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55 Cards in this Set

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How does globular actin polymerize into F-actin?
By binding ATP
Is F-actin polarized?
YES
What is necessary for globular actin to be competent for assembly?
ATP must be bound and have induced a conformational change.
What confers filament stability?
an ATP cap
Are subunit binding and ATP hydrolysis coupled?
No
From what end does Actin polymerize?
the plus end
What does the fact that the plus end is fast growing and minus end is slow growing allow?
Treadmilling
What does Actin use treadmilling for?
Locomotion of migrating cells
Define Critical concentration of actin:
The free subunit conc at which on and off rates of assembly are equal
At which end is the concentration of free Actin higher?
Minus end - that's where it falls off
What is the critical conc?
.5 uM
What are the concentrations at:
-minus end
-plus end
What state is achieved by this?
Minus = .8 uM
Plus = .1 uM
Steady state where plus end is always making, minus is always disintegrating
What state is favored in cells; polymerization, depolymerization, or equilibrium?
Polymerization - the conc of free globular actin is 1000x higher than the crit conc of 0.1
What disrupts F actin?
Cytochalasin
What stabilizes F actin?
Phalloidin
What is the big difference between Actin and IFs again?
No binding proteins for IFs; there are for Actins.
4 classes of Actin binding proteins:
1. Regulation
2. Severing
3. Cross-linking
4. Motor
What are the regulation ABPs?
-Thymosin B4
-Profilin
-Tropomodulin
-Capping protein
What are the Severing ABPs?
-ADF
-Cofilin
-Gelsolin
What are the crosslinking ABPs?
-Spectrin
-Dystrophin
-Fimbrin
-Filamin
-a-actinin
What is the motor ABP?
Myosin
Which end of f-actin does tropomodulin bind to regulate?
Minus end
Which end of f-actin does capping protein bind to regulate?
Plus end
What does Thymosin B4 bind?
Free g-Actin to prevent polymerization
What does profilin do?
Activates
How do bacteria and viruses take advantage of actin?
By inducing its polymerization and then using it for motility.
Phalloidins
Stabilize Factin by locking subunits together
Cytochalasins
depolymerize F-actin by binding to plus ends
What is the name of capping protein?
capZ
What is ARP2/3 complex?
Actin-related protein - another regulatory protein
How many actin binding sites do Crosslinking ABPs have?
Two - "bivalent"
What inhibits crosslinking with Actin?
Calcium
What determines how closely packed actin filaments will be when they bind an ABP?
The distance of protein between Actin binding sites
What would exhibit tight packing?
What would exhibit loose packing?
Tight = structural, microvilli
Loose = muscle (myosin xlink)
What stabilizes microvilli?
Core actin filaments crosslinked by Fimbrin and Villin, rooted in spectrin, keratin, and more actin.
Which ABP causes orthagonal crosslinking of Actin filaments?
FILAMIN
What is the product of actin orthagonal crosslinking?
A GEL that can dissolve into SOL
Why is Filamin important?
Because it anchors the actin crosslinked network to a glycoprotein that is attached to a platelet clot on the other side.
Does actin only play a role in stabilizing clots to the cytoskeleton?
No; it also links the skeletal muscle membrane to the cortical F-actin cytoskeleton.
What actin filament stabilizes the muscle myocyte's cytoskeleton to the basal lamina?
Dystrophin
What is the order of connecting molecules between Actin/myosin in the sarcomere and the Basal lamina?
Actin-Desmin-Actin cytoskeleton - Dystrophin - integral glycoproteins - laminin
What results when Dystrophin is defective?
The membrane tears during contraction
What is the name of the disease of membrane tearing during contractions?
Duchenne muscular dystrophy
Does actin always just link extracellular stuff to the intra cytoskeleton?
No; also links the cytoskeleton to the extracellular matrix.
What are myosin ATPase motors used for?
Moving actin microfilaments for moving vesicles, or protrusion of the cell surface.
What type of movement can be achieved via Actin microfilaments?
Amoeboid
What is the name of the model of amoeboid movement?
DENDRITIC NUCLEATION MODEL
Which form of myosin binds membranes for vesicle translocation?
Myosin I
Which form of myosin is involved in actin filament sliding?
Myosin II
What cell function is dependent on actin-based lamellipodia formation?
Macrophage phagocytosis
What disease results from defective phagocytosis?
Wiskott-Aldrich
What provides the push for membrane extrusion during phagoyctosis?
Actin polymerization
What activates Actin polymerization for phagocytosis?
Arp2/3 - activated by W (wiskott aldrich syndrome protein); Active ARP nucleates F-actin growth.
What stops the polymerization during phagocytosis?
Capping protein
How do pathogens (bacteria and viruses) harness Actin's movement?
By binding ARP2/3 at their tails