L18 Flashcards
What cellular components do not contain a membrane?
Cytoskeleton, ribosomes, cell wall.
Cytoskeleton
Dynamic network of soluble proteins packing the cell interior, primary function is to provide internal structure
What are the types of cytoskeleton filaments in the cytosol of the eukaryotic cell?
Actin filaments - micro filaments
Intermediate
Microtubules
What are actin filaments?
Small in diameter, twisted two-stranded structure, made of monomeric actin subunits.
What are microtubules?
Largest in diameter, hollow tube like structures made of dimeric subunits of alpha and beta tubulin
What are intermediate filaments?
Intermediate in diameter, assembled from different proteins depending on the cell
How are cytoskeleton filaments organised?
Into bundles and networks. In bundles, the filaments are packed in parallel arrays. In networks, the filaments crisscross. Cytoskeletal components are usually attached to plasma membrane proteins and form a skeleton that helps support the plasma membrane
What are the functions of the cytoskeleton?
Intracellular movement e.g. cytoplasm streaming - flow of the cytoplasm inside the cell, driven by forces from the cytoskeleton. Function is to speed up the transport of molecules and organelles around the cell. Whole cell movement e.g. host defences, muscle contraction.
What does cell movement require?
Arrangement of microfilaments or microtubules and/or motor proteins. Fuel (ATP) and proteins that convert the energy stored into ATP into motion.
What is not involved in cell movement?
Intermediate fibres, which instead play a role in cell adhesion
What mechanism have cells evolved for generating basic movement?
The assembly and disassembly of microfilaments and microtubules ; it is responsible for many changes in cell shape. The other mechanism requires motor proteins (myosins, kinesins and dyeneins).
Microfilaments - Actin structure
Made of actin, most abundant intracellular protein in most eukaryotic cells. It is encoded by a large, highly conserved, gene family. Single cell organisms 1-2 actin genes. Multicellular organisms multiple genes. Different genes in the family have different functions and are found in different levels.
What is G-actin?
A globular monomer, separated into two lobes by a deep cleft, lobes and the cleft compose the ATPase fold - the site where ATP and Mg2+ are bound. Cleft acts as a hinge that allows the lobes to flex relative to each other.
G-actin further
G-actin monomer can assemble into a chain, which forms F-actin a filamentous polymer. Which can disassemble to form G-actin again.
What is the organisation in the actin filament?
Subunits are organised as a helix (two intertwined strands), actin filaments are polarised with one end, - end, containing an exposed ATP-binding site. Actin filaments are organised into bundles and networks by a variety of bivalent cross-linking proteins. When cross-linked by a short protein actin filaments pack side by side to form a bundle. Long cross-linking proteins, are flexible and can cross-link actin filaments into a network.
Actin assembly
Within cells, the actin cytoskeleton is dynamic, with filaments able to grow and shrink rapidly. Actin filaments grow faster at their + end. Initial nucleation process acts as a focal point. In the initial nucleation phase, ATP-G-actin monomers slowly form stable complexes of actin. These nuclei are rapidly elongated in the second phase by the addition of subunits to both ends of the filament. After their incorporation into a filament, subunits slowly hydrolyse ATP and become stable ADP-F-actin.
Actin treadmilling
Actin subunits can flow through the filaments by attaching preferentially to the + end and dissociating preferentially from the - end of the filament. This process is called treadmilling. In this situation, the length of the filament remains constant.
How is actin polymerisation controlled?
It is regulated by proteins that bind to G-actin. Thymosin inhibits actin assembly, profilin promotes actin assembly, profilin binds to the G-actin coupled to ADP and catalysed the exchange of ADP with ATP. The ATP-G-actin-profilin complex can be linked to the plus end of the filament and profilin dissociates.
How does polymerisation of actin cause changes in cell shape?
Platelets change shape during blood clotting. Resting cells have a discoid shape. When exposed to clotting agents, cells settle on substratum, extend numerous filopodia, and spread out. Changes in morphology result from complex rearrangements of actin cytoskeleton, which is cross-linked to the plasma membrane
What are myosins?
Large superfamily of motor proteins that bind actin filaments
What are myosins responsible for?
Vesicular trafficking, cytoplasmic streaming, muscle contraction, cytokinesis.
How many members of the myosin gene family are present in eukaryotes?
Eight members, 1, 2 and 5 present in nearly all eukaryotic cells. 2 powers muscle contraction and cytokinesis. 1 and 5 take part in cytoskeleton-membrane interactions.
Myosins present in plants?
Plants do not have the same myosins as animal cells, 7, 11 and 13 are exclusively expressed in plants. Myosin 11 which may be the fastest myosin of all is implicated in cytoplasmic streaming.
What do all myosins consist of?
One or two heavy chains and several light chains. Head, neck, and tail domain organisation.
