Difference Between Actin and Myosin

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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 

Basis of Comparison

Actin

Myosin

Meaning

The proteins that form the thin bands in myofibrils are known as actin.

Myosin is a protein that is responsible for the thick bands found in myofibrils.

Structure

These are thinner (0.005 m in diameter) and shorter (2-2.6 m in length).

These are thicker (0.01 m in diameter) and larger (4-5 m in length).


Nature

They are Globular Proteins

They are Motor Proteins

Found in

Both the A and I bands of the sarcomere contain actin proteins.

Only the A bands of the sarcomere contain myosin proteins.

Molecular weight

Actin proteins have a lower molecular weight than myosin proteins.

Myosin proteins have a higher molecular weight than actin proteins.

Surface

Smooth

Rough

Abundance

More abundant.

Less abundant compared to actin.

Cross bridges

Absent

Myosin forms cross-bridges.

Proteins in filament

Actin protein

Tropomyosin protein

Troponin protein

Myosin protein
Meromyosin proteins

Association with ATP

Not associated with ATP molecules.

Remains associated with ATP molecules.

Sliding

Actin filaments slide into the H zone during contraction.

Myosin filaments do not slide into the H zone during contraction.

Location

Actin is present in muscle fibres, microfilaments, cell membrane, and cell wall.

Myosin is primarily found in muscle cells.

Muscle Contraction

Actin interacts with myosin to support muscle contraction.

Myosin initiates muscle contraction by generating a force by binding to the ATP molecule.


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.

FAQ (Frequently Asked Questions)

Q1. Discuss an Example of Actin.

Ans:

Microfilaments

  • Microfilaments, also known as actin filaments, are proteins that make up the cytoskeleton in cells.

  • Polymers of actin proteins that interact with other proteins in the cell are known as actin polymers.

  • Microfilaments have a diameter of around 7 nanometers and are made up of two strands of actin.

  • Microfilaments have a variety of functions, including cytokinesis, cell shapeshift, and cell motility.

  • Actin structures are also present adjacent to the plasma membrane, creating a structure that varies depending on the cell's activity, similar to the middle piece of mammalian sperm.

Q2. Discuss an Example of Myosin.

Ans: 

Myosin in Stereocilia

  • In the inner ear, myosin III proteins are found at the tip of the stereocilia.

  • The stereocilia are around 10-15 m long and have a structure similar to the microvilli in the intestine.

  • Stereocilia are mechanosensing organelles that respond to the fluid motion in the ear and perform hearing and balancing functions.

  • It forms a complex with a protein with a PDZ domain, which, along with actin proteins, aids the stereocilia's response to sound waves.

  • By shifting the cilia, myosin helps the stereocilia to respond to changes in sound waves and fluid motion.

Q3. State Some Similarities Between Actin and Myosin.

Ans: 

Some similarities between actin and myosin are-

  • Both are filaments made up of protein present in muscles.

  • Both types of proteins are required during muscle contraction.

  • Both need Calcium ions for muscle contraction.