establishment or maintenance of actin cytoskeleton polarity

Definition

Target type: biologicalprocess

Any cellular process that results in the specification, formation or maintenance of polarized actin-based cytoskeletal structures. [GOC:mah]

The establishment and maintenance of actin cytoskeleton polarity is a fundamental process in eukaryotic cells, essential for a wide range of cellular functions including cell migration, division, and morphogenesis. This process involves the generation of distinct actin structures with different properties and functions at opposite poles of the cell.

**Polarization of the actin cytoskeleton is achieved through the coordinated action of several key factors:**

1. **Nucleation and Assembly:** Actin monomers are added to existing filaments, primarily at the barbed (plus) end of the filament. Actin nucleation factors, such as the Arp2/3 complex, promote the formation of new actin filaments. These factors are often localized to specific regions of the cell, contributing to the localized assembly of actin filaments.

2. **Actin-binding Proteins:** A diverse array of actin-binding proteins regulate the dynamics of actin filaments by influencing their assembly, disassembly, and organization. These proteins include:
* **Profilins:** Promote actin monomer exchange, facilitating the addition of monomers to the barbed end of filaments.
* **Capping proteins:** Prevent further elongation of filaments by capping the barbed end.
* **Bundling proteins:** Cross-link actin filaments into parallel bundles, increasing filament stability and rigidity.
* **Severing proteins:** Break existing actin filaments, promoting depolymerization and filament turnover.

3. **Signaling Pathways:** Extracellular cues and intracellular signals activate signaling pathways that regulate actin polymerization and depolymerization. These pathways often involve small GTPases, such as Rho GTPases, which act as molecular switches to control the activity of downstream effectors.

4. **Polarity Cues:** Cells receive cues from their environment that define the direction of polarization. These cues can include gradients of growth factors, extracellular matrix components, and cell-cell contacts.

**Specific examples of actin cytoskeleton polarity in different cellular contexts:**

* **Cell migration:** In migrating cells, a leading edge (lamellipodium) forms at the front of the cell, driven by the localized polymerization of actin filaments. This creates protrusions that allow the cell to move forward.
* **Cell division:** During mitosis, the actin cytoskeleton forms a contractile ring that constricts the cell membrane, resulting in cell division. The positioning of the contractile ring is determined by the orientation of the microtubule spindle, which provides a polarizing cue.
* **Morphogenesis:** The organization of the actin cytoskeleton is crucial for shaping tissues and organs during development. For example, during gastrulation, the actin cytoskeleton drives the inward movement of cells to form the germ layers.

**The dynamic nature of actin polymerization and depolymerization is essential for maintaining the polarized state of the actin cytoskeleton. Continuous turnover of actin filaments ensures that the cytoskeleton is responsive to changes in cellular signals and environmental cues.
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