18.2: Cell movement

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Motor proteins, such as myosins, dyneins, and kinesins (figure 18.7), move along cytoskeletal filaments via a force-dependent mechanism that is driven by the hydrolysis of ATP molecules. Motor proteins propel themselves along the cytoskeleton using a mechanochemical cycle of filament binding, conformational change, filament release, conformation reversal, and filament rebinding. In most cases, the conformational change(s) on the motor protein prevents subsequent nucleotide binding or hydrolysis until the prior round of hydrolysis and release is complete.

Kinesin: Associated polypeptides, 2 small and 2 large, at one end connected to the motor domain by a stalk. The motor domain has two circles connected individually to the stalk. Myosin: A stalk connected to 2 branches containing 2 associated polypeptides and an oval shaped motor domain. Dynein: 9 Associated polypeptides of varying sizes, connected to two stalks with large circular motor domains with a small rectangle and circle attached. — Figure 18.7: Comparison of the three different motor proteins.

Myosin

Myosin can be characterized as conventional or unconventional, with characteristic head groups (that bind ATP) and unique tails. Myosin is essential for muscle contraction, and this occurs in striated muscle (skeletal and cardiac) after specific binding sites on the actin have been exposed in response to the interaction between calcium ions (Ca²⁺) and proteins (troponin and tropomyosin) that “shield” the actin-binding sites. Ca²⁺ is also required for the contraction of smooth muscle, although its role is different: here Ca²⁺ activates enzymes, which in turn activate myosin heads. All muscles require adenosine triphosphate (ATP) to continue the process of contracting, and they all relax when the Ca²⁺ is removed and the actin-binding sites are re-shielded.

Dynein

Dynein is a large motor protein that typically transports organelles (lysosomes or endosomes). It moves toward the minus end (α-tubulin) of microtubules, which is in the direction of the cell body.

Kinesin

Kinesin is a relatively small motor protein that moves membrane-bound cargo (e.g., vesicles). In contrast to dynein, most move toward the plus end (\(\beta\)-tubulin) of the microtubules, which is typically away from the cell body. Figure 18.8 nicely summarizes the location and general role of each of these motor proteins.

Cell with cell wall and plasma membrane on the outside and microtubules and actin microfilaments horizontally in the cell. Exocytic vesicles with MyoE and kinesin are moving to the right and forming a pile without bound kinesin. Endocytic recycling vesicles with dynein and MyoE are moving to the left. — Figure 18.8: Summary of the roles and movement of the motor proteins along various cytoskeletal elements.

Myosin can be found associated actin filaments, generally moving cargo (exocytic vesicles) away from the cell body. There is also a role in actin polymerization, which is essential for cellular motility.
Kinesin is associated with microtubules moving cargo away from the cell body.
In contrast, dynein, is associated with microtubules moving cargo (endocytic vesicles) toward the cell body.

References and resources

Text

Clark, M. A. Biology, 2nd ed. Houston, TX: OpenStax College, Rice University, 2018, Chapter 4: Cell Structure, Chapter 5: Structure and Function of the Plasma Membranes.

Karp, G., and J. G. Patton. Cell and Molecular Biology: Concepts and Experiments, 7th ed. Hoboken, NJ: John Wiley, 2013, Chapter 9: The Cytoskeleton and Cell Mobility.

Le, T., and V. Bhushan. First Aid for the USMLE Step 1, 29th ed. New York: McGraw Hill Education, 2018, 48–49.

Figures

Grey, Kindred, Figure 18.7 Comparison of the three different motor proteins. 2021. CC BY SA 4.0. Adapted from Aufbau der Motorproteine by keine Autoren genannt. CC BY SA 4.0. From Wikimedia Commons.

Grey, Kindred, Figure 18.8 Summary of the roles and movement of the motor proteins along various cytoskeletal elements. 2021. CC BY 4.0. Adapted from A simplified model for myosin V (MyoE) function at the hyphal tip in Aspergillus nidulans - journal.pone.0031218.g009B by Taheri-Talesh N, Xiong Y, Oakley BR. CC BY 2.5. From Wikimedia Commons.