Microfilaments and microtubules are key components of the cytoskeleton in eukaryotic cells. A cytoskeleton provides structure to the cell and connects to every part of the cell membrane and every organelle. Microtubules and microfilaments together allow the cell to hold its shape, and move itself and its organelles.
Formation and Structure
Structure of Microtubules
Microtubules are composed of globular proteins called tubulin. Tubulin molecules are bead like structures. They form heterodimers of alpha and beta tubulin. A protofilament is a linear row of tubulin dimers. 12-17 protofilaments associate laterally to form a regular helical lattice.
Formation of Microfilaments
Individual subunits of microfilaments are known as globular actin (G-actin). G-actin subunits assemble into long filamentous polymers called F-actin. Two parallel F-actin strands must rotate 166 degrees to layer correctly on top of each other to form the double helix structure of microfilaments. Microfilaments measure approximately 7 nm in diameter with a loop of the helix repeating every 37 nm.
Biological Role of Microtubules and Microfilaments
Functions of Microfilaments
- Microfilaments form the dynamic cytoskeleton, which gives structural support to cells and links the interior of the cell with the surroundings to convey information about the external environment.
- Microfilaments provide cell motility. e.g., Filopodia, Lamellipodia.
- During mitosis, intracellular organelles are transported by motor proteins to the daughter cells along actin cables.
- In muscle cells, actin filaments are aligned and myosin proteins generate forces on the filaments to support muscle contraction.
- In non-muscle cells, actin filaments form a track system for cargo transport that is powered by non-conventional myosins such as myosin V and VI. Non-conventional myosins use the energy from ATP hydrolysis to transport cargo (such as vesicles and organelles) at rates much faster than diffusion.
Functions of Microtubules
- Microtubules determine the cell structure.
- Microtubules form the spindle apparatus to divide the chromosome directly during cell division (mitosis).
- Microtubules provide transport mechanism for vesicles containing essential materials to the rest of the cell.
- They form a rigid internal core that is used by microtubule-associated motor proteins (MAPs) such as Kinesin and Dyenin to generate force and movement in motile structures such as cilia and flagella. A core of microtubules in the neural growth cone and axon also imparts stability and drives neural navigation and guidance.