Cytoskeleton Flashcards
List at least 5 functions of the cytoskeleton
- organize cells and their environment
- cell shape
- Mechanical strength
- Structures of locomotion
- Support of plasma membrane
- Scaffold for spatial organization of organelles
- Means of intracellular transport of organelles and other cargo
What are the physical properties of microtubules?
tubular (hollow-fiber)
up to many micro meters long
outer diameter of 25nm
flexible (but not resistant to stretching)
What are the functions of microtubules?
- Scaffolds for spatial organization of organelles in cell & cell movment
- Organelle movement
- Movement of cilia and flagella
(usually attached at one end to centrosome or Perinuclear Microtubule Organizing Center (MTOC))
What proteins form microtubules?
Alpha and beta tubulin
Can the GTP bound to the alpha tubulin hydrolyze?
No
Can the GTP bound to the beta tubulin hydrolyze?
Yes
Which end of the alpha-beta tubulin dimer is considered the positive end?
beta
- due to its capability to hydrolyze the GTP
What happens at the + end compared to the - end?
+ end tends to grow
- end tends to shrink
What is the tread-milling phenomenon?
Microtubule (MT) growth at the + end at the same rate as MT disassembly at the - end
What is dynamic instability?
Rapid shrinkage of MT from plus end until GTP containing dimers are added back (the GTP-rich cap)
How do MT capping proteins affect MT stability?
Increase it by protecting or adding GTP cap
Ho dow MT severing proteins affect MT stability?
Decrease it by exposing GDP rick parts
ex. spastin, katanin
What are centrioles?
- short cylinders at right angles to eachother
- divide and form mitotic spindle between them during mitosis
- bodies of cilia and flagella
Name 2 drugs that are known to modify MT polymerization dynamics
- Colchicine (from crocus like flower) - inhibit MT polymerization
- Vinblastine & Vincristine (madagascar periwinkle) - MT polymerization blockers
- Paclitaxel (taxol from pacific yew tree) - binds and stabilizes MT’s - tubule and tubulin aggregates
interest in cancer treatment
Explain the concept of molecular motors
the process whereby MT’s used energy from ATP hydrolysis to move cargo along MT in a “walking motion” toward the + or - end (using a mechanochemical cycle)
Which direction does Kinesin take cargo?
Toward the plus end
What motor moves cargo towards the - end?
Dyneins
What are the 3 types of mitotic spindle MT’s, and what do they do?
Astral MT:
radiate out from centrosomes
Kinetochore MT:
Attache to kinetochore from centromere of each duplicated chromosome
Overlap MT: interdigitate at the equator of the spindle
Which direction do the MT + ends face?
away from the centrosome
What are flagella?
Long MT’s
Propel sperm by undulating motion
What are cilia?
shorter
occur in large numbers
on apical surface of various epithelial cells
Beat in whip like motion in wave like pattern=> move fluids over surface of cells (ex. Respiratory tract)
What are the functions of intermediate filaments
Provide intracellular mechanical support
What are the 2 categories of of IF’s?
Cytoplasmic IF (including keratins, vimentins and neurofilament proteins) Nuclear lamins
More heterogeneous than MTs
How do nuclear lamins provide support?
form a meshwork that stabilizes inner membrane of nuclear envelop and anchors chromosomes and nuclear pores
What is the structure of IFs?
elongated molecules
extended central alpha-helical domain
parallel coiled-coil of alpha helixes
Paired diamers associate antiparallel and form staggered tetramers
Tetramers participate in IF polymerization
8 tetramers assemble into protofilaments and pack together laterally to form 1 IF
=> at all levels: IF cross section contains 32individual alpha-helix coils
What is a unique property of IFs?
32 individual alpha-helix coils at each cross sectional area
=> high tensile strength
Where are IFs often located?
intercellular junctions
Name 2 types of IF proteins and where they are located
Vimentins - Majority of cell types
Keratins - dominant components of epidermis and its appendages
Neurofilament proteins - vertebrate axons (controls axonal diameter)
Gilial fibrillary acidic protein (GFAP) - astrocytes in CNS (important in inflamatory/ degenerative processes of brain)
What pathology can result from keratin mutations?
epidermolysis bullosa simplex
- epidermis highly sensitive to mechanical stress (blisters)
- related diseases affecting IFs that anchor filaments in desmosomes have similar symptoms
Name a pahtology related to neurofilaments
Charcot-marie-tooth syndrome
- change in axonal transport of neurofilament subunits
- causes peripheral neuropahty
ALS
-abnormal assembly
Progeria syndromes (various) - mutation in lamin -> nucelar instability
What is a potential side effect of MT toxins used for chemotherapy?
peripheral neuropathy
- MT toxins block mytosis
- BUT also affect other MT functions like axoplasmic transport
What is a pathology associated with microtubule severing proteins?
