7,8. Cytoskeleton Flashcards
cytoskeleton
It is made up of filamentous proteins, provides mechanical stability for the cells and restricts the position of intracellular organelles. It is involved in intracellular transport processes, cell movement and cell division. It consists of filaments of various thickness (microtubules, microfilaments, intermediate filaments).
microtubule
Microtubules are approximately 25 nm in diameter. They are long cylindrical structures built from 13 parallel protofilaments, each composed of heterodimers of alpha- and beta-tubulin molecules. The dimers assemble in their GTP-bound state into protofilaments head-to-tail, to generate a distinct structural polarity. Microtubules are used by eukaryotic cells to organize the conmponents of their cytoplasm, serve as structural support and guide the intracellular transport of macromolecules and organelles. They are dynamic structures.
tubulin
Tubulins are the protein subunits of microtubules. They have globular structure. Alpha and beta tubulins can form a heterodimer with non-covalent binding, and these heterodimer molecules form the microtubules after polymerization. Gamma-tubulin molecules form rings in the pericentriolar matrix of centrosomes and serve as nucleation sites where microtubules start from, embedded with their minus end.
GTP cap
A microtubule has GTP-containing tubulin subunits at its growing (+) end, forming a GTP cap. During polymerization the incorporation of GTP-bound tubulin dimers is faster than the hydrolysis of GTP, which leads to the accumulation of GTP-bound tubulin dimers on the plus end of the microtubule. This cap provides for the relative stability of the + end.
GTP cap
A microtubule has GTP-containing tubulin subunits at its growing (+) end, forming a GTP cap. During polymerization the incorporation of GTP-bound tubulin dimers is faster than the hydrolysis of GTP, which leads to the accumulation of GTP-bound tubulin dimers on the plus end of the microtubule. This cap provides for the relative stability of the + end.
colchicine
It is plant alkaloid from autumn crocus (Colchicum autumnale). Via binding to the free tubulin subunits it can block the polymerization of microtubules and inhibit their dynamic instability. It can cause fast dissociation of the microtubules when it is used at high concentration.
microfilament
Thin flexible fibers of approximately 7 nm in diameter, made from globular actin (G-actin) molecules. Actin monomers assemble into polarized protofilaments, with ATP-bound G-actin preferentially joining the + (barbed) end. The actin filament can be viewed as two parallel protofilaments twisted around each other. Microfilaments are dynamic structures, and major constituents of all eukaryotic cells. They are essential for cell movement and contraction of muscle cells.
intermediate filament
Intermediate filaments have a diameter of about 10 nm and their major function is to provide
mechanical strength to cells and tissues. Cytoplasmic intermediate filaments are tissue specific, e.g. keratins occur mostly in epithelial cells, vimentins in mesenchymal and cancer cells, and neurofilaments in the nerve cells. Nuclear lamins are present in most eukaryotic cell
treadmilling
An important feature of microtubules (but also shown by actin filaments at high concentrations of G-actin-ATP). The speed of polimerization of microtubules at their positive end and the speed of depolymerization at their negative end is equal, thus the length of the microtubules does not change, but the position of the tubulin dimers in the microtubules changes in time, they stream from the plus end to the minus end. Microtubule treadmilling is necessary for the alignment and arrangement of chromosomes at the equator of the cell in metaphase.
motor proteins
Motor proteins convert chemical energy (usually of ATP) into kinetic energy, resulting in their movement along filaments. Major examples are: (1) Kinesin moves towards the positive end of a microtubule, in most cells transports cargo from the centre of the cells towards the periphery. (2) Dynein moves towards the minus end of a microtubule. In cilia, dynein forms the side arms in the axoneme that cause adjacent microtubule doublets to slide past one another. Both kinesin and dynein are important motors during cell division as well. (3) Myosin moves towards the plus end of actin filaments. Type II myosin is present at high amount in muscle cells. All three use ATP hydrolysis.
dynamic instability
Microtubules and microfilaments are dynamic structures that grow by adding tubulin dimers (using GTP energy) or actin monomers (using ATP energy) and shrinking by depolymerization. The sudden conversion from growth to shrinkage, or vice versa, in a microtubule or in an actin filament is called dynamic instability.
Lamellipodium, filopodium
Lamellipodia are flattened, sheet-like protrusions supported by a dynamically changing meshwork of actin filaments, which are extended at leading edge of a crawling animal cell. Dynamic reorganization of the meshwork is directed by various monomeric G proteins (Rho, Rac). Membrane bound spike-like actin bundles protruding form the lamellipodium are called filopodia.
Actin sequestering proteins
These proteins bind to actin monomers and/or filaments and modify the dynamic assembly/disassembly of the microfilaments. Thymosin is an actin monomer sequestering protein, which can block incorporation of monomers on both ends of the actin filaments. Profilin is an actin monomer sequestering protein, which promotes incorporation of the monomers on plus end of the filaments, but at the same time blocks polymerisation on the minus end. It also takes part in recirculation of the actin monomers.
Actin sequestering proteins
These proteins bind to actin monomers and/or filaments and modify the dynamic assembly/disassembly of the microfilaments. Thymosin is an actin monomer sequestering protein, which can block incorporation of monomers on both ends of the actin filaments. Profilin is an actin monomer sequestering protein, which promotes incorporation of the monomers on plus end of the filaments, but at the same time blocks polymerisation on the minus end. It also takes part in recirculation of the actin monomers.
Actin binding toxins
Toxins that bind actin and can block dynamic actin assemby/disassembly are valuable research tools. Examples are: (1) Cytochalasin: binds to the plus end of the actin filament and blocks polymerization. (2) Phalloidint is a toxin isolated from the death cap mushroom Amanita phalloides. It binds to and stabilizes filamentous actin. Fluorescent dye conjugated to phalloidin can be used for labeling and visualization of actin filaments in the cells.
