Cytoskeleton Flashcards
Wht type of protein strucutes are the cytoskeleton made up of?
- Microfilaments(actin)
- Intermediate filaments
- Microtubules
Actin Microfilaments
- 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)
How are microfilaments assembled?
- 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

Microfilament assembly based on concentrations
- 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
General charecteristics of microfilament
- 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
Drugs that affect actin polyermzation
- 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
Actin binding proteins
- 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
Types of actin binding proteins?
- 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
Thymosin
- Thymosin
- Captures actin monomers and prevents actin monomers from being polymerized
- Actin biding molecules that control treadmilling
Profilin
- 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
Gelsolin
- 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
Cofilin and Arp2/3
- 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
Phalloidin and Latrunculins
- 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

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

Intermediate filaments charecteristics
- 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
Intermediate filament assembly
- 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

Intermediate filaments functions
- Form a cytoplasmic network in most cells
- associate with other cytoskeletal elements to form a scaffolding that organizes the internal structure of the cell
Intermediate filament types
Type I
Type II
Type I: Acidic keratins
Type II: Neutral to basic keratins
Intermediate filament types
Type III
- Vimentin
- desmin
- Glial fibrillary acidic protein
- Peripherin
Intermediate filaments
Type IV ,V,VI
- Type IV:Neurofilaments
- Type V:Nuclear lamins
- Type VI: Nestin
Charecteristics of MIcrotubules
- 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
Treadmilling and dynamic instability as it relates to microtubules
- 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

Treadmilling and dynamic instability as it relates to microtubules
more specifically with concentration of tubulin GTP
- 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

Factors that inhibit microtubuels polymerization
- 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
Factors that stabilize microtubules
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

cytoskeleton functions
- 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
mitotic center related to the mitotic appartus
- 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
mitotic spindle related to the mitotic appartus
- 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
Kinetochores
- formed by several proteins assembled on centromeric DNA during mitosis and meiosis
centromere
- The chromosomal site where the kinetochore assembles

intracillary transport

- 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
- 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
- retrograde transport of cargos along a microtubule is mediated by cytoplasmic dynein
- dissassembly of the raft protein complex-cargo molecular motor machinery
Axonal transport

- 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

