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44 Cards in this Set
- Front
- Back
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Cellular Movement |
• Cell motility involves – Movement of a cell or organism through the environment – Movement of the environment past or through the cell – Movement of components in the cell • Contractility, used to describe shortening of muscle cells, is a specialized form of contractility |
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Motile Systems |
• To generate movement, MTs and MFs provide a scaffold for motor proteins or mechanoenzymes that produce motion at the molecular level • This is an active process that requires ATP – ATP may instigate changes in shape of a protein that mediates movemen |
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Two eukaryotic motility systems |
• 1. Interactions between motor proteins and microtubules – E.g., fast axonal transport in neurons, or the sliding of MTs in cilia and flagella • 2. Interactions between actin and members of the myosin motor proteins – E.g., muscle contraction |
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Intracellular Microtubule-Based Movement |
MTs provide a rigid set of tracks for transport of a variety of organelles and vesicles Traffic toward the minus ends of MTs is considered "inbound"; toward the plus end is "outbound" |
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Kinesin and Dynein |
Microtubule-associated motor proteins walk along the MTs and provide the force needed for movement Each motor protein will have a preferred direction of movement |
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Example: Axonal Transport |
Fast axonal transport, involves movement of vesicles and organelles along MTs Organelles can be observed moving along filaments through axoplasm (cytoplasm of axons) at rates of about 2 μ m/sec |
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Two proteins responsible for fast axonal transport |
Kinesin I is involved in ATP- dependent transport toward the plus ends (away from the centrosome), called anterograde axonal transport Cytoplasmic dynein moves particles (cargo) in the opposite direction, called retrograde axonal transport |
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Motor Proteins Move Along Microtubules by Hydrolyzing ATP |
• Kinesins consist of three parts – A globular head region that attaches to MTs – A coiled helical region – A light-chain region involved in attaching the kinesis to other proteins or organelles |
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Kinesin movement along MTs |
• Kinesin movement looks like "walking" with the two globular head domains taking turns as the front foot • Each kinesin molecule exhibits processivity – It can move long distances along a MT before detaching from it by releasing bound ADP and acquiring a new ATP, so that the cycle repeats |
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Dyneins Can Be Grouped into Two Major Classes: |
• Cytoplasmic dynein moves toward the minus ends of MTs – It is associated with a protein complex called dynactin, which helps link it to cargo • Axonemal dyneins include at least four different types |
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Microtubule Motors and the Endomembrane System |
• MT motors are important for dynamically shaping the complicated endomembrane system – E.g., ER membrane extensions can be moved along MTs • The vesicles to and from the Golgi complex are carried by MT motors on microtubule tracks |
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Axoneme: Cilia |
• Cilia: are about 2–10 μm long and have a diameter of 0.25 μm and occur in large numbers on the surface of ciliated cells They occur in both unicellular and multicellular eukaryotes Cilia display an oarlike pattern of beating, generating a force parallel to the cell surface |
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Axoneme: Flagella |
Flagella move cells through a fluid environment Diameter =0.25 μm, length up to 200 μ m They are limited to one or a few per cell Move with a propagated bending motion |
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Structure of Cilia and Flagella |
Ciliaandflagellashareacommon structure, the axoneme Itisconnectedtoabasalbodyand surrounded by an extension of the cell membrane Betweentheaxonemeandbasalbody is a transition zone in which the MTs take on the pattern characteristic of the axoneme |
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The basal body |
Thebasalbodylookslikea centriole, with 9 sets of tubular structures around the circumference Eachsetisatripletwiththree MTs that share common walls |
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Axonemes |
Axonemes have a characteristic 9+2 pattern, with 9 outer doublets and 2 MTs in the center, the central pair |
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Axoneme Accessory proteins |
• the A and B tubules share a wall in which tektin is a major component • Each A tubule has a set of sidearms that project from each of the outer |
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Axonemal dynein |
• The dynein arms occur in pairs, one inner and one outer arm • Less frequently, adjacent doublets are joined by interdoublet links that limit the extent of relative movement of doublets, which are linked to each other by a protein called nexin |
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Radial spokes |
At regular intervals, radial spokes project inward toward the central pair The spokes are thought to be important in translating the sliding of MTs into the bending of the axoneme along with nexin |
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Microtubule Sliding Within the Axoneme Causes Cilia and Flagella to Bend |
• The sliding-microtubule model: sliding of MTs relative to each other is converted into localized bending because the doublets are connected to the central pair and to each other |
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Dynein Arms Are Responsible for Sliding |
The driving force for MT sliding is provided by ATP hydrolysis that is catalyzed by the dynein arms dynein is responsible for the MT sliding; the dynein arm attaches to and detaches from the B tubule cyclically Axonemal dynein has multiple subunits, the largest three having ATPase activity |
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Crosslinks and Spokes Are Responsible for Bending |
To bend the doubles must somehow be restrained so that there is resistance to sliding but there is deformation instead Resistance in bending is provided by: – the radial spokes that connect the doublets to the central pair – nexin crosslinks between doublets |
