Actin is the primary protein that forms microfilaments, which are fundamental components of the cell’s internal scaffolding, known as the cytoskeleton. This dynamic network provides structural support and enables various cellular movements within eukaryotic cells.
What is Actin?
Actin is an abundant, multi-functional globular protein found in nearly all eukaryotic cells. It exists as a free monomer, referred to as globular actin or G-actin, before assembling into larger structures. G-actin has a molecular weight of approximately 42 kilodaltons (kDa) and a diameter of about 4 to 7 nanometers (nm). Its concentration within a mammalian cell can range from 10 to 100 micromolar, corresponding to millions of monomers.
Each G-actin monomer has two distinct lobes separated by a deep cleft. This cleft contains an “ATPase fold” that binds to a magnesium ion and a molecule of ATP (adenosine triphosphate). The binding of ATP or ADP (adenosine diphosphate) is necessary to stabilize each actin monomer, preventing its denaturation.
Microfilaments Explained
Microfilaments, also known as actin filaments, are the thinnest components of the cytoskeleton, measuring about 7 nm in diameter. Found in the cytoplasm of eukaryotic cells, they are composed primarily of actin polymers. They are characterized by their dynamic nature, constantly assembling and disassembling in response to cellular needs.
Microfilaments provide structural support and contribute to the mechanical stability of the cell. Their flexible yet strong framework allows them to resist compressive and tensile forces, helping cells maintain their shape.
The Dynamic Duo: Actin and Microfilaments in Action
Individual actin monomers (G-actin) polymerize to form filamentous actin (F-actin). This process begins with a nucleation phase where a small aggregate, often three G-actin proteins, comes together. An elongation phase follows, where more actin monomers rapidly add to both ends of the growing filament, particularly the faster-growing “barbed” or plus end.
As G-actin monomers join the filament, the ATP bound to them is hydrolyzed to ADP, and a phosphate group is released. This ATP hydrolysis contributes to the dynamic behavior of microfilaments, allowing rapid assembly and disassembly. This constant remodeling enables microfilaments to perform a wide array of functions within the cell, including:
- Maintaining cell shape by forming a dense network beneath the plasma membrane, known as the cell cortex, which helps resist deformation.
- Facilitating cell movement, such as amoeboid movement and cell crawling, by serving as tracks for motor proteins like myosin.
- Enabling muscle contraction, where they form thin filaments that slide past myosin filaments.
- Assisting in cell division (cytokinesis), forming an actomyosin ring that pinches the cell into two daughter cells.
- Supporting intracellular transport, acting as “roads” for the movement of vesicles and organelles.
The Cytoskeleton’s Full Picture
The cytoskeleton is a comprehensive cellular support system, extending throughout the cytoplasm of eukaryotic cells. While actin microfilaments are a significant component, the cytoskeleton also includes two other main types of protein filaments: microtubules and intermediate filaments. These three filament types work in concert to provide the cell with its structural integrity, enable movement, and organize its internal contents.
Microtubules are the largest cytoskeletal components, hollow tubes made of tubulin proteins that help the cell resist compression and serve as tracks for transport. Intermediate filaments, with a diameter between microfilaments and microtubules, provide mechanical strength and anchor organelles in place. Together, these filament systems support the cell’s adaptability and functionality.