Table of Contents
Both microtubules and microfilaments are components of a cell’s cytoskeleton. Microtubules, microfilaments, and intermediate filaments compose the cytoskeleton. Tubulin proteins are polymerized to produce microtubules. They provide the cell with mechanical support and help to intracellular transport. Polymerization of actin protein monomers produces microfilaments. They contribute to the mobility of the cell on the surface. Microfilaments are double-stranded helical polymers composed of actin proteins, whereas microtubules are long, hollow cylinders composed of tubulin protein units.
What are Microtubules?
- Microtubules are one of the three types of cytoskeletal filaments found in eukaryotic cells. They are composed of tubulin protein subunits that polymerize into long, hollow, cylindrical structures.
- Microtubules play a crucial role in maintaining cell shape and structure, as well as in cell division, intracellular transport, and cilia and flagella movement. They also form part of the mitotic spindle, a structure that helps separate chromosomes during cell division.
- In addition to their structural role, microtubules are involved in intracellular transport. They act as tracks for motor proteins, which move along the microtubules to transport vesicles, organelles, and other cargo within the cell. They are also involved in the movement of cilia and flagella, which are hair-like structures that protrude from the surface of some cells and are involved in cell motility and sensory functions.
- Microtubules are dynamic structures that can rapidly polymerize and depolymerize in response to cellular signals. This dynamic behavior allows cells to rapidly reorganize their microtubule networks in response to changes in their environment.
- Disruption of microtubules can lead to a variety of cellular defects, including changes in cell shape and motility, defects in cell division, and impaired intracellular transport. Drugs that interfere with microtubule formation or function are commonly used in research to study the role of microtubules in various cellular processes.
Structure of Microtubules
- Microtubules consist of the globular protein tubulin.
- Tubulin molecules have structures like beads.
- They generate alpha and beta tubulin heterodimers.
- A protofilament is composed of a single row of tubulin dimers.
- 12 to 17 protofilaments produce a regular helical lattice by connecting laterally.
Functions of Microtubules
- The cell structure is determined by microtubules.
- During cell division, microtubules create the spindle machinery that divides chromosomes directly (mitosis).
- Microtubules serve as a transport system for vesicles containing important cell components.
- Microtubule-associated motor proteins (MAPs) such as Kinesin and Dyenin exploit their hard internal core to create force and movement in motile structures such as cilia and flagella. A core of microtubules in the neural development cone and axon is also responsible for stability and navigation and guiding of neurons.
What are Microfilaments?
- Microfilaments, also known as actin filaments, are one of the three types of cytoskeletal filaments found in eukaryotic cells. They are composed of actin protein subunits that polymerize into long, thin, flexible fibers.
- Microfilaments play a crucial role in maintaining cell shape and structure, as well as in cell division, movement, and intracellular transport. They also form part of the cell cortex, a specialized region of the cell membrane that helps maintain the shape of the cell and regulate cellular processes.
- During cell division, microfilaments form a contractile ring that pinches the cell membrane inward, eventually separating the dividing cell into two daughter cells. They also play a key role in cell movement, as they are involved in the formation of lamellipodia and filopodia, which are projections that extend from the cell and allow it to move.
- Microfilaments are also important in muscle contraction, as they interact with myosin to generate force. In addition, they play a role in intracellular transport by forming a network of microfilaments that act as tracks for motor proteins.
- Disruption of microfilaments can lead to a variety of cellular defects, including changes in cell shape and motility, defects in cell division, and impaired intracellular transport. Drugs that interfere with microfilament formation or function are commonly used in research to study the role of microfilaments in various cellular processes.
Structure of Microfilaments
- Isolated microfilament subunits are known as globular actin (G-actin).
- G-actin subunits assemble into F-actin polymers, which are long filamentous polymers.
- To produce the double helix structure of microfilaments, two parallel F-actin strands must spin 166 degrees in order to layer appropriately.
- Microfilaments have a diameter of roughly 7 nm and a helix loop that repeats every 37 nm.
Functions of Microfilaments
- Microfilaments constitute the dynamic cytoskeleton, which provides structural support for cells and connects the inside of the cell to the exterior in order to transmit information about the external environment.
- Cell motility is provided by microfilaments. e.g., Filopodia, Lamellipodia.
- Via actin cables, motor proteins transfer intracellular organelles to daughter cells during mitosis.
- Myosin proteins exert pressures on actin filaments to support muscular contraction in muscle cells.
- In non-muscle cells, actin filaments create a transport mechanism powered by non-conventional myosins such as myosins V and VI. Non-conventional myosins utilise the energy derived from ATP hydrolysis to carry cargo (such as vesicles and organelles) at significantly quicker rates than diffusion.
Differences Between Microfilaments and Microtubules
|Structure||Double Helix Structure||Helical lattice Structure|
|Size||7 nm in diameter||20-25 nm in diameter|
|Composition||Mostly consists of the contractile protein actin.||Made of protein tubulin subunits. They are designated as alpha and beta subunits.|
|Strength||Flexible and relatively strong. Resist buckling as a result of compressive forces and filament fracture as a result of tensile stresses.||Stiff and resist bending forces.|
|Associated Proteins||Actin monomer-binding proteins, filament cross-linkers, actin-related protein 2/3 (Arp2/3) complex and filament-severing proteins are involved in the regulation of the dynamics of microfilaments.||MAPs, +TIPs and motor proteins are the associated proteins regulating the dynamics of microtubules.|
|Function||Micro-filaments are smaller and thinner and mostly help cells move||Microtubules are shaped similarly but are larger, and help with cell functions such as mitosis and various cell transport functions.|