Filamentous actin (F-actin) is a ubiquitous protein found in nearly all living cells, playing a fundamental role in their structure and activities. Actin exists in two primary forms: globular G-actin and polymerized, thread-like F-actin. This article explores the diverse functions of filamentous actin.
Building Blocks of Cellular Structure
Filamentous actin forms through the polymerization of individual G-actin monomers. These monomers assemble into a double-stranded helical filament, resembling two twisted strings of beads. This helical structure is approximately 7 nanometers in diameter and exhibits polarity, with a “plus” end where G-actin subunits are added more rapidly, and a “minus” end where they are added more slowly or removed.
F-actin serves as a primary component of the cytoskeleton, a network of protein filaments providing structural support. Located predominantly beneath the plasma membrane, F-actin forms a dense network known as the cell cortex. This cortical network determines and maintains cell shape, provides mechanical stability, and helps cells resist external forces. The dynamic assembly and disassembly of these filaments allow cells to adapt their shape and respond to their environment.
Dynamic Roles in Cell Function
The dynamic nature of filamentous actin allows it to participate in many cellular processes beyond structural support. Its ability to rapidly assemble, disassemble, and reorganize is central to many cell activities.
Filamentous actin is involved in cell movement, facilitating processes like amoeboid movement and cell crawling. It drives the formation of membrane protrusions such as lamellipodia and filopodia at the leading edge of migrating cells, which cells use to explore and move across surfaces. F-actin polymerization at these leading edges generates the force needed for protrusion and cell migration.
In muscle cells, F-actin forms thin filaments that interact with myosin, a motor protein, to generate force for muscle contraction. This interaction, described by the sliding filament theory, involves myosin heads binding to actin filaments and pulling them past each other, shortening the muscle unit. This coordinated movement of actin and myosin is responsible for all muscle activity.
F-actin also plays a part in cell division, specifically during cytokinesis, the physical division of the cytoplasm. It forms a contractile ring inside the cell membrane at the cell’s equator. This ring, composed of actin and myosin, constricts like a drawstring, pinching the cell into two daughter cells.
Beyond movement and division, F-actin serves as tracks for the movement of organelles and vesicles within the cell, a process known as intracellular transport. Motor proteins, such as certain types of myosin, move along these F-actin tracks, transporting cellular components to their destinations. This transport system is important for maintaining cellular organization and delivering materials throughout the cell.
F-actin contributes to cell adhesion, which involves how cells attach to each other and to the extracellular matrix. It forms a link between the cell’s internal machinery and adhesion complexes at the cell surface. This connection provides mechanical stability to adhesion sites and influences cell signaling pathways related to cell-to-cell and cell-to-matrix interactions.
Beyond Basic Biology
Disruptions in the normal dynamics of filamentous actin can have implications for cellular health and biological function. When F-actin assembly or disassembly is imbalanced, it can lead to cellular dysfunctions.
For example, alterations in F-actin dynamics are implicated in immune responses, affecting how immune cells move and interact with pathogens. Changes in the actin cytoskeleton can impair the ability of immune cells to migrate to infection sites or engulf foreign invaders. Such disruptions can compromise the body’s defense mechanisms.
F-actin is also relevant in processes like wound healing, where coordinated cell movement and shape changes are important for tissue repair. An organized F-actin network supports the migration of cells, such as fibroblasts, which help close wounds and rebuild tissue structure.
Some pathogens have evolved mechanisms to manipulate the host cell’s F-actin for their own benefit. Certain bacteria, for instance, can hijack the actin polymerization machinery to propel themselves within the host cell or facilitate entry into new cells. This manipulation highlights the importance of F-actin to both host and pathogen survival.