Biology 101: Microfilaments vs Microtubules in Cell Biology

Microfilaments vs Microtubules: What is the difference between microfilaments and microtubules?

Microfilaments and microtubules are both important elements of the cytoskeleton, maintaining the structure and integrity of the cell, but there are several differences between microfilaments and microtubules. In this article, we share this information on microfilaments versus microtubules.

Microfilament and Microtubulues are made from different individual subunits. Microfilaments are made from ACTIN, while microtubules are made from TUBULIN.

Microfilaments = Actin

Microtubules = Tubulin

Difference in Microtubule and Microfilament Reorgnization

The process of adding on or removing monomers from microfilaments and microtubules differs. For microfilaments, actin monomers can be added to either end of the filament, including the barbed end (positive end) and the pointed end (negative end). For microtubules, unlike microfilaments, tubulin monomers are only added and removed on the positive end. In microtubules, a tubulin heterodimer consisting of beta-tubulin and alpha-tubulin is added or removed each time. Microtubules have gamma-tubulin proteins at the minus ends of the complex that prevent removal or addition of tubulin on that minus end. In fact, during mitosis and meiosis, gamma tubulin complexes are loosely organized around centrioles.

Microfilaments: Added on either end (plus or minus)

Microtubules: Added only on the plus end

Microtubules: have gamma-tubulin proteins on the minus end, prevents addition or removal of tubulin

Mitosis requires 2 centrosomes whose spindles are made up of microtubules, not microfilaments. These microtubules in the spindles help position chromosomes in the middle of the cell during metaphase and pull them apart during anaphase.

Microtubules grow and shrink only on the plus end. When the plus end is capped with GTP bound tubulin, the microtubule is stable and no longer grows. When the plus end is capped with GDP bound tubulin at the end, microtubule is unstable and can shrink.

Another difference between microtubules and microfilaments is that for microfilaments, nucleating complexes are involved in initiating filament formation. There are 3 proteins involved in filament formation: actin, Arp2, and Arp3. They make a nucleating complex, bind to either minus or plus end on the microfilament.

Microfilament Regulating Actin Assembly Rates

Actin-profilin complex causes rapid plus-end growth. Actin-thymosin complex inhibits binding and plus-end growth. Profilin and thymosin compete with each other for the binding of actin monomers and the promotion of microfilament assembly.

Microfilament: Arp2 + Arp3 + Actin = Nucleating complex to start formation

Microfilament: Actin-thymosin = Inhibit vs Actin-profilin = Proliferate/Grow

Difference in Examples: Microfilament vs Microtubule

Actin/microfilament in microvilli of small intestines

Platelet activation: actin filament plays a role here in change of cellular shape

Movement of neutrophils and lamellipodiums driven by actin polymerization

Example of Microtubules: sperm movement, mitosis and meiosis with spindles and centrosomes, cilia on trachea, modified cilia on rod cells in the eye

Difference in Diseases: Microfilament vs Microtubule

Disease of Microfilaments: mutation in cardiac myosin gene can cause atrial hypertrophy.

Disease of Microtubules: Kartagners Disease with mucus, respiratory problems because cilia not working, unique symptom is invertus of organs, heart on the right side, flipped organs

Difference in Motor Proteins between Microfilaments and Microtubules

Actin microfilaments: myosin is the main motor.

Microtubules: kinesins and dyneins are the main motor proteins. Kinesins deliver away from the center of the cell, to the outside. Dynein delivers and transports things back to the cell, towards the middle of the cell.

Dynein causes cilia to bend, creating motive force. In isolated doublet microtubules, dynein causes microtubules to slide across each other. Dynein can also cause microtubules to bend if there are linking proteins.

Microtubule: CILIA in trachea/lungs/eyes/kidney

Microfilament: MICROVILLI in small intestines

Similarity between Microfilament and Microtubules

Both help control cell shape and organization.

Both microtubules and microfilaments promote cellular movement.

Both microtubules and microfilaments serve as tracks for motor proteins.

Both have protein subunits that bind to nucleotides like ATP or GTP. True similarity! Remember earlier how we talked about microtubules with GTP caps and microfilaments with ATP with actin and myosin heads.

Last similarity of microtubules and microfilaments is that they are both part of the awesome cytoskeleton of our cells.

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