Microfilaments Compared to the other parts of the cytoskeletons, the microfilaments contain the thinnest filaments, with a diameter of approximately 7 nm. Microfilaments are part of the cytoskeleton that are composed of protein called
actin. Two strands of actin intertwined together form a filamentous structure allowing for the movement of motor proteins. Microfilaments can either occur in the monomeric G-actin or filamentous F-actin. Microfilaments are important when it comes to the overall organization of the plasma membrane. Actin filaments are considered to be both helical and flexible. They are composed of several actin monomers chained together which add to their flexibility. They are found in several places in the body including the microvilli, contractile rings, stress fibers, cellular cortex, etc. In a contractile ring, actin have the ability to help with cellular division while in the cellular cortex they can help with the structural integrity of the cell. ; Microfilament Polymerization Microfilament
polymerization is divided into three steps. The nucleation step is the first step, and it is the rate limiting and slowest step of the process. Elongation is the next step in this process, and it is the rapid addition of actin monomers at both the plus and minus end of the microfilament. The final step is the steady state. At this state the addition of monomers will equal the subtraction of monomers causing the microfilament to no longer grow. This is known as the critical concentration of actin. There are several toxins that have been known to limit the polymerization of actin.
Cytochalasin is a toxin that will bind to the actin polymer, so it can no longer bind to the incoming actin monomers. Actin originally attached in the polymer is still leaving the microfilament causing depolymerization.
Phalloidin is a toxin that will bind to actin locking the filament in place. Monomers are neither adding or leaving this polymer which causes the stabilization of the molecule.
Latrunculin is similar to cytochalasin, but it is a toxin which will bind to the actin monomers preventing it from adding onto the actin polymer. This will cause the depolymerization of the actin polymer in the cell. ; Actin Based Motor Protein- Myosin There are several different proteins that interact with actin in the body. However, one of the most famous types of motor proteins is
myosin. Myosin will bind to these actins causing the movement of actin. This movement of myosin along the microfilament can cause muscle contraction, membrane association,
endocytosis, and organelle transport. The
actin microfilament is composed of three bands and one disk. The A band is the part of the actin that will bind to the myosin during muscle contraction. The I band is the part of the actin that is not bound to the myosin, but it will still move during muscle contraction. The H zone is the space in between two adjacent actin that will shrink when the muscle begins to contract. The Z disk is the part of the microfilament that characterizes the overall end of each side of the
sarcomere, a structural unit of a
myofibril. ; Proteins Limiting Microfilaments These microfilaments have the potential to be limited by several factors or proteins.
Tropomodulin is a protein that will cap the ends of the actin filaments causing the overall stability of the structure.
Nebulin is another protein that can bind to the sides of the actin preventing the attachment of myosin to them. This causes stabilization of the actin limiting muscle contraction.
Titin is another protein, but it binds to the myosin rather than the actin microfilament. Titin will help stabilize the contraction and myosin-actin structure.
Microtubules Microtubules are the largest type of filament, with a diameter of 25 nm wide, in the cytoskeleton. A single microtubule consists of 13 linear microfilaments. Unlike microfilaments, microtubules are composed of a protein called tubulin. The tubulin consists of dimers, named either "αβ-tubulin" or "tubulin dimers", which polymerize to form the microtubules. These 10 nm filaments are made up of polypeptide chains, which belong to the same family as intermediate filaments. Intermediate filaments are not involved with the direct movement of cells unlike microtubules and microfilaments. Intermediate filaments can play a role in cell communication in a process known as crosstalk. This cross talk has the potential to help with the mechanosensing. This mechanosensing can help protect the cell during cellular migration within the body. They can also help with the linkage of actin and microtubules to the cytoskeleton which will lead to the eventual movement and division of cells. Lastly these intermediate filaments have the ability to help with vascular permeability through organizing continuous adherens junctions through plectin cross-linking. ; Classification of Intermediate Filaments Intermediate filaments are composed of several proteins unlike microfilaments and microtubules which are composed of primarily actin and tubulin. These proteins have been classified into 6 major categories based on their similar characteristics. Type 1 and 2 intermediate filaments are those that are composed of keratins, and they are mainly found in epithelial cells. Type 3 intermediate filaments contain vimentin. They can be found in a variety of cells which include smooth muscle cells, fibroblasts, and white blood cells. Type 4 intermediate filaments are the neurofilaments found in neurons. They can be found in many different motor axons supporting these cells. Type 5 intermediate filaments are composed of nuclear lamins which can be found in the nuclear envelope of many eukaryotic cells. They will help to assemble an orthogonal network in these cells in the nuclear membrane. Type 6 intermediate filaments are involved with nestin that interact with the stem cells of central nervous system. == References ==