Introduction
To understand the role of actin-myosin, some knowledge of muscle contraction is needed. Muscle cells are responsible for all cellular and molecular motions in the body. Smooth muscles, cardiac muscles, and skeletal muscles are the three types of muscle cells present in vertebrates. Smooth muscles are responsible for involuntary movements in the body, while cardiac muscles are responsible for constantly pumping our hearts, and skeletal muscles are responsible for all sorts of voluntary movements.
Skeletal muscles contain a large number of muscle fibres, which are a collection of various cells that fused to form a single large cell during development. Numerous nuclei can be found in muscle cells, and myofibrils, which are cylindrical bundles of thick and thin filaments, can be found in the cytoplasm. The thin filament is made up of actin, whereas the thick filament is made up of myosin, and both are grouped into sarcomeres, which are repeated chains of proteins.
What is Actin?
Actin is a group of globular proteins that are present in the majority of eukaryotic cells and assist in the body's form, structure, and mobility.
What is Myosin?
Myosin is a motor protein superfamily that, along with actin proteins, is responsible for muscle fibre contraction.
Actin and Myosin are involved in a variety of cellular movements, the most noticeable of which is muscle contraction, which serves as the best model for understanding the function of actin and myosin.
Now that we are familiar with the role of Actin and Myosin in muscle contraction, let us familiarize ourselves with the major difference between actin and myosin filament.
Difference Between Actin and Myosin
Before we jump on to discuss all the Actin and Myosin differences, we need to discuss two main differences in detail.
Difference Between Myosin and Actin on the Basis of Structure
Actin forms a small filament with a diameter of 2-2.6 um and a thickness of 0.005 um. Troponin* and tropomyosin* are contained in actin (protein) whereas, Myosin forms a 4.5 um long filament with a thickness of 0.01 um. Meromyosin* is found in myosin (protein).
Troponin: Troponin is a form of protein found in your heart's muscles. Troponin is a protein that isn't usually present in the bloodstream. It is released into the bloodstream when heart muscles are weakened/damaged.
Tropomyosin: Tropomyosin, a polymer of a second protein, is an important component of most actin filaments in animals. Tropomyosins are a broad family of actin filament integral components that play a key role in controlling actin filament activity in both muscle and nonmuscle cells.
Meromyosin: ‘Mero’ means ‘part of’. Meromyosin refers to a part of the myosin protein.
Difference Between Myosin and Actin Filament on the Basis of Nature
Actin proteins are globular proteins, also known as spheroproteins, which are spherical ("globe-like") proteins. Unlike fibrous or membrane proteins, they are somewhat water-soluble (forming colloids in water). Whereas;
Myosin proteins are proteins that function as motors. They are a type of molecular motor that can pass through animal cells' cytoplasm. They use ATP hydrolysis to turn chemical energy into mechanical work.
Difference Between Actin and Myosin
Conclusion
As a result, it is said that myosin and actin collaborate during muscle contractions, with myosin serving as the precursor protein in the conversion of chemical energy (ATP) to mechanical energy. So, apart from muscle contraction, actin and myosin play an important role in cell biology by engaging in cell division, nonmuscle cell functions, and so on. Myosin is smoother and has deeper striations than actin. The sliding filament theory can explain how muscle contractions function.
FAQs on Difference Between Actin and Myosin
1. What is the main difference between actin and myosin filaments?
The main difference is that actin forms the thin filaments in muscle cells, while myosin forms the thick filaments. During muscle contraction, the thick myosin filaments act like motors, pulling the thin actin filaments towards the center, which causes the muscle to shorten.
2. What are actin and myosin?
Actin and myosin are the two primary types of proteins found within our muscle cells. They are essential for a muscle's ability to contract and generate force. You can think of them as the engine (myosin) and the rope (actin) that work together to create movement.
3. What are the key functions of actin and myosin proteins?
Their main function is to enable muscle contraction. However, they also have other important roles in the body:
- Actin filaments (also called microfilaments) are crucial for maintaining cell shape, enabling cell movement, and participating in cell division.
- Myosin proteins function as molecular motors, converting chemical energy from ATP into mechanical force, not just in muscles but for other cellular transport tasks as well.
4. Are there any similarities between actin and myosin?
Yes, despite their differences, actin and myosin share some key characteristics. Both are proteins found in muscle fibres that are essential for movement. Furthermore, their interaction during muscle contraction is dependent on the presence of calcium ions (Ca²⁺) and requires energy from ATP.
5. How do actin and myosin interact to make a muscle contract?
This interaction is explained by the Sliding Filament Theory. The heads of the myosin filaments attach to the actin filaments, forming a structure called a cross-bridge. Using energy from ATP, the myosin heads then bend and pull the actin filaments inward. This sliding action shortens the muscle's basic unit (the sarcomere), and when millions of sarcomeres shorten together, the entire muscle contracts.
6. What is the difference between an individual actin protein and an actin filament?
An individual actin protein is a single, ball-shaped molecule known as G-actin (Globular actin). When many of these G-actin molecules link together to form a long chain, it is called F-actin (Filamentous actin). The thin filaments in our muscles are composed of two F-actin strands twisted around each other.
7. Why is ATP essential for the interaction between actin and myosin?
ATP (adenosine triphosphate) plays two critical roles. First, the breakdown of ATP provides the energy that prepares the myosin head to bind to actin and perform the 'power stroke' that pulls the actin filament. Second, a new ATP molecule must bind to the myosin head to make it detach from actin after the pull. Without this detachment, muscles would remain locked and stiff.
8. Can actin and myosin be found in cells other than muscle cells?
Yes, absolutely. While they are famous for their role in muscles, both proteins are found in almost all eukaryotic cells. Actin is a fundamental part of the cell's cytoskeleton, providing structural support. Myosin often acts as a motor protein to transport substances along these actin tracks, helping with processes like cell division and internal organisation.
9. What would happen if there was a defect in the myosin proteins in a person's body?
A defect in myosin could lead to serious health issues. If the defect is in the myosin of skeletal muscles, it can cause muscle weakness or diseases called myopathies. If it affects myosin in the heart, it could lead to cardiomyopathy, a severe heart condition. Defects in other specialised myosins can even cause issues like deafness, as they are vital for the function of cells in the inner ear.
