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Cytoskeleton is composed of network of proteins, and tubules in the cell cytoplasm: actin/microfilaments, microtubules and intermediate filaments
- Importance: movement of proteins from ER?Golgi via microtubules
- Intercellular motility
- Important for Cell division
- Role in Protein structure and shape
- 1.Cell and intracell motility
- 2.cell shape
- 3. cell to cell interactions
- 4. cytokinesis
- 5. cell signaling
- 6. adhesion,
- Structure: Smallest cytoskeletal element composed of actin filaments. Actin monomers bind to ATP and form ACTIN TRIMERS. Helical structure, depends on ATP hydrolysis for polymerization
- ASSEMBLY: consist of polar ends, Barbed (+) are fast growing ends with more ATP coming in, and negative pointed (-) end with more ADP binding occurs (some assembly) and higher Dissociation rate.
- Assembly depends on Mg, actin monomer concentration and ATP
- Compotent actin: ATP bound monomer, allowing assembly VS. NON-ATP monomers (non-assembly component)
- Treadmilling: Net gain of Assembly of Actin trimers (ATP bound) at postive barbed end with net loss of Actin (ADP bound) at negative pointed end
- ARP2/3 (actin related proteins) filaments, are involved in nucleating actin filaments to bring together (assembly) and form branching off points for more actin to assemble
- Role: Cell and tissue and nuclear structure and integrity (NO motility)
- Assembly: Disassembly at either end
- IF made of protein monomers, 6 classes: (and assoc. diseases):
- 6 classes
- 1. Keratin (epithelial) (2 classes, acid and basic): blistering diseases
- 2. Desmin (muscle): Myopathies
- 3. Vimentin (mesenchymal): None...mouse still viable
- 4. Neurofilaments (nerves): None...associated w/ALS
- 5. Lamins (nuclear envelope): Muscular dystrophy, Progeria..rapid aging
- Role: Similar to actin, but NOT involved in Adhesion
- Structure: Composed of Alpha and beta tubulin (heterodimer forming)
- Gamma tubulin is a major component of microtubule organizing complex (MTOC)..aka centrosome organizing center
- Only grow and disassemble in a head to tail array at the POSITIVE (+) end (the alpha tubulin
- GTP binds to Alpha and Beta initially. The Alpha tubulin has GTP that is STABLE..so NOT exchangeable whereas Beta tubulin has GTP that can be hydrolyzed or exchangeable.
- Composed of 13 protofilaments: polar, like actin, and + end assembly, - end is nucleations site
- in vivo no assembly at - end
- Tubulin has Similar to structure Actin, but beta-tubulin has multiple isoforms (b1-4, beta 1 is ubiquitous and the other three are specific to various developmental stages)
motor proteins & cell motility: 3 major classes and what type of filaments do each move along:
- 1.Myosin: Transports vesicles along ACTIN filaments, used for muscle contraction
- 2.Kinesin: Walks towards the plus end of MICROTUBULES, anterograde axonal transport
- 3.Dynein: Walks towards the negative end (retrograde) of MICROTUBULES, motor for cilia and flagella retrograde axonal transport
- Do all motor proteins "walk" in the same direction along cytoskeleton filaments? If no, what is the significance of directionality for the cell? Provide some examples of motors that walk in different directions.
- The structure of these proteins dictate if they will be moving in the forwards or reverse direction.
- Myosin: most walk towards the barbed (positive) end of actin filaments (Myo 6 goes towards neg). Most of these are dimers, but some monomers
- Kinesin: most walk towards the Positive end, except for 14, which is C terminal directed, walking towards the negative end. Most dimerize except for Kinesin 3 (monomer)
- Dyneins: mostly all walk towards negative end of microtubules. All are dimers, or trimers
Compare and contrast the movement generation mechanisms of the myosin and kinesins.
