Protein filaments are elongated, fibrous protein structures that form the cytoskeleton of cells or function in extracellular support. These filaments provide mechanical strength, shape, motility, and intracellular transport in both eukaryotic and prokaryotic cells.
Main Types of Protein Filaments (in Eukaryotic Cells):
| Type of Filament | Subunit Protein | Function |
|---|---|---|
| Actin filaments (microfilaments) | Actin | Cell shape, movement, division |
| Intermediate filaments | Varies (e.g., keratin, vimentin) | Mechanical strength, structural support |
| Microtubules | Tubulin (α and β) | Organelle movement, chromosome segregation, cilia/flagella motion |
1. Actin Filaments (Microfilaments):
- Diameter: ~7 nm (smallest)
- Structure: Two intertwined strands of actin
- Functions:
- Provide cell shape and support
- Enable cell motility (e.g., lamellipodia, filopodia)
- Involved in cytokinesis (cell division)
- Support intracellular transport
2. Intermediate Filaments:
- Diameter: ~10 nm
- Structure: Rope-like fibers made from various proteins depending on cell type:
- Keratins (epithelial cells)
- Vimentin (mesenchymal cells)
- Neurofilaments (neurons)
- Lamin A/C (nuclear lamina)
- Functions:
- Maintain cell integrity and resist mechanical stress
- Anchor organelles
- Support the nuclear envelope
3. Microtubules:
- Diameter: ~25 nm (largest)
- Structure: Hollow tubes of α- and β-tubulin dimers
- Functions:
- Act as tracks for motor proteins (kinesin, dynein)
- Form mitotic spindle during cell division
- Form the core of cilia and flagella
- Help in positioning organelles
Protein Filaments in Prokaryotes:
Though prokaryotes lack a classic cytoskeleton, they possess cytoskeletal-like protein filaments, such as:
- FtsZ (tubulin-like) – cell division
- MreB (actin-like) – maintains rod shape
- Crescentin (intermediate filament-like) – cell curvature
Functions of Protein Filaments (General):
- Maintain cell shape and structure
- Enable cell movement and intracellular transport
- Organize the position of organelles
- Facilitate cell division
- Enable interactions with the extracellular environment
Summary Table:
| Filament Type | Size | Subunit | Function |
|---|---|---|---|
| Actin filaments | ~7 nm | Actin | Shape, motility, division |
| Intermediate filaments | ~10 nm | Keratin, vimentin, etc. | Mechanical support |
| Microtubules | ~25 nm | α/β-tubulin | Transport, mitosis, cilia/flagella |