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Comparing motions of cilia and flagella |
The wave-like motion of the flagellum of a sperm cell from a tunicate. – Note that waves of constant amplitude move continuously from the base to the tip of the flagellum. The beat of a cilium, which resembles the breast stroke in swimming. |
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Intracellular Actin-Based Cell Movement: |
• Two forms of movement occur as a result of actin function: 1. Cell contraction 2. Cell crawling |
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Actin Molecular motors: Myosins |
Myosins are ATP-dependent motors that exert force on actin filaments Currently there are 24 known classes of myosins All have at least one polypeptide chain called the heavy chain, with a globular head group attached to a tail of varying length |
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The myosin light chains |
Myosins typically contain small polypeptides bound to the head group These light chains play a role in regulating the ATPase Some myosins bind actin in the tail region, others bind membranes |
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Myosin functions |
• Myosins function in a wide range of cellular events, including – Muscle contraction – Maintainingauditorystructures in humans |
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Type II myosins |
Type II myosins have two heavy chains (each with a globular head, a hinge region, a rodlike tail) and four light chains They use ATP hydrolysis to cause actin filaments to slide past myosin molecules, resulting in contraction of a cell or group of cells |
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Many Myosins Move Along Actin Filaments in Short Steps |
Myosin II is an efficient motor that walks along actin The two heads walk along a protein filament, with the use ATP hydrolysis to change their shape |
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Structure of skeletal muscle cells |
Each muscle fiber contains numerous myofibrils, each of which is divided along its length into repeating units called sarcomeres Each sarcomere contains bundles of thin filaments (containing actin, troponin and tropomyosin) and thick filaments (containing myosin) |
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Thick Filaments |
Each thick filament consists of hundreds of molecules of myosin, oriented in opposite directions in the two halves of the filament The myosin is arranged in staggered fashion Protruding heads of myosin molecules contact the adjacent thin filaments, forming cross-bridges |
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Thin Filaments |
• Thin filaments contain three proteins: F-actin, intertwined with tropomyosin and troponin |
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The Sliding-Filament Model Explains Muscle Contraction |
• According to the model, muscle contraction is due to thin filaments sliding past thick filaments, with no change in length of either |
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Muscle contraction |
• Muscle contraction is the net result of a set of repeated events involving the myosin head It binds to actin subunits on the thin filament It undergoes an energy-requiring change in shape that pulls the thin filament Then it breaks the association with the actin filament and associates with a site farther along the filament closer to the Z line |
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The contraction cycle |
see picture on pg12 |
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Actin-Based Motility in Non-muscle Cells |
• Cell Migration via Lamellipodia i.e. cell crawling, Involves Cycles of: 1. Protrusion of the cells leading edge 2. Attachment to a substrate |
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Extending Protrusions |
To crawl, cells extend protrusions at their front, or leading edge A thin sheet of cytoplasm is a lamellipodium and a thin-pointed protrusion is a filopodium |
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Retrograde flow |
• Retrograde flow results from actin assembly at the growing tip of the protrusion and rearward translocation of filaments toward the base |
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Cell Attachment |
Attachment, or adhesion, of the cell to the substrate is necessary for cell movement New sites of attachment are formed at the front of the cell, and contacts at the rear must be broken Attachment sites are complex structures involving attachment of transmembrane proteins to proteins inside and outside of the cell |
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Integrins – transmembrane proteins needed for attachment |
Integrins on the outside of cells attach to extracellular matrix proteins Inside the cell integrins are connected to actin filaments via linker proteins The integrin-dependent attachments are called focal contacts |
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Translocation and Detachment |
Contraction at the rear of the cell squeezes the cell body forward and releases the attachments at the rear Contraction, due to actin-myosin interactions, is under control of Rho, which activates non-muscle myosin II at the rear of the cell For movement to occur, new attachments must be balanced by loss of old ones |
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Chemotaxis |
Directional migration occurs through the formation of protrusions predominantly on one side of a cell Diffusible molecules can act as cues for directional migration; when a cell moves in response to a chemical gradient, it is called chemotaxis – Chemoattractants: cells move toward a higher concentration of the diffusible molecules – Chemorepellants: cells move toward a lower concentration of the diffusible molecules |
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Amoeboid Movement |
• Amoebas and white blood cells exhibit a type of crawling called amoeboid movement, which is accompanied by protrusions of pseudopodia • Involves Cycles of Gelation and Solation of the Actin Cytoskeleton – Gelation: as a pseudopodium is extended, more material streams forward and congeals at the tip – Solation: at the rear of the cell, cytosol changes to a more fluid state and streams forward |
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Movement of Components Within the Cytoplasm of Cells |
Cytoplasmic streaming: an actomyosin- dependent movement of cytoplasm in the cell In plants the process is called cyclosis; a dense set of aligned microfilaments is found near sites where cyclosis occurs |
