Microfilaments: Structure and Mechanisms of Movement
Microfilaments are formed by globular molecules of the protein actin, which form a double helix 5–7 nm thick. In the cytoplasm, they create a dense network, and beneath the plasmalemma, they form a pronounced cortical network. Actin filaments attach to the plasmalemma via intermediate proteins, such as α-actinin, and connect to one another at the network nodes using filamin.
Cell movement and changes in shape are carried out through two main mechanisms:
- Change in MF length — occurs due to the polymerization or depolymerization of actin (e.g., during pseudopodia formation).
- Interaction with myosin — sliding of actin filaments relative to myosin, resembling muscular contraction.
These mechanisms provide cell migration, phagocytosis and pinocytosis, axonal growth in neurons, and cytokinesis. A specific case of microfilament organization is found in the microvilli of the intestine and renal tubules, whose framework of forty MFs operates via ATP-dependent filament sliding.
Intermediate Filaments and Their Tissue Specificity
Intermediate filaments have a diameter of 10 nm, occupying an intermediate position between microfilaments and microtubules. Their primary role is a supportive (structural) function. A unique feature of IFs is the strict tissue specificity of their protein composition, making them a valuable diagnostic marker:
- Epithelium: protein keratin (forms structures known as tonofilaments).
- Connective tissue, endothelium, and smooth myocytes: protein vimentin.
- Muscle tissue: protein desmin.
- Nerve cells: specific proteins that form neurofilaments.
Inside the cell, cytoplasmic IFs are located parallel to the nucleus and weave into cellular junctions. In the nucleus, a special class of intermediate filaments forms the lamina on the inner surface of the membrane, which serves as an attachment site for chromosomes.
Microtubules: Organization, Transport, and Division
Microtubules are hollow cylinders with an outer diameter of 24 nm, the wall of which consists of 13 tubulin subunits. They radiate from the cell center to the periphery and are stabilized by MAP proteins. MTs exhibit pronounced polarity: the minus-end is usually anchored at satellites in the cell center, while the plus-end serves as a growth zone through tubulin polymerization.
Main functions of microtubules:
- Maintenance of the rigid cell framework.
- Intracellular transport: MTs serve as tracks along which motor proteins dyneins and kinesins move, transporting vesicles and organelles at the expense of ATP energy.
- Cell division: formation of the mitotic spindle and ensuring chromosome segregation.
Pharmacological agents, such as the alkaloid colchicine, cause microtubule depolymerization, causing the cell to lose its shape and completely blocking mitosis.
Briefly: answers to topical questions
What accessory proteins cross-link microtubule doublets in the axoneme of cilia and flagella?
The connecting elements for microtubule doublets in the axoneme are dynein arms and radial spokes. The structure of the axoneme of cilia and flagella is supported by the following accessory formations:
- Dynein arms — formed by the protein dynein, extending from each peripheral doublet to the neighboring doublet.
- Radial spokes — extend from the peripheral microtubule doublets to the central sheath.
The diameters of the cytoskeleton elements are easy to remember in ascending order: MF (5–7 nm) — IF (10 nm) — MT (24 nm). The formula "5-10-24" helps to quickly navigate the dimensions of non-membranous organelles.
Class and exam questions
What are the main differences in size and chemical composition of the cytoskeleton elements?
Microfilaments are 5–7 nm thick and consist of the protein actin. Intermediate filaments have a diameter of 10 nm and feature a tissue-specific protein composition (keratin, vimentin, desmin, etc.). Microtubules have a diameter of 24 nm and consist of the protein tubulin.
How is substance transport carried out along microtubules?
Microtubules serve as tracks for transport along their outer surface. The movement of organelles and vesicles is carried out by specialized motor proteins — kinesins and dyneins, which utilize ATP energy.
What is the function of the nuclear lamina?
The nuclear lamina consists of a specific class of intermediate filaments located on the inner surface of the nuclear membrane. Its main function is to ensure secure chromosome attachment.
What else to study on this topic
- Molecular Organization of Satellites and Kinetochores
- The Role of MAP Proteins in Microtubule Stabilization
- Mechanisms of Phagocytosis and Pinocytosis at the Microfilament Level
- The Involvement of Vimentin and Desmin in Tumor Diagnostics
- The Effect of Colchicine on Cell Division and the Mitotic Apparatus
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