Week 3 Flashcards
Cytoskeleton
- Dynamic intracellular structure establishes order in the cell, and organizes cells in their environment
- Formed by 3 families of proteins: microfilaments, microtubules, intermediate filaments
Microtubule Structure
- alpha/beta tubulin heterodimer
- forms long protofilaments that mind side by side and assemble into microtubules
- Protofilaments not stable → get broken down if they aren’t assembled into microtubules
B end =
A end =
B end = + end, grows and shrinks from this end, has GTP exposed here
A end = - end, anchor end, GTP not exposed and thus not hydrolyzed to GTP
Stabilizing end-binding proteins bind microtubules and…
keep protofilaments together even without GTP cap
Microtubule Function (4)
- Cellular cytoskeleton
- Intracellular transport - scaffold for cell organization and movement of organelles (including chromosomes)
- Cell division
- Cilia – movement of flagella and cilia
Microtubule motor proteins
Dynein
Kinesin
Microfilament structure
helical filament
composed of actin
Grows from + end, and shrinks from - end
- end has ATP binding pocket
- requires stepwise nucleation
Microfilament function
cell movement and contraction
Microfilament motor proteins
myosins
ATP favors _________, and GTP favors ___________
ATP –> Microfilament
GTP –> Microtubule
Intermediate Filament Structure
Complex rope
- a-helix monomers bound together in rope-like structure
- can be made from vimentin, keratin, neurofilament (NO mixing)
Function of intermediate filaments
mechanical stability
Microtubules polymerize/depolymerize from the _______ and where _______ is bound
+ end, where GTP is bound
GTP cap acts to…
stabilize microtubules, removal of cap or GTP hydrolysis –> depolymerization
DYNAMIC INSTABILITY
Microtubule severing enzymes structure
GTPases
6 subunits with active hole in the middle
Microtubule severing enzymes act to…
-Cause depolymerization by cutting Tubulin C-terminus tail
Hereditary Spastic Paraplesia
neuro-degenerative disease caused by mutation in severing enzyme (messes up neat organization of axonal microtubules)
Paclitaxel
- binds and stabilizes microtubules causing Tubulin aggregates → block mitosis
- Cancer treatment
Microtubule Molecular Motor
Organelles transported long distances within cells, using MTs as “tracks” and motor proteins
Kinesin transports cargo toward ______
Dynein transports cargo toward ________
Kinesin –> + end
Dynein –> - end
Kinesin uses _____ to walk along microtubules.
Kinesin domains (2)
ATP
Domains:
-Cargo-binding domain (uses adaptor molecules to carry lots of different cargos
-Head domain (motor domain) - hydrolyzes ATP and reversibly binds MTs with conformational change (power stroke)
Three types of MTs in mitosis
1) Astral MTs
2) Kinetochore MTs
3) Overlap MTs
Astral MTs
radiate out from centromeres
Kinetochore MTs
attached to kinetochore, formed at centromere of each duplicated chromosome
Overlap MTs
- interdigitate at equator of spindle
- Contain double headed kinesins that walk toward opposite + ends, pulling chromosomes apart
Microtubules in Mitosis act to…
Segregate replicated chromosomes during mitosis
Actin Filament Nucleation is the formation of _______ but can also use _____ and _____ to form pseudo nucleation centers
G-actin homotrimers (VERY slow)
Arp2/3 and FH2 (Foramin)
Arp2/3
mimics two actin monomers – allows polymerization to begin with only 1 actin binding to Arp2/3
-Generates branched actin filaments
FH2 (Foramin)
binds two actin monomers to form nucleation center and
-Generates straight bundle actin filaments
Arp2/3 and FH2 are regulated by ________. They are active when bound to _____ and inactive when bound to ______
small monomeric GTPases
GTP (active)
GDP (inactive)
Steps of Actin filament formation (2)
- Nucleation
2. Extension/Retraction (spontaneous)
Actin formation is regulated by (4)
1) G-actin concentration
2) ADP/ATP exchange
3) Capping
4) Depolymerization/Severing
Actin cytoskeleton determines epithelial cell _______.
