Actin is a highly abundant protein found in nearly all eukaryotic cells. It is a fundamental component of the cytoskeleton, an intricate network of protein filaments and tubules that provides structural support and enables various cellular functions. Actin’s diverse roles underpin many dynamic processes within living organisms.
Understanding Actin’s Structure and Identity
Actin is a globular protein. In its monomeric form, it is referred to as globular actin (G-actin). G-actin monomers bind to each other, forming head-to-tail interactions that allow them to polymerize into long chains.
When G-actin monomers polymerize, they form filamentous actin (F-actin), also known as microfilaments. These F-actin filaments have a double-stranded helical structure, resembling two intertwined ropes. Each G-actin monomer within the filament is oriented in the same direction, giving the F-actin filament a distinct polarity with two different ends: the plus (or barbed) end and the minus (or pointed) end. G-actin monomers add more readily to the plus end, influencing filament growth. The assembly and disassembly of these filaments are regulated by various actin-binding proteins and involve ATP hydrolysis.
Actin’s Essential Roles in Cellular Processes
Actin’s dynamic nature allows it to perform many functions within cells.
One of its most recognized roles is in muscle contraction, where it interacts with another protein called myosin. In muscle cells, actin filaments are organized into repeating units called sarcomeres, and the sliding of actin and myosin filaments past each other, driven by myosin’s motor activity, generates the force for muscle contraction.
Beyond muscle, actin contributes to cell shape and rigidity. A dense network of actin filaments beneath the plasma membrane provides mechanical support, determining the cell’s overall form and consistency.
Actin also plays a role in cell movement, such as amoeboid movement and cell migration. Cells extend temporary protrusions, like lamellipodia and filopodia, at their leading edge through controlled actin polymerization. These extensions attach to a surface, and the cell body retracts, enabling the cell to crawl.
During cell division, actin is involved in cytokinesis, the process where a single cell divides into two daughter cells. A contractile ring composed of actin filaments and myosin II assembles underneath the plasma membrane at the cell’s equator. The contraction of this ring pulls the membrane inward, pinching the cell in half to complete the division.
Actin filaments also serve as tracks for intracellular transport. Motor proteins, particularly unconventional myosins, can move along these actin tracks, carrying membrane vesicles and organelles to their correct locations within the cell. This directed movement is crucial for the organized distribution of cellular components.
Furthermore, actin participates in endocytosis and exocytosis, processes involving the formation and movement of vesicles. In endocytosis, actin helps form vesicles that bring substances into the cell, while in exocytosis, it aids in the movement and fusion of vesicles that release substances from the cell. The dynamic assembly and disassembly of actin filaments support these membrane remodeling events.