Cytoskeleton

Question 1

Wht type of protein strucutes are the cytoskeleton made up of?

Answer 1

• Microfilaments(actin)
• Intermediate filaments
• Microtubules

Question 2

Actin Microfilaments

Answer 2

• Highly conserved among eukaryotes
• are 7 nm thick
• filaments may be up to 7um in length
• filaments are organized into bundles and 3-D networks
• Bind to specific transmembrane proteins either directly or indirectly
• Exist as monomers(G-actin) and long chains(F-actin)
• constitute three major varieties
  
  • a-actin(found in muscle tissue)
  • B-actin(non-muscle actin)
  • y-actin(non-muscle actin)

Question 3

How are microfilaments assembled?

Answer 3

• First step in actin polymerization is nucleation
  
  • A trimer is formed
  • Additional actin monomers can then be added to either end
• Actin polymerization is reversible
• ATP-actin associates with the growing(plus or barbed)ends ,and the ATP is hydrolyzed to ADP following polymerization
• Rate at whic hmonomers are added to the growing filament is proportional to the cytosolic concentration of actin monomers
• ADP-actin dissociates more readily from filaments than ATP-actin
• Rate of addition of new G-actin filaments occurs more rapidly and at a lower concentration at the plus(barbed or polymerization) end
• Minus end(pointed depolymerization end) is the slower growing end

Question 4

Microfilament assembly based on concentrations

Answer 4

• Very low concentrations of G-actin favor the disassembly of actin filaments
• intermediate concentrations favor a dynamic equilibrium between he minus end and hte plus end=treadmilling
• This results in equilbirium and zero net growt
• Higher concentrations of G-actin favor net addition at both ends and therefore growth of the actin filament
• Actin microfilaments consists of a double helical chain of G-actin subunits

Question 5

General charecteristics of microfilament

Answer 5

• Each actin monomer(G actin) has a binding stie for ATP , which binds tightly to G-actin
• Each actin monomer can bind tightly with two other actin monomers to form filamentous actin( F actin)
• Because all actin monomers are oriented in the same direction, actin filaments display polarity

Question 6

Drugs that affect actin polyermzation

Answer 6

• Cytochalasins
  
  • Bind to barbed ends
  • block elongation
  • can inhibit movements
  • block growth or elongation
  • can go through all steps except telophase
• Phalloidin
  
  • Binds to actin filametns and prevents dissociation
  • can be labeled with fluorescent dyes to allow visualization of actin filaments

Question 7

Actin binding proteins

Answer 7

• Actin is a very common and very ubiquitous protein found in all cells, but in spite of the fact that it has pretty much the same structure in all cells , it is involved in a large number of kinds of functions
• Diversity of function of actin filaments in different regions of a cell is determined by actin-binding proteins and not by actin itself

Question 8

Types of actin binding proteins?

Answer 8

• Spectrin
  
  • Found in RBCs
  • binds cortical cytoskeleton to the plasma membrane
  • Important in deforming and reforming the Red blood cell and binds to the cell membrane itself, which helps make the red blood cells deformable
• Dystrophin
  
  • Binds cortical cytoskeleton to the plasma membrane
• Villin and Fimbrin
  
  • Cross-links in microvilli
• Calmodulin and Myosin I
  
  • Cross-links actin to plasma membrane in microvilli
• a-Actinin
  
  • Cross-links stress fibers and connects actin to protein-plasma membane complex complexes
• Filamin
  
  • Cross links actin at wide angles to form screen like gels

Question 9

Thymosin

Answer 9

• Thymosin
  
  • Captures actin monomers and prevents actin monomers from being polymerized
• Actin biding molecules that control treadmilling

Question 10

Profilin

Answer 10

• Binds to actin monomers and prevents monomers from being polymerized
• Facilitates exchange of bound ADP for ATP-which favors polymerization
• Note that only ATP-actin monomers can be assembled into F-actin
• Actin binding molecules that control treadmilling
• regulates filament assembly by catalyzing the exchange of G-actin bound ADP for ATP and promoes the transfer of actin monomers from thymosin to the barbed end of the actin filament

Question 11

Gelsolin

Answer 11

• Destabilizes F-actin and caps actin filaments,preventing loss and addition of G-actin
• In presence of calcium ion,fragments actin filaments and remaisn bound to plus ends
• Actin binding molecuels that control treadmilling
• severs actin filaments and bidns to the newly formed plus end, blocking further polymerization

Question 12

Cofilin and Arp2/3

Answer 12

• Cofilin
  
  • Triggers depolymerization of ADP-bound actin at the minus end
• Arp2/3
  
  • initiates growth of F actin from sides of existing filament-causes branching
  • complex of seven proteins– initiates the growth of F-actin from the side of a preexisiting filament
• Actin bindign molecuels that control treadmilling

Question 13

Phalloidin and Latrunculins

Answer 13

• Phalloidin
  
  • Prevents depolymerization by binding to actin filaments
  • fluorescent labeled phalloidin is used to stain actin filaments in cells.
• Latrunculins
  
  • Binds to G-actin and induces F-actin deoplymerization
  • disrupt actin filaments by binding to G actin and inducing directly F actin depolymerization

Question 15

Cytochalasins

Answer 15

• bind to the fast growing end(plus end), preventing further addition of G-actin
• a cytochalasin cap is formed