Hereditary spastic paraplesia
What is G-actin?
Microfilament
- filamentous polymer of actin monomer
- ~7nm in diaeter
What induces G-actin to form F-actin?
presence of divalent cations and ATP
What are 4 important roles of actin?
muscle contraction
cell mobility
epithelial polarity
cell division
What are some of the actions of the accessory proteins that regulate actin?
60+ accessory proteins
- G- actin concentration (profilin)
- ADP to ATP exchange (profilin)
- Polymerization
- disassembly
- stabilize
- Crosslink
- sever (ADF/ Cofolin)
- cap (gelsolin)
What protein nucleates actin in a linear pattern?
Formin
What protein nucleates actin and causes it to branch?
Arp2/3
What is Rho
monomeric GTP activated by extracellular signal which induced actin polymerization
What is the role of actin at tight junctions?
It acts as an intracellular anchor to the tight strands forming the impenetrable cell junction
- connects cytoplasm of adjacent cells
What are adherens junctions?
protein complexes that occur at cell cell junctions between epithelial and endothelial cells
- more basal than tight junctions
What is actin’s role in adherens junctions?
actin links cytoplasmic face of the junctions
What are microvilli
microscopic protrusions in cell membranes that increase surface area in order to maximize functions such as absorption, secretion, adhesion and mechanical transduction
How is actin related to microvilli?
A tight MF is the core of microvilli
- Actin bundles are held together by villin and fimbrin (cross linking proteins)
- actin bundles are linked laterally to plasma membrane by myosin-I
What is microvilli inclusion disease?
loss of microvilli
Describe myosin II (involved in muscle contraction)
Head region with ATPase activity and actin binding sites & tail region
- ATPase at + end
- tail binds to other molecules
- forms hetero-oligomers ( 2 heavy chains, 2 copies each of 2 light chains)
- combine to form thick filaments in muscle
Explain the power stroke between actin and myosin during muscle contraction
ATP binds myosin head where it is hydrolyzed by ATPase -> myosin is activated and goes into extended position (high energy)
Myosin head binds to exposed actin filament
Myosin releases ADP and phosphate -> returns to low E position (bringing actin with it)
ATP binds myosin head-> actin released -> cycle restarts
Contraction = hundreds of myosin heads in same direction at the same time
What is the function of myosin V?
cargo carrier to transport vessicles
What complex regulates lamelipodia formation?
Arp2/3
What complex regulates filopodia formation?
formins
How does actin induce cell movement using the lamellipodium?
polymerizes at + end to protrude lamellipodium -> unpolymerized actin moves toward plus end -> Myosin II causes contraction of - end
focal contacts contain integrins
What family of extra cellular molecules act to control actin organization and cell shape? What super family are they a part of?
Rho family
superfamily of Fas GTPases
When are Ras GTPases active v. inactive?
Active in GTP bound state
Inactive in GDP bound state
What is the role of Rho in relation to actin
causes formation of stress fibers & focal adhesions
What is the role of Rac activation?
formation of veils
What is the role of Cdc42?
protrusion of fillopodia
What is the role of actin in cell division?
actomyosin ring forms -> constricts -> forms clevage furrow -> separates the 2 daughter cells
highly regulated -> determine symmetry of daughter cells
What cells undergo asymmetric cell division?
erythroblast -> reticulcyte + macrophage
2n meakaryocyte -> 4n (abortive cytokenisis)-> up to 128n polyipolod megakaryocytic (abortive cytokenisis) -> platelets
spermatagonia -> sermatatid -> sperm + residual body
Give an example of cell motility in the context of development
migration of neural crest cells from extoderm adjacent to neural tube -> cover whole body as pigment cells
nerve fibers grow over long distances -> tipped by nerve growth cone (has ameboid movement)
Give an example of cell motility in the context of infection
polymorphonuclear leukocytes must exit blood vessels and migrate into tissues to reach infection site
As cancer cells become malignant they migrate and invade healthy tissues (metastasis) -> eventually establishing tumors in locations distant from origin
What is wiskott-aldrich syndrome? (WAS)
rare x linked immunodeficiency
- result of WASp mutations
SX: throbyocytopenia (low platelet # & size)
- recurrent infections
defective lamellipodia/ platelet formation
macrophages, neutrophil leukocytes are migration & chmotaxix deficient
What is lissencephaly?
Defect in brain development:
- no gyri -> smooth surface
- improper neuronal migration
- loss of function of n-cofilin
- severe mental retardation
