Difference Between Actin and Myosin

The key difference between actin and myosin is that actin exists as thin, short filaments while myosin exists as thick, long filaments in myofibrils of the muscle fibers.

Actin-myosin contractile system is the main contractile system of all muscular tissues, and it works based on the interactions between the two proteins – the actin and myosin. Furthermore, these two proteins exist as filaments in muscles, and their association is mainly responsible for the muscular movements.

CONTENTS

1. Overview and Key Difference
2. What is Actin 
3. What is Myosin
4. Similarities Between Actin and Myosin
5. Side by Side Comparison –  Actin vs Myosin in Tabular Form
6. Summary

What is Actin?

Actin is the most abundant protein in the muscle fibers, and it is responsible for muscle contraction. It can exist in two distinct forms within the cell. They are globular actin (G-actin) or filamentous actin (F-actin). G-actin is a ≈43kDa protein that can bind ATP and polymerize to form microfilaments known as F-actin filaments. F-actin filaments have appositive (+) ends and negative (-) ends. Both ends are highly dynamic, but have different on/off rates; growth of filaments occurs primarily at the positive end as it has a much higher “on” rate.

Actin filaments are highly cross-linked and bundled by proteins such as α-actinin to increase their structural integrity. The cellular actin network owes its highly dynamic nature to the actin-interacting proteins that facilitate its assembly, stabilization, and disassembly.

What is Myosin?

Myosins are a family of motor proteins associated with actin. Actin-myosin complexes generate the cellular forces used in cell contractility and migration. The majority of myosins are (+) end motors, i.e., they move along actin filaments towards the (+) end. There are several different types of myosins, and each participates in specific cellular functions. Myosin “heavy chains” consist of one or more head, neck, and tail domains.

Functionally, myosins also strengthen the actin network by cross-linking actin fibers. Myosin uses ATP to produce energy; thus, it initiates the muscle contraction by forcing its head towards the actin fiber. One myosin molecule produces about 1.4 pN of force when it changes the confirmation.

What are the Similarities Between Actin and Myosin?

• Actin and myosin are two proteins existing as filaments.
• They are present in muscle cells.
• Also, muscle contraction is a result of actin and myosin interaction and their association.
• Besides, they are arranged longitudinally in myofibrils.

What is the Difference Between Actin and Myosin?

Actin filaments are thin, short filaments, and myosin filaments are thick, long filaments. So, we can take this as the key difference between actin and myosin. Besides, actin filaments occur in two forms: monomeric G-actin and polymeric F-actin. Whereas, myosin molecule has two components: a tail and a head. The tail is formed of heavy meromyosin (H-MM) while the head is formed of light meromyosin (L-MM). Thus, this is another difference between actin and myosin.

Moreover, a further difference between actin and myosin is that the actin forms both A and I bands whereas myosin forms only A bands (A-band forms the dark anisotropic band of myofibril, and I-band forms the light isotropic band of myofibril). Additionally, ATP only binds to myosin ‘head’, and it does not bind to actin. Furthermore, unlike actin, myosin produces a force by binding ATP in order to initiate muscle contractions. Hence, this is also a difference between actin and myosin.

Below infographic on the difference between actin and myosin provides more differences between both comparatively.

Summary – Actin vs Myosin

Actin and myosin are two types of proteins present in muscle cells. Actin makes thin and short filaments in the myofibrils while myosin makes thick and long filaments. Both types of protein filaments are responsible for muscle contraction and movements. They interact with each other and aid in muscle contractions. Moreover, there are comparatively more actin filaments present in the muscle fibers. Furthermore, actin filaments join with Z lines and slide into H zones, unlike myosin filaments. However, myosin filaments form cross bridges, unlike actin filaments. Thus, this summarizes the difference between actin and myosin.

Reference:

1. Cooper, Geoffrey M. “Actin, Myosin, and Cell Movement.” Current Neurology and Neuroscience Reports., U.S. National Library of Medicine, 1 Jan. 1970, Available here.

Image Courtesy:

1. “F-actin filaments in cardiomyocytes” By Ps1415 – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Actin-myosin” By Jeff16 – Own work (CC BY-SA 4.0) via Commons Wikimedia