Does this in two ways:
polarity
Gate function: prevents movement of particles between cells
Sense Function: prevents proteins that need to be on apical side from diffusing across to basolateral side
Actin also plays a critical role in ________ formation
microcilli
extensions of apical surface, actin cytoskeleton runs into microvilli giving them support (Foramin dependent)
Examples of Asymmetric cell division (4)
o Red Blood Cells – no nucleus
o Generation of platelets
o Spermatogenesis
o Epithelial cells
rho
monomeric GTPase, activates formin, that forms long bundles of actin that makes contractile actinomysin ring
-regulated by astral microtubules
Actinomysin ring
o Key during last stages of cell division (Cytokinesis)
o Contraction of ring drives formation of cleavage furrow and separation of daughter cells
o Timing of contraction and ring formation are highly regulated and important for symmetry of cell division
Cell movement
- Lemellipodium (leading edge) senses extracellular signal that binds cell surface → activate monomeric GTPases → activate Arp2/3 → polymerize branched actin → pushes plasma membrane forward
- Extending one end, contracting/depolymerizing other end
Muscle sliding filament theory
- Myosin (thick filament) assembles with actin (thin filament)
- Myosin in middle
- Myosin heads bind to actin and pull forward, causing distance between Z discs to contract and causes muscle to contract
-ATP-driven walk of myosin heads along actin filaments
Myosin
actin-binding motor proteins
-Structurally similar to kinesins
- Neck region - ATPase activity and actin binding site
- Coiled coil domain – where dimerization occurs
Classes of signaling molecules (2)
Lipophilic
Hydrophilic
Lipophilic
- EX) Steroid Hormones
- Can penetrate the membrane → Can have intra-cellular receptors
- No cellular storage because can cross membrane
- Release controlled by synthesis only
- Slow
Hydrophilic
- EX) peptides, protines, amino acids
- Cannot penetrate the membrane → receptor must be on cell surface
- Can be stored intracellularly in vesicles
- Release is controlled via vesicle release
- Fast
Tools of signaling pathways (5)
1) Receptors
2) Second Messengers
3) Protein modification
4) protein-protein binding
5) GTP/GDP exchange
Phosphodiesterases (PDEs)
- Constitutive signal terminator
- Hydrolyze cAMP or cGMP → AMP/GMP
PKG
cGMP-dependent protein kinase, phosphorylates PDES and enhances cGMP binding
Cooperative binding of PDEs
Cooperative binding - catalytic and non-catalytic binding domains (binding at one increases affinity for cGMP at the other)
Mechanisms for signal termination (5)
1) Extracellular signaling molecule taken up, reused, broken down, or diffuse to reach a too low concentration
2) Termination initiated by another signal
3) Enzymes dedicated to turn off signals
4) Signaling proteins with built in terminators
• EX) G-proteins (ras-like proteins) – slow GTPases that reverse their active GTP-bound state
5) Receptor desensitization, receptor internalization
Nodes (EX)
-Points in a network that receive multiple inputs and/or multiple outputs
- Ca2+ : most extensive node in signaling
- Involved in linking “specific” upstream signals with “specific” downstream outputs in many different pathways
Acquired resistance to TKIs occurs by…
- Second site mutations: in EGFR arising or selected in patients who initially benefit from therapy, but then acquire resistance and disease progression
- Up regulation of different RTK pathways: can allow cancer to bypass block by using a different RTK pathway → drug no longer effective
EGFR
Promotes cell growth and proliferation
- Target for cancer therapeutics
- Over expressed in tumors
- Increased EGFR correlates with poorer clinical outcome in breast, lung, head, and neck
Two main mechanisms of RTK-targeted anti-cancer agents
antibodies
TKIs
Antibodies in RTK-targeted anti-cancer drugs
Primary role of antibodies is to block ligand binding to EGFR and prevent receptor dimerization
Tyrosine Kinase Inhibitors (TKIs) in RTK-targeted anti-cancer drugs
(TKIs) inhibit catalytic activity (usually) by binding (and blocking) in ATP binding site