THE CYTOSKELETON - MICROTUBULES, INTERMEDIATE FILAMENTS, MICROFILAMENTS

the cytoskeleton of a eukaryote specifically of an animal cell has three kinds of cytoskeletal filaments which provide the structure aid in movement and help with transport within the cell microtubules organize the positions of organelles and direct intracellular transport intermediate filaments are rope like fibers found along the interface of the nuclear envelope and they also build a network of cables which connect the cells of epithelial sheets these filaments provide mechanical strength microfilaments or actin filaments are most concentrated right beneath the plasma membrane at the so called cortex of the cell and they control the outer shape of the cell and are important in locomotion actin can form several kinds of cell surface projections including micro villi lamellipodia and philip Odia these help move cells over solid substrates the three kinds of cytoskeleton filaments work in concert with countless accessory proteins which attach the filaments to each other and to other cell components and direct their assembly distribution and disassembly the cytoskeleton is not a static structure but rather is dynamic able to change or persist to suit the cell's needs this is because the cytoskeletal structures are composed of tiny polar subunits that can rapidly assemble and disassemble thanks to weak non covalent linkages into polymers these macro molecular components of the cytoskeleton filaments are constantly in flux and nucleation of a cytoskeletal polymer is the rate-limiting step since the small initial aggregate is less stable and more likely to fall apart it is important to note that cytoskeletal filaments do not build by the addition of a single subunit at a time multiple proto filaments are being built constantly which then associate laterally through non covalent or hydrophobic interactions this composite structure is much harder to break some cells in your body require rapid rearrangement of your cytoskeleton while others require the maintenance of stable structures a white blood cell pursuing a bacterium is able to rapidly move thanks to a perpetually shifting leading edge of actin meanwhile and established neuron or epithelial cell requires a more stable cytoskeletal structure in the case of epithelial tissues the cytoskeleton also maintains polarity over the course of the cell's lifetime specialized cell surface protrusion face the lumen from the apical surface providing more surface area for the transfer of nutrients while the basal lateral surface is much flatter let's discuss each type of cytoskeleton filament in more detail in this video we will mostly focus on microfilaments and microtubules microfilaments composed of the globular protein actin are a double helix six nanometers in diameter they are organized into linear bundles 2d networks and 3d networks loosely orthogonal e cross linked with properties of semi solid gels actin molecules are tightly bound to an ATP molecule when they are added to a growing polymer the ATP is soon hydrolyzed to ADP this hydrolysis makes it more likely to dissociate from the end of the filament if actin molecules are added quickly enough the actin filament can acquire what's called an ATP cap tred milling is observed in actin filaments and microtubules mostly in actin filaments in this process the filament appears to be moving but really one end is growing while the other is shrinking growth of the polymer proceeds until the concentration of free monomer is such that the growth at the positive end equals disassembly at the negative end actin filaments are typically nucleated at the plasma membrane and external signals can trigger this nucleation nucleation is promoted by the ARP complex ARP stands for actin related proteins because they are 45% identical to actin the ARP complex is also known as the ARP 2 3 complex it nucleates actin filament growth from the minus end so the plus end can grow rapidly ARP nucleates most efficiently when it is attached to the side of another actin filament the result is a gel-like tree like web with 70 degree angles at the leading edge actin filaments become capped by capping protein which means they can neither grow nor shrink meanwhile cofilin causes d polymerization getting rid of older filaments this causes the actin filament network as a whole to move forward despite individual filaments remaining stationary actin filament nucleation can also be triggered by Foreman's however the actin filaments nucleated by this accessory protein are not branched instead the filaments form parallel bundles these parallel bundles formed for instance during cell division they form the cleavage furrow that helps to daughter cells pinch off actin has many many accessory proteins these are just a few of the most important ones intermediate filaments are ten nanometers in diameter and are more stable than actin filaments they are composed of intermediate filament proteins which are a diverse family of proteins that are elongated in fibrous you can think of them as cables they help the cell maintain its shape by bearing tension they anchor organelles and structure the nuclear lamina they are important in epithelial tissues where together with proteins and desmosomes they form cell to cell connections check out my video on junctions to learn more while intermediate filaments resist tension microtubules mostly resist compression microtubules are hollow tubes composed of tubulin dimers which are made of two globular proteins alpha and beta tubulin these heterodimers spontaneously bind together forming a proto filament thirteen such protofilaments arranged together into a cylinder forming a microtubule a microtubule is 23 nanometers in diameter and the inside diameter is 15 nanometers while microfilaments have some bend to them microtubules are much more rigid hence they are long and straight microtubules are polar molecules the positively charged end with beta subunits exposed grows relatively quickly while the negatively charged end with alpha subunits exposed grows relatively slowly as with actin different kinds of proteins alter properties of growing microtubule ends remember how we mentioned that actin molecules are bound to ATP and then this ATP gets hydrolyzed to ADP soon after the actin molecule gets added on to a growing filament well microtubules are similar except they have gtp tightly bound to the tubulin heterodimer again once the dimer binds into a growing microtubule the gtp soon hydrolyzes to GDP this reduces the affinity of the subunit to lateral proto filaments as well as the subunits in front and behind it hence increasing the odds of it dissociating dynamic instability occurs due to differences between the two polar ends of the microtubule if the rate of addition of subunits exceeds the rate of hydrolysis of gtp to GDP the microtubule acquires a GTP cap a microtubule without a GTP cap d polymerizes around 100 times faster than one with a GTP cap hence a microtubule with a GTP cap grows rapidly if the cap is lost because nucleotide hydrolysis occurs more quickly than addition then catastrophe occurs and the microtubule begins to shrink an event called a rescue occurs if gtp bound subunits are added to the shrinking end fast enough to form a new cap microtubule nucleation occurs primarily near the nucleus since microtubules extend from the center of the cell they establish a general coordinate system then the cell can use various measuring mechanisms to organize itself microtubules are nucleated and organized by microtubule organizing centers or MTO C's centrosomes containing a pair of centrioles at right angles to one another are the primary MTO C's they are centrally located organelles that act as the spindle pole during mitosis and meiosis which separates the chromosomes rapid reorganization of the cytoskeleton occurs during cell division after chromosomal replication we can see the bipolar mitotic spindle as mentioned previously actin is responsible for the contractile ring that pinches the cell into basal bodies are also MTO sees and are found in cilia and flagella cilia and flagella have the same cross-section featuring a 9+2 structure meaning nine doublet microtubules and two single microtubules the nine doublets are in a ring and their position relative to one another is maintained thanks to necks in between them there is also an inner and an outer Dinan arm motor proteins that allow one doublet to move along the other I will go into more detail on the structure of cilia and flagella in a later video in fact this video is just an introduction to the cytoskeleton and I'll be making several more in-depth videos on the topic please subscribe to see them when they're uploaded

Channel: Neural Academy

Views: 159,002

Keywords: cytoskeleton, microtubules, actin, microfilaments, intermediate filaments, tubulin, accessory proteins, proteins, locomotion, lamellipodia, filopodia, basal bodies, centrosomes, centrioles, cilia, flagella

Id: 5DKZiSJeoV4

Length: 9min 10sec (550 seconds)

Published: Fri Jan 25 2019