Cytoskeleton Pt 1 Flashcards
Cytoskeleton is a
supportive network of fibers, filaments, and associated proteins.
is dynamic and adaptable
Main functions of the cytoskeleton
Cell Shape: Neurons branch out, amoebas
Change shape when they move towards a food source
Strength: Mechanical strength (supportive), intermediate filaments support nuclear envelope
Cell movement: Organelles move around within cell while attached to cytoskeletal structures. Motor proteins can move vesicles along cytoskeletal filaments
Solid State Biochemistry: Metabolic pathways
anchored to cytoskeletal platform in liquid medium (increase reaction rates of enzymes, ex. glycolysis)
Microtubules Overview
largest (25nm diameter), composed of tubulin protein (allows for follow rigid structure)
Support (allows vesicular transport, “tracks” in cell)
Position organelles
Framework/necessary for cell division/ mitotic spindle
Forms various structures ex. Cilia
Intermediate filaments Overview
mid-sized (10nm diameter). Constructed from a number of different subunit proteins
Line up- creases rope like structure - mechanical strength.
Actin filaments Overview
smallest (6 nm diameter), made up of actin protein.
Muscle contraction
Dynamics/ Rearrangements allow for cell movements - leading edge
Present under plasma membrane, plasma cortex
Accessory proteins Overview
Regulate and link the filaments to other cell components, as well as to each other
ex. mussel contractions
filament polymerizing and depolymerizing
Microtubules are composed of
tubulin subunits (hetero dimer)
alfa and beta subunits
monomers are synthesized and rapidly self associate to form dimers
Have structural polarity - two sides are different - different dynamics on either side
Microtubule Protofilament formation
Heterodimers line up to form it
Beta tubulin - Plus end
Alfa tubulin - Minu end
Have head to tail arrangement - all point in same direction
Alfa Tubulin
GTP bound CANNOT be hydrolyzed or exchanged (integral part of structure)
Betta Tubulin
GTP or GDP bound, exchangeable/can be hydrolyzed (dynamic part of structure)
Formation of microtubule from protofilament
13 protofilaments - to make microtubule
NON-covalent interactions
longitudinal contacts between dimers
also lateral contact - keeps structure together
middle of microtubule is harder to break because it ahs more interactions
Nucleation of microtubule
slower phase - built from ring - grows from base
Makes the platform/template to build from
elongation of microtubule
polymerization of microtubule
Steady state of microtubule
also equilibrium - balance between tubulin addition and disassociation
length of microtubule does not change
There is a Preferential growth of Microtubules at the ___ end
Plus end
Plus end favored for elongation
Minus end biased against elongation
Microtubule dynamics are Influenced by
binding and hydrolysis of GTP (β tubulin allows for)
Tubulin subunits at end of microtubule (T or D form) depends on
Rates of GTP hydrolysis and tubulin addition
In microtubules, when the rate of subunit addition is high
(filament growing rapidly), likely new subunit will be added BEFORE nucleotide in previous subunit is hydrolyzed (tip remains in T form, forms a GTP cap)
In microtubules, when the rate of subunit addition is low
hydrolysis may occur before next subunit is added, tip in D form
Dynamic instability in microtubules is when
one end is growing and/or the other end is shrinking
Microtubule catastrophe
Change from growth to shrinkage. Hydrolysis more rapid then subunit addition.
Microtubule rescue
Change from shrinkage to growth, GTP subunits add to shrinking end, new GTP cap
Microtubule T end promotes
polymerization
Microtubule D end promotes
depolymerization
GTP hydrolysis changes subunit conformation and weakens binding affinity in the polymer
How is a GTP cap formed
If the rate of subunit addition is high—and thus the filament is growing rapidly—then it is likely that a new subunit will be added to the end of the polymer before the GTP in the previously added subunit has been hydrolyzed.
In this case, the tip of the polymer remains in the T form, forming a GTP cap.
Why do we need dynamic instability?
May reduce time required to find a target (formation of mitotic spindle, microtubules search for chromosomes).
Microtubules could explore the interior of the cell.
How is a microtubule polar
The subunits point in the same direction (head to tail, alfa - betta +)
What keeps tubulin together in the microtubule?
