Table of Contents
- Actin Definition
- Myosin Definition
- Key differences (Actin vs Myosin)
- Examples of Actin
- Examples of Myosin
- References
Actin Definition
Actin is a group of globular proteins, widely found in eukaryotic cells, playing a crucial role in providing shape, structure, and mobility.
- It is present in all eukaryotic organisms except for nematode sperm.
- Actin proteins are highly conserved and engage in more protein-protein interactions than any other known protein.
- Unlike other proteins, actin can transition between two states: monomeric (G-actin) and filamentous (F-actin).
- This transition is regulated by nucleotide hydrolysis, ions, and various actin-binding proteins.
- Actin serves as the monomeric unit for two distinct cytoskeletal filaments: microfilaments and the contractile apparatus.
- It plays essential roles in cell division, motility, and signaling.
- The dense network of microfilaments allows cells to remodel in response to environmental stimuli, such as increasing adhesion to form tissues.
- Vertebrates express three main actin isoforms:
- Three α-isoforms found in skeletal, cardiac, and smooth muscles.
- β- and γ-isoforms, present in both muscle and non-muscle cells.
- In muscles, actin filaments are short, measuring between 2–2.6 µm in length and approximately 0.005 µm in diameter.
- Actin filaments in muscles are separated by actin-binding proteins like α-actinin, which binds two actin filaments while maintaining space for myosin.
- α-Actinin is a key component of the contractile apparatus in muscles.
- Each actin filament in a muscle fiber consists of two intertwined strands of actin protein.
Myosin Definition
Myosin is a superfamily of motor proteins that, along with actin, plays a fundamental role in muscle fiber contraction.
- It is classified as a motor protein because it functions as an enzyme that converts chemical energy into mechanical energy. Myosin acts as an ATPase, moving along actin filaments by coupling ATP hydrolysis with conformational changes.
- All myosins consist of one or two heavy chains and multiple light chains. Genomic studies have identified 13 different myosin types. Each myosin molecule shares a highly conserved structural organization, including:
- A head domain, which serves as an actin-activated ATPase responsible for generating movement.
- A neck domain, associated with several regulatory light-chain subunits.
- An effector tail domain, which varies among myosin types and determines their specific cellular functions.
- Myosin filaments are larger than actin filaments, measuring approximately 4–5 µm in length and 0.01 µm in diameter.
- In muscle cells, myosin primarily exists as Myosin II, a polymeric molecule with a long rod-like tail domain that assembles into thick bipolar filaments. The dimeric head domain of Myosin II alone is capable of generating movement in the presence of ATP.
- Beyond muscle contraction, myosins also perform various functions depending on the specific myosin type and the organism. Despite variations across species, the structure and function of myosin remain highly conserved, with mammalian myosin being compatible with the actin of unicellular organisms.
Key Differences: Actin vs Myosin
| Basis for Comparison | Actin | Myosin |
|---|---|---|
| Definition | A globular protein abundant in eukaryotic cells, contributing to shape, structure, and mobility. | A family of motor proteins that, along with actin, enables muscle contraction. |
| Location in Sarcomere | Found in both A and I bands. | Restricted to A bands. |
| Size | Shorter (2–2.6 µm) and thinner (0.005 µm in diameter). | Longer (4–5 µm) and thicker (0.01 µm in diameter). |
| Nature | Globular protein. | Motor protein. |
| Molecular Weight | Lower molecular weight. | Higher molecular weight. |
| Abundance in Muscle Cells | More abundant than myosin. | Less abundant, with one myosin per actin filament. |
| Surface Texture | Smooth. | Rough. |
| Filament Composition | Contains actin, tropomyosin, and troponin. | Composed of myosin and meromyosin. |
| Cross-bridges | Absent. | Present. |
| Association with ATP | Not associated with ATP. | Directly associated with ATP. |
| Filament Ends | One end is free, the other is attached to Z lines. | Both ends are free; the head domain interacts with ATP. |
| Movement During Contraction | Slides into the H zone. | Does not slide. |
| General Location | Found in muscle fibers, microfilaments, cell membranes, and cell walls. | Primarily found in muscle cells. |
| Role in Muscle Contraction | Interacts with myosin to support contraction. | Initiates contraction by binding to ATP and generating force. |
Examples of Actin
Microfilaments
- Microfilaments, also known as actin filaments, are protein structures that contribute to the cytoskeleton of a cell.
- These filaments are composed of actin protein polymers that interact with various cellular proteins. Each microfilament has a diameter of approximately 7 nm and consists of two intertwined strands of actin.
- Microfilaments serve multiple functions, including cytokinesis, cell shape modulation, and cell motility. Additionally, actin structures located near the plasma membrane adapt based on the specific function of the cell, such as forming specialized structures in the middle piece of mammalian sperm.
Examples of Myosin
Myosin in stereocilia
- Myosin III proteins are located at the tips of stereocilia in the inner ear.
- Stereocilia, which are structurally similar to the microvilli of the intestinal tract, measure approximately 10–15 µm in length. These specialized mechanosensing organelles detect fluid motion in the ear, playing a crucial role in hearing and balance.
- Myosin III forms a complex with PDZ domain-containing proteins, working alongside actin to facilitate the response of stereocilia to sound waves. By enabling movement of the cilia, myosin helps stereocilia adapt to changes in sound waves and fluid motion.
- Additionally, genes encoding myosin proteins have been identified in other regions of the ear, such as the cochlea.
References
- Dominguez, R., & Holmes, K. C. (2011). Actin structure and function. Annual review of biophysics, 40, 169–186. https://doi.org/10.1146/annurev-biophys-042910-155359
- Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 18.3, Myosin: The Actin Motor Protein.Available from: https://www.ncbi.nlm.nih.gov/books/NBK21724/