Myosin head and tails
Myosin head domains have ATPase activity. ATP hydrolysis allows myosin movement along an actin filament. Myosins role is related to its tail domain. Tail domains 1, 5, 6, 11 bind the plasma membrane or the membranes of intracellular organelles. Myosin 2 diners associate to form bipolar thick filaments, close packing of myosin 2 molecules into thick filaments allows many head domains to interact simultaneously with actin filaments. In non-muscle cells the myosin 2 actin bundle are referred to as stress fibres and are responsible for cell and tissue migration and adhesion.
Myosin 2 in muscle contraction
Skeletal muscle consists of bundles of myofibers. Each myofiber contains millions of myofibrils that are comprised of longitudinally aligned sarcomeres. The sarcomere is the minimum contractile unit. Myosin heads walk along actin filaments in discrete steps. Generally, longer neck domain of a myosin, the greater its step.
ATP hydrolysis allows myosin movement along an actin filament
ATP is hydrolysed into ADP + Pi resulting in myosin attaching to the actin subunit. The phosphate is released causing the head to change position pulling the myofilament. The ADP is released and ATP attaches resulting in detachment of the myosin from the actin. Cycle is repeated.
What is cell migration initiated by?
Cell migration is initiated by lamellipodium, in vertebrate cells, which are large, broad membrane protrusions at leading edge of a cell.
How does microfilament based cell movement occur?
When acting filaments at the leading edge are rapidly cross-linked into bundles and networks. Lamellipodium adheres to the substratum, moves forward and then the trailing edge de-adheres.
What are Microtubules?
Hollow tubes formed from a wall of usually 13 tubulin molecule columns.
What is tubulin made of?
Tubulin is a diner of the monomers alpha and beta tubulin
What are the functions of microtubules?
Maintenance of cell shape, cell motility, chromosome movement, organelle movement
Microtubules structure
Alpha tubulin monomer is bound to GTP which is noexchangable. Beta tubulin monomer is bound to GDP which is exchangeable with GTP. GTP or GDP bound to beta tubulin modulates the addition of tubulin subunits
Microtubules assembly contains three steps, what are they?
Protofilaments assemble from alphabeta tubulin subunits. Protofilaments associate to form the wall or the microtubule. The addition of more subunits to the ends of the Protofilaments elongates the microtubule
Protofilaments
In Protofilaments, one end will have the alpha subunits (-) while the other will have beta subunits (+). Microtubule elongation is faster at the + end
Microtubule-organising centre (MTOC)
Microtubules form at a central site, the MTOC, which contains a major site of microtubule nucleation. Animal cells - MTOC is usually a centrosome, sometimes but not always contains a pair of centrioles. Fungi - MTOC is called the spindle pole. Plants - nuclear envelope appears to act as a MTOC.
Kinesin and dynein powered cell movements
Microtubules function as tracks in the intracellular transport of various types of cargo. Two families of motor proteins, kinesins and dyneins, mediate transport.
Kinesins
Composed of heavy chains and two light chains, most kinesins are + end-directed motors. Divided into cytosolic and mitiotic kinesins based on the cargo they transport. Cytosolic kinesins - transport of organelles and vesicles. Mitotic kinesins - participate in spindle assembly and chromosome segregation in cell division.
Dyneins
Composed of two or three heavy chains complexes and several intermediate and light chains. Cytosolic dynein, role in the movement of vesicles and chromosomes. Axonemal dynein - responsible for the beating of cilia and flagella. Dynein cannot mediate cargo transport alone requires dynactin, that links vesicles and chromosomes to the dynein light chains. Cytosolic dyneins are - end-directed motor proteins that bind cargo through dynactin.
What are intermediate filaments
Found in all animals but not plants and fungi, fibrous proteins supercooled into thicker cables. Protein subunits depend on cell type. Associated with the nuclear and plasma membrane principal function is structural. Formation of nuclear lamina, not involved in cell movement.
What are ribosomes?
Site of protein synthesis in eukaryotes and prokaryotes. Thousands to millions per cell depending on activity. Found in cytosol, rough ER, mitochondria, chloroplasts. 2 subunits made up of RNA molecules and proteins.
Ribosomes in prokaryotes
Smaller 25nm, large subunits 50S, small subunits 30S.
Ribosomes in eukaryotes
Larger 30nm, large subunits 60S, small subunit 40S.
What is the cell wall?
Structural layer that can be tough, flexible, and sometimes rigid. Provides structural support and protection, and acts as a filtering. Absent in animals, present in most other eukaryotes. Major function prevent over-expansion of the cell when water enters. Composition depends on cell type
What does the cell wall consist of?
Middle lamella lies between cells : calcium and magnesium pectates. Primary cell wall consists of : hemicellulose, pectin, cellulose microfibrils add strength.
