- The cortex of a cell is an active region beneath the plasma membrane.
- It is responsible for ingesting extracellular materials, extension during cell movements and constriction duiring cell division.
- Actin undergoes treadmilling where it is assembled on one end and disassembled on another end.
Actin §
- Thinner actin-based structures organized into branching networks.
- Actin is responsible for extracellular and intracellular motile processes such as moving the cell around a medium or moving vesicles and performing phagocytosis.
- Actin also plays a role in determining cell shape and provides structural support.
- An actin filament is a two-stranded structure with two helical grooves running along its length
- It has a barbed structure with a positive and a negative end. This arises because all the monomers within an actin filament are pointed in the same direction.
- Actin is an ATPase so its assembly involves ADP. The assembly of the filament relies on the concentration of ATP and actin.
- Has the motor protein myosin
- These molecules tend to move towards the barbed end of an actin filament.
- Conventional / Type II Myosins are the primary motors for muscle contraction but they are also used for splitting a cell during cell division, generating tension at focal adhesions, and cell migration.
- Type II myosins tend to assemble so that the ends of their tails point towards the center of the filament and the globular heads point away.
- This results in a bipolar filament which can pull actin filaments towards one another.
- Unconventional Myosins are smaller and unable to assemble into filaments in vivo. They instead operate as individual protein molecules.
- These myosins move in a hand-over-hand molecule similar to kinesins.
- Myosin VI is unique in that it moves to the pointed end of an actin filament.
Actin-Binding Proteins §
- These proteins give the actin filament structure for a particular purpose.
- They affect the localized assembly or disassembly of the actin filaments, their properties, and their interactions with one another.
- Nucleating Proteins - responsible for speeding up the nucleation or formation of the actin filament.
- Arp 2/3 - actin-related proteins that scaffolds the formation of actin, generating networks of branched actin filaments
- Formins - generate unbranched filaments found in focal adhesions. They also promote rapid elongation of filaments.
- Spire - binds multiple actin monomers to form an actin nucleus.
- Monomer Sequestering Proteins - prevents the near complete polymerization of soluble actin monomers into filaments.
- This regulates the concentration of actin depending on whether or not polymerization / depolymerization is favored.
- Thymosins - prevents actin-ATP monomers from polymerizing.
- Capping Proteins - regulates the length of actin filaments by forming a cap that blocks loss and gain of subunits.
- Monomer-binding Proteins - allows for the polymerization of actin monomers into filaments.
- Profilin - promotes the growth of actin filaments.
- Depolymerization Proteins - These proteins allow depolymerization, allowing for dynamic changes in cytoskeletal structure
- Cofilins - fragments actin filaments and promotes depolymerization at the pointed ends.
- Cross-Linking Proteins - alters the 3D organization of actin filaments.
- Examples: Filamins, Villins, Fimbrin
- Filament-Severing Proteins - binds to the side of an existing filament and breaks it in two. They may either promote incorporation of actin monomers or cap the fragments they generate.
- Membrane-Binding Proteins - Proteins that link the filaments to the plasma membrane by attaching to a plasma membrane indirectly. These control the contractile, expanding motion of the cell.
- Examples: Vinculin, Ezrin, Radixin, Moesin, Spectrins.
Links §