The structure and composition of microfilaments


Microfilaments, also known as actin filaments, are thin, flexible protein fibers found in the cytoplasm of eukaryotic cells. They play a crucial role in cell structure, movement, and various cellular processes. In this article, we will explore the structure and composition of microfilaments to understand their functions and contributions to cellular biology.

1. Composition of Microfilaments

1.1 Actin Proteins

Microfilaments are primarily composed of actin proteins. Actin is a globular protein that polymerizes to form long, helical chains, known as F-actin (filamentous actin). Each actin protein consists of a small subunit called G-actin (globular actin). G-actin monomers assemble end-to-end, forming a polymerized filament.

1.2 Actin-binding Proteins

In addition to actin proteins, microfilaments interact with various actin-binding proteins. These proteins can regulate the assembly, disassembly, and organization of microfilaments. Examples of actin-binding proteins include tropomyosin, which stabilizes the microfilament structure, and cofilin, which promotes the disassembly of actin filaments.

2. Structure of Microfilaments

2.1 Double Helical Structure

Microfilaments have a double helical structure, with two strands of actin proteins twisting around each other. The actin monomers within the microfilament are oriented in the same direction, creating a polarity. The fast-growing end is called the plus end, while the slow-growing end is called the minus end.

2.2 Filament Bundles and Networks

Microfilaments can bundle together, forming larger structures within the cell. Actin-binding proteins, such as fimbrin and alpha-actinin, help organize and stabilize these bundles. Microfilaments can also form extensive networks, where they crisscross and interact with other cellular components. These networks provide structural support and help determine cell shape.

3. Functions of Microfilaments

3.1 Cell Shape and Structure

Microfilaments contribute to the maintenance of cell shape and structure. They provide mechanical support and help define cell boundaries. Microfilaments also interact with other cytoskeletal components, such as intermediate filaments and microtubules, to form a dynamic cytoskeleton that maintains cell integrity.

3.2 Cell Movement

Microfilaments are essential for cell movement. They play a key role in cell crawling, contraction, and cytokinesis (cell division). Microfilaments, along with myosin motor proteins, generate the force required for cell motility. Actin polymerization at the leading edge of the cell allows for the extension of protrusions, such as lamellipodia and filopodia, which facilitate cell movement.

3.3 Intracellular Transport

Microfilaments are involved in intracellular transport processes. They serve as tracks for the movement of vesicles, organelles, and other cellular cargo. Motor proteins, such as myosins, interact with microfilaments and transport cargo along the filament tracks, allowing for precise delivery within the cell.

Frequently Asked Questions (FAQs)

Q1: How do microfilaments differ from microtubules?

Microfilaments are thinner and more flexible compared to microtubules. Microfilaments are primarily composed of actin proteins, while microtubules are composed of tubulin proteins. Microfilaments are involved in cell movement and maintain cell shape, while microtubules are involved in intracellular transport and cell division.

Q2: Can microfilaments be found in prokaryotic cells?

Microfilaments are predominantly found in eukaryotic cells. Prokaryotic cells, such as bacteria, have a different cytoskeletal structure and lack the actin-based microfilament system.

Q3: How are microfilaments involved in muscle contraction?

In muscle cells, microfilaments interact with myosin motor proteins to generate muscle contraction. Myosin binds to actin filaments and uses ATP energy to slide along the filaments, causing the contraction of muscle fibers.


Microfilaments, composed of actin proteins, are essential components of the cytoskeleton in eukaryotic cells. They form double helical structures, organize into bundles or networks, and interact with actin-binding proteins. Microfilaments contribute to cell shape and structure, facilitate cell movement, and participate in intracellular transport processessuch as vesicle movement. Understanding the structure and composition of microfilaments is crucial for unraveling their functions and their significance in cellular biology. Their involvement in various cellular processes highlights their importance in maintaining cell integrity and enabling cellular activities.

Frequently Asked Questions: Microfilaments

1. What are microfilaments?

Microfilaments, also known as actin filaments, are thin, thread-like structures found in the cytoplasm of cells. They are made up of protein subunits called actin and are a vital component of the cell’s cytoskeleton. Microfilaments play a crucial role in various cellular processes, including cell movement, cell shape maintenance, and intracellular transport.

2. What is the structure of microfilaments?

Microfilaments are composed of actin protein subunits that polymerize to form long, helical chains. Each actin subunit consists of a globular head region and a slender tail region. These subunits assemble end-to-end, forming a filamentous structure. Microfilaments have a diameter of about 7 nanometers (nm) and can exist as single filaments or bundle together to form larger structures.

3. What is the function of microfilaments?

Microfilaments serve various important functions in cells, including:

  • Cell movement: Microfilaments are involved in cell motility processes such as crawling, contraction, and cell division. They provide the structural framework for cellular protrusions like lamellipodia and filopodia, which enable cell movement and migration.
  • Cell shape and support: Microfilaments contribute to the maintenance of cell shape and provide mechanical support to cells. They help define the cell’s overall structure and help it withstand external forces.
  • Intracellular transport: Microfilaments are involved in the transport of vesicles and other cellular components within the cell. They function in conjunction with motor proteins to move cargo along the filament tracks.
  • Muscle contraction: Microfilaments, particularly in muscle cells, are responsible for generating the contractile force required for muscle contraction. The interaction between actin and myosin proteins enables the sliding of microfilaments, resulting in muscle fiber contraction.

4. How do microfilaments contribute to cell division?

During cell division, microfilaments play a role in the formation of the contractile ring, a structure that helps divide the cytoplasm of the parent cell into two daughter cells. The contractile ring is composed of microfilaments and associated proteins. Contraction of the ring, guided by microfilament dynamics and motor proteins, leads to the cleavage of the cell membrane and the separation of the two daughter cells.

5. Can microfilaments be targeted for medical purposes?

Microfilaments and their associated proteins are potential targets for certain medical interventions. For example, drugs that inhibit actin polymerization or disrupt microfilament dynamics can be used to control cell motility in diseases such as cancer metastasis. Additionally, some drugs target microfilaments in muscle cells to treat conditions like muscle spasms or disorders characterized by abnormal muscle contractions.

These are some of the frequently asked questions about microfilaments. If you have more specific questions or need further information, feel free to ask!