The cytoskeleton is the cell's structural backbone, made up of microfilaments, microtubules, and intermediate filaments. These components work together to maintain cell shape, enable movement, and facilitate various cellular processes.
Understanding the cytoskeleton is crucial for grasping how cells function mechanically. This knowledge ties into the broader study of molecular motors and cytoskeletal mechanics, revealing how cells move, divide, and respond to their environment.
Cytoskeletal components: structure and composition
Microfilaments
- Composed of actin monomers that polymerize to form thin, flexible filaments
- Have a diameter of about 7 nm
- Actin monomers are globular proteins (G-actin) that assemble into filamentous actin (F-actin)
- Actin filaments are polarized, with a plus end (barbed end) and a minus end (pointed end)
- Actin polymerization occurs more rapidly at the plus end, while depolymerization occurs at the minus end
Microtubules
- Hollow, cylindrical structures composed of α- and β-tubulin dimers
- Polymerize to form rigid, tubular filaments with a diameter of about 25 nm
- Tubulin dimers assemble in a head-to-tail fashion to form protofilaments
- Typically, 13 protofilaments associate laterally to form a microtubule
- Microtubules are polarized, with a plus end (β-tubulin exposed) and a minus end (α-tubulin exposed)
- Microtubule polymerization is more rapid at the plus end, while depolymerization occurs at the minus end
Intermediate filaments
- Composed of various proteins, such as keratins, lamins, and vimentin
- Form rope-like filaments with a diameter of about 10 nm
- Intermediate filament proteins have a central α-helical rod domain flanked by non-helical head and tail domains
- Monomers assemble into dimers, which then form tetramers and higher-order structures
- Intermediate filaments are more stable and less dynamic than microfilaments and microtubules
- The composition of intermediate filaments varies depending on the cell type and the specific functions they perform (keratin filaments in epithelial cells, neurofilaments in neurons)
Functions of cytoskeletal components
Microfilaments and microtubules
- Microfilaments are primarily involved in cell movement, cell division, and maintaining cell shape
- Microtubules are essential for intracellular transport, cell division, and maintaining cell polarity
- Both microfilaments and microtubules are highly dynamic structures that can rapidly assemble and disassemble
- Microfilaments form contractile bundles (stress fibers) and networks (cortical actin) that generate forces for cell movement and shape changes
- Microtubules serve as tracks for motor proteins (kinesins and dyneins) to transport organelles and vesicles
Intermediate filaments
- Provide mechanical strength and resistance to shear stress
- Maintain the structural integrity of the cell and its organelles
- Anchor organelles and provide mechanical strength to the cell
- Help maintain cell shape and resist mechanical stress
- Intermediate filaments are more stable and less prone to reorganization compared to microfilaments and microtubules
Coordination of cytoskeletal components
- All three cytoskeletal components work together to maintain cell shape, enable cell movement, and facilitate various cellular processes
- Microfilaments and microtubules often interact and coordinate their functions (microtubule-actin crosstalk during cell division and migration)
- Intermediate filaments provide a structural framework that supports and integrates the functions of microfilaments and microtubules
Cytoskeleton's role in cell shape and movement
Cell shape maintenance
- Microfilaments form a cortical network beneath the plasma membrane, providing mechanical support and maintaining cell shape
- Microtubules provide structural support for the cell and help maintain cell polarity by directing the distribution of organelles and other cellular components
- Intermediate filaments anchor organelles and provide mechanical strength to the cell, helping to maintain its shape and resist mechanical stress
- The coordinated action of all three cytoskeletal components is essential for cell shape changes during processes like cell division and differentiation
Cell movement
- Actin-myosin interactions in microfilaments generate contractile forces that enable cell movement, such as in muscle contraction and cell migration
- Microfilaments form protrusive structures (lamellipodia and filopodia) at the leading edge of migrating cells
- Microtubules provide structural support and help maintain cell polarity during migration
- Intermediate filaments provide mechanical strength and resist the forces generated during cell movement
- The coordinated regulation of cytoskeletal dynamics is crucial for efficient cell migration (actin polymerization at the leading edge, myosin-driven contraction at the rear)
Cytoskeleton dynamics and regulation
Dynamic nature of the cytoskeleton
- The cytoskeleton is a highly dynamic structure that undergoes constant remodeling in response to cellular needs and environmental cues
- Microfilaments and microtubules exhibit dynamic instability, rapidly switching between periods of growth and shrinkage
- The assembly and disassembly of cytoskeletal components are tightly regulated by associated proteins and signaling pathways
- Cytoskeletal remodeling is essential for various cellular processes, including cell division, cell migration, and intracellular transport
Regulation by associated proteins
- Actin-binding proteins, such as profilin, cofilin, and Arp2/3 complex, regulate the assembly, disassembly, and organization of microfilaments
- Microtubule-associated proteins (MAPs), such as tau and MAP2, stabilize microtubules and regulate their dynamics
- Motor proteins like kinesins and dyneins facilitate transport along microtubules
- Intermediate filament-associated proteins, such as plectin and desmoplakin, help to organize and link intermediate filaments to other cellular structures
- Post-translational modifications, such as phosphorylation and acetylation, of cytoskeletal proteins and their associated proteins play a key role in modulating cytoskeletal dynamics and function
- Rho GTPases (Rho, Rac, and Cdc42) are key regulators of cytoskeletal dynamics, controlling actin polymerization, myosin activity, and microtubule stability