Non covalent interactions
longitudinal interactions (inc binding affinity between alfa and betta)
Latter associations (between profilamants, alfa-alfa, betta-betta)
Difference between beta and alpha tubulin within the dimer?
Both bind to GTP
GTP B tubulin can be hydrolyzed
in Alfa tubulin - cannot be hydrolyzed, is part of dimer structure
what does Taxol do
it binds to microtubule and stabilizes the polymerized from - preferentially killing dividing cells by not allowing cell division dynamics
( it is from dried bark of Pacific yew tree)
What must happen at the end of the MT in order for it to stop shrinking and start growing?
(Rescue, change from shrinking to growing)
GTP cap needs to be established
GTP addition exceeds rate of hydrolysis
How would an increase in GTP-tubulin concentration affect this switch from shrinking to growing?
Rate of addition of GTP tub grater at high tub concentrations
frequency of switching from shrinking to growing will increase with increases in tub concentration
What would happen if GDP, but no GTP, is present in solution?
Continue to shrink and eventually disappear
GDP tub - binding affinity much lower
Microtubule nucleation is the ____formation of microtubules
de novo
Microtubules are nucleated from an
intracellular location called ___ which is enriched with ____
(MTOC):
Microtubule-organizing center
(enriched with γ tubulin)
the γ tubulin small complex has
2 accessory proteins bind to two γ tubulin
Formation of ___ , serves as template for microtubule nucleation
of spiral ring of γ tubulin molecules
(creates a microtubule with 13 protofilaments)
spiral ring of γ tubulin associates with additional accessory proteins to form
γ tubulin ring complex: nucleate from minus end
Centrosome is a
well defined MTOC in many animal cells, located near nucleus, from which MT are nucleated at their minus ends
Centrioles are
Embedded within centrosome, cylindrical structure consists of MTs arranged in barrel shape
can nucleate (have gama tubulin)
In Cells: The Minus-End of Microtubules are Linked to
MT-Organizing Centers (MTOC’s)
The plus end of microtubules in interphase vs Mitosis
Interphase - Post to edge
Mitosis - duplicate and Post migrate to edge of cell
Intermediate Filaments are found in ___
Regulated by ___
Provide____
Found in vertebrates, nematodes, mollusks (not in every eucaryotic cell)
Often regulated by phosphorylation/ dephosphorylation events
Prominent in cytoplasm of cells, subjected to mechanical stress: provide mechanical strength for squishy animals
Examples of Intermediate Filaments
Nuclear Lamins - support the nuclear envelope
Keratins - Skin/ Hair
Structure of Intermediate Filament
subunits of elongated proteins - coil together making coiled coil dimer (are amphipathic, hydrophobic interactions)
Coiled-coil dimer - The two ends are the same (do NOT have structural polarity)
Two coiled coil dimers make a staggered tetramer - arranged in opposite orientations
8 tetramers pack laterally to form filament
Large number of polypeptides lined up together, with strong lateral hydrophobic interactions (coiled coils), gives a ropelike character
Easily bent, difficult to break.
Nuclear Lamins make up a
meshwork lining the inner membrane of the nuclear envelope
Progeria results from
Point Mutation in the Lamin A gene (codes for the lamin protein intermediate filament)
Progeria causes
a Fragile nuclear membrane, low tolerance to mechanical stress
Accelerated Aging
Progeria signs include
growth failure,
loss of body fat and hair,
aged-looking skin,
stiffness of joints,
hip dislocation,
generalized atherosclerosis,
cardiovascular (heart) disease
and stroke
is fatal
Keratins provide
Mechanical strength to epithelial tissue by anchoring the intermediate filaments at sites of cell-cell contact (desmosomes) or cell-matrix contact (Basal side, Basal lamina) (hemidesmosomes)
Tough coverings for animals, skin, hair, nails, claws, and scales
Defective Keratins in basal cell layer of epidermis causes ___
Epidermolysis bullosa simplex
Skin blisters in response to very slight mechanical stress, which ruptures basal cells
Epidermolysis bullosa simplex treatment
lost of gauze and wrappings to prevent friction