Question 16

Intermediate filaments charecteristics

Answer 16

• 8-10nm thick
• abundant in cells subject to mechanical stress
• provide tensile strength in cells such as neurons and muscle
• strengthen epithelial cells as dsmosomes and hemidesmosomes
• All have a common monomer consisting of a central a-helical rod flanked by head and tail domains
• head and tail domains determine specific functions

Question 17

Intermediate filament assembly

Answer 17

• Central rod of two polypeptides form a coiled dimer
  
  • Rods are aligned tail-to-tail and head-to-head
• dimers associate in a staggered antiparallel fashion to form tetramers
  
  • becasue of hte antiparallel association of the dimers, polymerized filaments do not have distinct ends
  • therefore , they are more stable than actin and do not demonstrate dynamic behaviors such as treadmilling
• Tetramers assemble end to end to form protofilaments
• Eight protofilaments are wound together to form filaments

Question 18

Intermediate filaments functions

Answer 18

• Form a cytoplasmic network in most cells
• associate with other cytoskeletal elements to form a scaffolding that organizes the internal structure of the cell

Question 19

Intermediate filament types

Type I

Type II

Answer 19

Type I: Acidic keratins

Type II: Neutral to basic keratins

Question 20

Intermediate filament types

Type III

Answer 20

• Vimentin
• desmin
• Glial fibrillary acidic protein
  
  • Peripherin

Question 21

Intermediate filaments

Type IV ,V,VI

Answer 21

• Type IV:Neurofilaments
• Type V:Nuclear lamins
• Type VI: Nestin

Question 22

Charecteristics of MIcrotubules

Answer 22

• 25um in diamter
• composed of tubulin dimers
  
  • alpha +beta unit
• protofilametns are longitudinal rows of tubulin dimers
• microtubules consist of 13 protofilaments arranged parallel to form a cylinder with hollow core
• Protofilaments have a fast growing plus end and a slow-growing minus end

Question 23

Treadmilling and dynamic instability as it relates to microtubules

Answer 23

• Tubulin dimers with GTP bound to the B-tubulin associate with the growing end
• Plus end:
  
  • grows more rapidly than minus end in presence of low calcium ion concentration
• After polymerization , GTP is hydrolyzed to GDP and the tubulin is less stable
• Dimers at the minus end dissociate
• GTP cap enables further addtion of tubulin dimers
• dyanimc instability - refers to the alternate phases of slow growth and rapid depolymerization
  
  • results from the hydroylsis of GTP-tubulin dimers, release of hydrolyzed phosphate, and subsequent relase of GDP-tubulin subunits

Question 24

Treadmilling and dynamic instability as it relates to microtubules

more specifically with concentration of tubulin GTP

Answer 24

• At high concentrations of tubulin-GTP , the dimers are added more rapidly than GTP is hydrolyzed , and the microtubule grows
• If concentration of tubulin-GTP drops , GTP at the plus end is hydrolyzed and dimers are lost

Question 25

Factors that inhibit microtubuels polymerization

Answer 25

• colchicines
  
  • binding to tubulin dimers prevents their assembly into microtubules
• colcemid
• vincristine-anti cancer drug
• vinblastin=used to treat hotchikins lymphoma
  
  • used in anti-tumor therapy inhibit tubulin polymerization

Question 26

Factors that stabilize microtubules

Answer 26

taxol -given for breast cancer

• binds to microtubules prevent their depolymeriation
• disrupts mitosis by affecting the dynamic assembly and disassembly of the mitotic spindle required for separation of chromsomes into daughter cells

Question 27

cytoskeleton functions

Answer 27

• cell movement
• support and strength for the cell
• phagocytosis
• mitotic spindle formation
• cytokinesis
• cell-to-cell and cell-to extracellular matrix adherence
• changes in cell shape

Question 28

mitotic center related to the mitotic appartus

Answer 28

• three components:
  
  • microtubule organizing center surrounding a pair of
  • centrioles and
  • radiaitng microtubules(astral microtubuels)
• These three components anchor the mitotic center to the plasma membrane

Question 29

mitotic spindle related to the mitotic appartus

Answer 29

• two major classes of microtubules originating in the mitotic center
  
  • Kinetochore microtubules-anchored to the centromeres of hte metaphase chromosomes
  • polar microtubules-which overlap with each other in the center of the cell and are not attached to chromosomes

Question 30

Kinetochores

Answer 30

• formed by several proteins assembled on centromeric DNA during mitosis and meiosis

Question 31

centromere

Answer 31

• The chromosomal site where the kinetochore assembles

Question 32

intracillary transport

Answer 32

• 1.anterograde transport of cargos along a microtubule is mediated by kinesin
  
  • need to add tubulins to the plus side
  • kinesin runs twoard the plus end and cytoplasmic dynein runs toward the negative end
    
    • they will carry other molecules to the plus end and come back
• 2. retrograde transport of cargos along a microtubule is mediated by cytoplasmic dynein
     * raft protein complex may provide a mechanism for the transport of multiple cargos
• 3. dissassembly of the raft protein complex-cargo molecular motor machinery

Question 33

Axonal transport

Answer 33

• Anterograde transport of a vesicle along a microtuble is mediated by kinesin
  
  • tail of kinesin binds to the vesicles and deposit the vesicle at the terminal end of the axon
  • when they do this the vesicle membrane becomes part of the terminal membrane
  • do an encodcytossi to remove excess membrane via cytoplasmic dynein and goes back to cell
• Retrograde transport of a vesicle along a microtubule is mediated by cytoplasmic dynein

Question 34