Difference Between Actin and Myosin


Main Difference

The main difference between actin and myosin is that actin protein is the leading supplier of the contractile property of muscle and other cells whereas myosin is working as a motor, hydrolyzing adenosine triphosphate (ATP) to discharge energy in such a way that a myosin filament moves along an actin filament, initiating the two threads to slide past each other.

Actin vs. Myosin

Both actin and myosin play a physical and enzymatic role in muscle contraction and intracellular motility. Actin is present in both A and I bands whereas myosin is present in A band of the sarcomere. Actin contains thinner (0.005 mn), but shorter (2 -2.6 mn) filaments whereas myosin contain thicker (0.01 mn) but longer (4.5 mn) filaments. Cross bridges are absent in actin, existing on a smooth surface but cross bridges are present in myosin, living on a rough surface. Actin is more numerous than myosin filaments, six of them surround each myosin filament, but myosin is fewer in number than actin filaments. The actin filament is free at one end and joins to Z -line at another end on the other hand myosin filament is open at both the ends. Actin consists of 3 proteins such as actin, tropomyosin, and troponin while myosin consists of 2 proteins like myosin and meromyosin. Actin filament slides into H-zone in muscle contraction, but myosin does not slide during muscle contraction.


Comparison Chart

Protein that forms thin contractile strands in muscle cellsProtein that forms the thick contractile strands in muscle cells
Size of Filaments
Thin (0.005 μm), and short (2 – 2.6 μm) filamentThick (0.01 μm), and long (4.5 μm) filament
Present in A and I bandsPresent in A bands of a sarcomere.
Regulatory Proteins
Tropomyosin and troponinMeromyosin
Cross Bridges
Cross bridges are not presentCross bridges are present
Great in numberOne myosin filament arises per six actin filaments.
Slide into H zone during contractionDo not slide during contraction
Free at one endFree at both ends

What is Actin?

Actin discusses a protein that forms a thin contractile filament in muscle cells. It is the richest protein in eukaryotic cells. Actin is incredibly conserving protein. Two forms of the actin are monomeric and filamentous. Under physical conditions, the monomeric is readily polymerize to form filaments by using the energy from ATP. The polymerization of actin filaments initiates from both ends of the filament; the ratio of the polymerization is not equal in each end and results in an inherent polarity in the filament. The relationship of the tropomyosin and troponin stabilizes the actin filament. The nature and movement of the cell depend on the actin filaments. The central role of actin filaments is to form the active cytoskeleton of a cell. The cytoskeleton gives physical support and links cell to its surroundings. Actin filaments are involving in the development of filopodia and Lamellipodia that help the cell motility. Actin filaments help in the transportation of organelles to the daughter cells during mitosis. The compound of thin filaments in muscle cells produces forces, supporting the contraction of the muscles.


What is Myosin?

Myosin discusses a protein that forms the thick contractile filaments in muscle cells. Myosin plays a physical and enzymatic role in muscle contraction and intracellular motility. All myosin molecules are comprising as one or two heavy chains and many light chains. Three domains can identify in this protein: head, neck, and tail. The head area is circular and comprises actin and ATP binding sites. The neck region comprises α-helical. The tails contain almost three hundred myosin molecules from the shaft of the thick filament. Myosin is a superfamily of proteins which fix actin, hydrolyze ATP and most are locating in muscle cells. The myosin heads of these molecules develop outward toward the thin filaments like the paddles of a rowboat. The tail site comprises the binding sites for different molecules. There are 18 classes of myosin. Thirteen different types of myosin can identify as myosin I, II, III, IV, etc. The myosin I involve in the transportation of vesicles. Myosin II is responsible for muscle contraction. The contraction of muscles is describing as the sliding filament theory. The thin actin filaments glide over a thick myosin filament, producing tension in the muscle. Each myosin thick filament is surrounding by thin actin filaments, and each thin filament is surrounding by thick filaments. Numerous of these filament bundles make up the functional portion of a muscle cell.

Key Differences

  1. Actin denotes to a protein that forms a thin contractile filament in human muscles while myosin refers to a protein that forms the thick contractile filaments in muscles cells.
  2. Actin makes a thin (0.005 μm), short (2 – 2.6 μm) filament but myosin makes a thick (0.01 μm), long (4.5 μm) filament.
  3. Actin filaments consist of tropomyosin and troponin while myosin filaments consist of meromyosin.
  4. Actin filaments are present in A and I bands conversely myosin filaments are present in A bands of a sarcomere.
  5. Actin filaments do not form cross bridges on the other hand myosin filaments build cross bridges.
  6. The exterior of the actin filaments is smooth, but the surface of the myosin filaments is rough.
  7. Actin filaments are numerous whereas one myosin filament occurs per six actin filaments.
  8. Actin filaments are free at one end whereas myosin filaments are free at both ends.
  9. Actin filaments slide into H zone during a contraction, but myosin filaments do not slide during contraction.


Above this discussion, it concludes that actin and myosin are two types of proteins that form contractile filaments in muscle cells. Actin makes thin and short filaments while myosin makes thick and long filaments. Both actin and myosin are present in eukaryotic cells, forming the cytoskeleton and involving in the movement of molecules.

Janet White

Janet White is a writer and blogger for Difference Wiki since 2015. She has a master's degree in science and medical journalism from Boston University. Apart from work, she enjoys exercising, reading, and spending time with her friends and family. Connect with her on Twitter @Janet__White