unit 6 - cytoskeleton Flashcards
cytoskeleton is found in
only eukaryotic cells, almost always animal cells
three components of cytoskeleton
actin filaments (diameter ~6 nm), intermediate filaments (diameter ~12 nm), microtubules (diameter ~24 nm)
major classes of intermediate filaments
- keratin filaments in epithelial cells. most abundant type of intermediate filament
- vimentin and related filaments in connective tissue, muscle cells, and glial cells of nervous system
- neurofilaments in nerve cells
- nuclear lamins under the nuclear envelope of all animal cells, provides struture
functions of intermediate filaments
- abundant in epidermal/skin cells to allow them to stretch without breaking, preventing mechanical damage. cross cells and anchor in the desmosomes
- help stabilize organelles in their positions, support and provide shape to cells
- strengthen the nuclear envelope
intermediate filiments composition
rope-like, made of coiled-coiled tetramers
great tensile strength
Epidermolysis bullosa
genetic disease of intermediate filaments. mutations in keratin genes. layers of epidermal cells not joined together well, skin is extremely prone to mechanical damage. severe blisters as the skin shears off
nuclear lamins
inside inner surface of the nuclear envelope
- disassemble via phosphorylation (weakens the binding) in each cell division, reassemble via dephosphorylation in each daughter cell
- provide mechanical support, helps organize chromatin, anchors nuclear pore complexes, regulates DNA replication and cell division
how do intermediate filament structures form?
- 2 fibrous monomers with long alpha-helical regions coil around each other to form a dimer
- two dimers stagger and form noncovalent associations to form a tetramer
- two tetramers bind noncovalently side to side, more two-tetramer groups bind end to end to form a protofilament
- eight tetramers bind noncovalently, side to side and end to end, and twist together to form an intermediate filament
progeria
- caused by a defect in a certain type of nuclear lamina
- children with progeria age prematurely– wrinkled skin, loss of teeth and hair, die of cardiovascular disease by late teens
- potential cause: nuclear instability leads to impaired cell division, increased cell death, less capacity for tissue repair
structure of microtubules
long, rigid, hollow cylinders made of the protein tubulin (dimer made of alpha-tubulin and beta-tubulin)
- can rapidly assemble and disassemble
- tubulin dimers stack to form ** proto-filaments ** with a plus end (beta subunit) and a minus end (alpha subunit)
- proto-filaments form the cylindrical walls of the microtubule (with plus and minus ends)
where in the cell are microtubules found?
- minus ends are attached to the microtubule-organizing center (centrosome) in the center of the cell, plus ends extend into the cell cytoplasm
- compose cilia and flagella when these structures are present in a cell
function of microtubules
- create a system of tracks for vesicles and organelles to travel along
- anchor organelles to a specific location in the cell
- form the spindle fiber and help separate chromosomes during mitosis
- power cell movement (cilia and flagella)
dynamic instability of microtubules
each microtubule grows (polymerization) and shrinks (depolymerization) independently of its neighbors. continuously changing. allows for rapid ‘remodeling.’
centrosomes are continuously shooting out new microtubules in an exploratory fashion (like roots!)
centrosome structure
pair of centrioles, stacked perpendicularly, surrounded by a protein matrix
-matrix contains gamma-tubulin rings that act as ** nucleation site ** (starting point) for the growth of a microtubule to extend outward from
centriole structure
cylindrical, composed of 9 sets of short microtubule triplets
process of tubulin dimer addition to microtubules
- free tubulin dimers are bound by GTP (both the alpha and beta subunits)
- dimers attach to the growing microtubule, bound tightly to one another because of the GTP (shape?)
- GTP is hydrolyzed to GDP in older beta subunits as the chain continues. bonds weaken.
catastrophe!!!
process of microtubules disassociating
rescue process
keeps microtubules from shrinking when they lose their GTP cap by adding more GTP dimers so that the microtubule starts to grow again
regulated by MAPs or microtubule-associated proteins
kinesin 13
disassociate GTP-bound tubulin from the plus end of the microtubule to induce catastrophe
CLASP proteins
rescue microtubules from catastrophe by clamping down around the microtubule to stop disassembly, restart growth
capping protein
permanently fix microtubules to specific locations in the cell. prevent depolymerizatin by capping the plus end.
how do microtubules grow and shrink?
GROW: if polymerization at the plus end occurs faster than GTP hydolysis of the older subunits, a ** GTP cap ** forms and growth continues
SHRINK: if hydrolysis of GTP occurs faster than polymerization at the plus end, the ‘GTP cap’ is lost and the unstable GDP-bound tubulin rapidly dissassociates
microtubule-associated proteins
regulate dynamic instability of microtubules by binding to the plus end. accelerate growth by increasing incorporation of GTP-bound tubulin
kinesin motor proteins
- move towards plus end of microtubule, away from the centrosome and cell body
