inside cells
Provides mechanical force for cell division
3. Types of cytoskeletal filaments:
Microfilaments (7nm diameter actin protein)
Microtubules (25nm diameter tubulin proteins)
Intermediate Filaments (10nm diameter IF proteins)
MICROFILAMENTS:
Extremely fine strands of the protein actin. Each actin filament consists of two strings of bead-
like subunits twisted together like a rope. The globular subunits (G-actin) are stabilized by
calcium ions and associated with ATP molecules to provide energy for contraction.
Actin filaments are best demonstrated histologically in skeletal muscle cells where they form a
stable arrangement with another type of filamentous protein called myosin. Contraction occurs
when both of the protein filaments slide relative to one another due to the rearrangement of the
intercellular bonds, fueled by ATP.
Actin filaments have structural polarity with plus end and minus end. Actin binds ATP or
ADP (growth is faster on ‘barbed end’ (plus end) where the ATP—actin binds are more
favorably.
Actin filaments assemble by nucleation (new actin filaments are formed)
Formation of G-Actin trimer is energetically unfavorable and so is rate limiting for
nucleation of actin filaments
, Hydrolysis of ATP-actin to ADP-actin decreases stability of actin filament
ATP hydrolysis is not driving polymerization but acts as a timer for filament stability and
disassembly.
The cells contains small proteins, such as thymosin and profilin, that bind to actin monomers in the
cytosol, preventing them from adding to the ends of actin filaments. By keeping actin monomers in
reserve until they are required, these proteins play a crucial role in regulating actin polymerization.
When actin filaments are needed, other actin-binding proteins such as formins and actin-related
proteins (ARPs) promote actin polymerization.
There are a great many actin-binding proteins in cells. Most of these bind to assembled actin filaments
rather than to actin monomers and control the behavior of the intact filaments (Figure 17–32 Actin-
binding proteins control the behavior of actin filaments in vertebrate cells. Actin is shown in
red, and the actin-binding proteins are shown in green.). Actin-bundling proteins, for example,
hold actin filaments together in parallel bundles in microvilli; others cross-link actin filaments together
in a gel-like meshwork within the cell cortex—the specialized layer of actin-filament-rich cytoplasm
just beneath the plasma membrane. Filament-severing proteins fragment actin filaments into shorter
lengths and thus can convert an actin gel to a more fluid state. Actin filaments can also associate with
myosin motor proteins to form contractile bundles, as in muscle cells. And they often form tracks
along which myosin motor proteins transport organelles, a function that is especially conspicuous in
plant cells.
