Detailed Structure of Microfilaments
Microfilaments are dynamic, helical polymers primarily composed of actin protein. They play a key
role in maintaining cell shape, enabling motility, and intracellular transport.
1. Actin Monomers:
• Actin Types: Actin exists in two forms:
◦ G-actin (Globular actin): A single, soluble monomeric form.
◦ F-actin (Filamentous actin): A polymerized, helical form of actin.
• Structure of G-actin:
◦ Each actin monomer is ~42 kDa in size.
◦ It has an ATP/ADP binding cleft, which regulates its polymerization.
2. Filament Formation:
Microfilaments are formed by the polymerization of G-actin into F-actin. This process involves the
following steps:
a. Nucleation
• The first and rate-limiting step in microfilament formation.
• Three actin monomers combine to form a stable trimer, which acts as a nucleus for further
polymerization.
b. Elongation
• Actin monomers are added to the ends of the filament.
• Polarity:
◦ Plus (+) end (Barbed end): Rapid growth occurs here. ATP-actin is preferentially
added.
◦ Minus (−) end (Pointed end): Slower growth; ADP-actin is more common.
c. Steady State
• A balance is achieved between polymerization at the plus end and depolymerization at the
minus end, known as treadmilling.
3. Polarity of Microfilaments:
• Intrinsic Polarity:
◦ Actin filaments have two distinct ends due to the orientation of actin monomers.
◦ This polarity is crucial for directional processes like intracellular transport and cell
movement.
4. Helical Structure:
• Helical Arrangement:
◦ F-actin forms a right-handed helix with a periodicity of ~37 nm.
◦ Each turn of the helix consists of ~13 actin subunits.
• Stability:
, ◦ ATP-actin at the plus end stabilizes the filament, while ADP-actin at the minus end
destabilizes it.
5. Dynamic Behavior:
Microfilaments are highly dynamic due to the continuous assembly and disassembly of actin
subunits, regulated by ATP hydrolysis:
• ATP Hydrolysis:
◦ ATP-actin is polymerized into the filament.
◦ After incorporation, ATP is hydrolyzed to ADP, reducing the affinity of actin
monomers for each other.
6. Accessory Proteins:
• Actin-binding proteins regulate the structure and dynamics of microfilaments:
◦ Nucleating Proteins: Help in nucleation (e.g., formin, Arp2/3 complex).
◦ Severing Proteins: Fragment filaments (e.g., gelsolin, cofilin).
◦ Capping Proteins: Stabilize filament ends (e.g., capZ).
◦ Cross-linking Proteins: Create networks or bundles (e.g., fimbrin, filamin).
7. Bundling and Networks:
• Microfilaments are organized into two main structures within cells:
◦ Bundles: Parallel filaments packed together (e.g., in microvilli).
◦ Networks: Cross-linked filaments forming a mesh (e.g., in the cell cortex).
Functions of Microfilaments
1. Cell Shape Maintenance:
◦ Provide mechanical support to the plasma membrane.
◦ Form a dense network beneath the cell membrane, known as the cell cortex.
2. Cell Movement (Motility):
◦ Facilitate amoeboid movement by polymerization and depolymerization cycles.
◦ Participate in processes like lamellipodia and filopodia formation in migrating cells.
3. Intracellular Transport:
◦ Act as tracks for myosin motor proteins, enabling organelle and vesicle transport.
◦ Example: Transport of vesicles during exocytosis and endocytosis.
4. Cytokinesis:
◦ Form the contractile ring during cell division, which helps in separating the
cytoplasm into two daughter cells.
5. Specialized Structures:
◦ Found in microvilli of intestinal epithelial cells, increasing surface area for
absorption.
◦ Contribute to the structure of cilia and flagella indirectly through anchoring.
Microfilaments are dynamic, helical polymers primarily composed of actin protein. They play a key
role in maintaining cell shape, enabling motility, and intracellular transport.
1. Actin Monomers:
• Actin Types: Actin exists in two forms:
◦ G-actin (Globular actin): A single, soluble monomeric form.
◦ F-actin (Filamentous actin): A polymerized, helical form of actin.
• Structure of G-actin:
◦ Each actin monomer is ~42 kDa in size.
◦ It has an ATP/ADP binding cleft, which regulates its polymerization.
2. Filament Formation:
Microfilaments are formed by the polymerization of G-actin into F-actin. This process involves the
following steps:
a. Nucleation
• The first and rate-limiting step in microfilament formation.
• Three actin monomers combine to form a stable trimer, which acts as a nucleus for further
polymerization.
b. Elongation
• Actin monomers are added to the ends of the filament.
• Polarity:
◦ Plus (+) end (Barbed end): Rapid growth occurs here. ATP-actin is preferentially
added.
◦ Minus (−) end (Pointed end): Slower growth; ADP-actin is more common.
c. Steady State
• A balance is achieved between polymerization at the plus end and depolymerization at the
minus end, known as treadmilling.
3. Polarity of Microfilaments:
• Intrinsic Polarity:
◦ Actin filaments have two distinct ends due to the orientation of actin monomers.
◦ This polarity is crucial for directional processes like intracellular transport and cell
movement.
4. Helical Structure:
• Helical Arrangement:
◦ F-actin forms a right-handed helix with a periodicity of ~37 nm.
◦ Each turn of the helix consists of ~13 actin subunits.
• Stability:
, ◦ ATP-actin at the plus end stabilizes the filament, while ADP-actin at the minus end
destabilizes it.
5. Dynamic Behavior:
Microfilaments are highly dynamic due to the continuous assembly and disassembly of actin
subunits, regulated by ATP hydrolysis:
• ATP Hydrolysis:
◦ ATP-actin is polymerized into the filament.
◦ After incorporation, ATP is hydrolyzed to ADP, reducing the affinity of actin
monomers for each other.
6. Accessory Proteins:
• Actin-binding proteins regulate the structure and dynamics of microfilaments:
◦ Nucleating Proteins: Help in nucleation (e.g., formin, Arp2/3 complex).
◦ Severing Proteins: Fragment filaments (e.g., gelsolin, cofilin).
◦ Capping Proteins: Stabilize filament ends (e.g., capZ).
◦ Cross-linking Proteins: Create networks or bundles (e.g., fimbrin, filamin).
7. Bundling and Networks:
• Microfilaments are organized into two main structures within cells:
◦ Bundles: Parallel filaments packed together (e.g., in microvilli).
◦ Networks: Cross-linked filaments forming a mesh (e.g., in the cell cortex).
Functions of Microfilaments
1. Cell Shape Maintenance:
◦ Provide mechanical support to the plasma membrane.
◦ Form a dense network beneath the cell membrane, known as the cell cortex.
2. Cell Movement (Motility):
◦ Facilitate amoeboid movement by polymerization and depolymerization cycles.
◦ Participate in processes like lamellipodia and filopodia formation in migrating cells.
3. Intracellular Transport:
◦ Act as tracks for myosin motor proteins, enabling organelle and vesicle transport.
◦ Example: Transport of vesicles during exocytosis and endocytosis.
4. Cytokinesis:
◦ Form the contractile ring during cell division, which helps in separating the
cytoplasm into two daughter cells.
5. Specialized Structures:
◦ Found in microvilli of intestinal epithelial cells, increasing surface area for
absorption.
◦ Contribute to the structure of cilia and flagella indirectly through anchoring.