- All 3 use similar principles to generate force, utilizing ATP
- Both come from NTPase superfamily
- Myosin and kinesin have similar structures, they evolved from common ancestor, calcium is involved in both of these mechanisms. They both are involved in intracellular movement, both anterograde. Myosin is, however, also involved in sliding the microfilaments in muscle contraction and cytokinesis
- Myosin mechanism involves the the first step of ATP hydrolysis, with Pi still attached. The myosin binds to actin here, and once Pi is released this is the POWER STROKE. The ADP is also released, then the binding of ATP causes the myosin to RELEASE from actin.
- Kinesin is similar to myosin 5, in which 2 motor domains act in sequence to generate a step by step movement along microtubule. In this case however, the BINDING of ATP to the LEADING head causes the POWER STROKE of the lagging head to bind ahead and become the leading head. What occurs is ADP is released from one domain which is attached to MT, and when ATP comes in to replace it, it triggers the other domain to shift and attach ahead of the first. Then that first domain hydrolyzes ATP and releases the phosphate from its group, and as its neck linker is unzippering from the first head, the ADP from the second domain is released and ATP comes in to continue the cycle.
Compare and contrast the movement generation mechanisms of the kinesins and dyneins.
- Dyneins are dimers or trimers, whereas kinesins exist in dimers and can also be monomers
- Kinesins are from NTPase superfamily, and Dyneins are from AAA ATPase superfamily
- They have similar POWER STROKES, both occurring when ATP binds to the domain
- They both are involved in intracellular motility, with dynein retrograde and kinesin anterograde
- Dyneins can cause sliding of microtubules in axonemes (cytoskeletal structure of appendages of cilia and flagella)
- Dynein mechanism involves the binding of ATP, which induces the POWER STROKE, followed by the release of ADP. The ATP is then hydrolyzed and both ADP and Pi are released, followed by ATP binding, which continues the cycle.
- Cilia and flagella are basically the same structure but vary in length and type of beat generated with cilia being shorter and flagella longer. Cells with flagella may have one to several and cells with cilia have several hundred. They are both used by for cellular movement. Internally cilia and flagella have the same structure, the axonemes are templated of basal bodies.
- Axoneme is the cytoskeletal structure of the flagella and cilium
- Difference of motile vs non-motile cilia structurally is the central doublet microtubules in the MOTILE-cilia
- Briefly describe the centriole/centrosome cycle in animal cells. Be sure your answer includes explanation of the distinction between the mother and daughter centrioles.
pair of these is found at the center of the centrosome in animal cells.
composed of both a mother and daughter centriole, with the mother centriole containing distal and sub-distal appendages containing ninein. The daughter centriole lacks the appendages.
space surrounding centrioles
the site of centrioles maturation. Gamma-TuRC is a major component of the PCM, which aids in assembly of MTs by acting as a template for assembly (gamma TuRC includes many proteins as well as ?-tubulin) as cell enters mitosis.
- During S phase: Both the mother and daughter centrioles undergo replication. The mother centriole becomes the old mother and the daughter centriole becomes the new mother. With each mother centriole and new daughter centriole forms. Cdk2-cyclin and tubulins are involved here to aid in this replication.
- During G2 phase: Mother and daughter fully replicated. The centrioles now move to opposite poles of the cell during prophase, and the original daughter of new mother centrosome gains the APPENDAGES.
OLD mother centriole distinguished from the centrosome with the NEW mother centriole:
staining with ODF2/cenexin. The centrosome with the OLD mother centriole will stain BRIGHTER vs. the NEW.
What is asymmetric cell division and where, that is in what cell types, does it most commonly occur? Why is it important?
- Asymmetric cell division is an unequal division of committed stem cells. The asymmetry can be either functional (in that the cells do different things) or structural (size, shape, orientation of cell components). It is seen most often in committed stem cell lines were one cell of the division will remain a stem cell and the other cell will begin to differentiate.
- In neural stem cells: one cell remains a stem cell and the other enters a neuronal developmental pathway. Asymmetric division also is used in the establishment of tissue organization: by allowing for the MAINTENANCE of the stem cell line while also producing DIFFERENTIATING cells that will differentiate into specialized tissue cells.