Exam 4 - Actin slides Flashcards
Master Actin slides for final.
Fibers of actin filament are made of what?
Polymerized protein
What is Phalloldin?
F-Actin stabilizer
What is cytochalasins?
F-Actin destabilizer
MTs had GTP nucleotide activity, F-Actin has what?
ATP activity
Where is F-actin located?
Cytoplasm, Beta-actin in Nucleus
Where are myosin motors located?
Muscle, cytoplasm, cell periphery
What is the directionality of myosin’s?
mostly barbed end (+), except Myosin VI
Barbed end of Microfilament is + or -?
Plus end
Pointed end of F-Actin is + or -?
Minus end
What is NOT a feature shared by both MTs and actin filaments?
Both depolymerize rapidly when their (+) end cap hydrolyzes a nucleotide.
What are 3 features shared by both MTs and actin filaments?
- Polar
- Sensitive to the free subunit concentrations of their (+) ends
- Plus end is the faster growing end.
What is G-actin?
Globular actin
How is F-actin formed?
G-actin polymerizes with nucleus to form F-actin
What does the rate and direction of growth depend on in actin?
Free actin concentration
How would you specifically block F-actin elongation from the pointed end?
Add a binding factor for the barbed end of G-actin
What are the functions of actin?
- Structural support
- Contraction
- Migration
Small G protein activation does what?
Regulates actin organization
Rac interacts with ____.
Lamellipodia
Rho interacts with ____.
Stress fibers
Cdc42 interacts with ____.
Filopodia
What are actin stress fibers?
Contractile bundles of actin filaments and myosin that are anchored to focal adhesions at the plasma membrane and function in cell adhesion.
What does ADF/Cofilin do?
Actin-associated protein: actin filament disassembly (ADF = actin depolymerizing factor)
What does ARP2/3 do?
Actin-associated protein: Actin filament polymerizing/branching
What does Gelsolin/Villin do?
Actin-associated protein: Actin filament severing
What does the spectrin family do?
Actin-associated protein: Actin filament cross-linking
What does the myosin family do?
Actin-associated protein: multiple motor functions in cytoplasm and muscles
What does tropomyosin do?
Actin-associated protein: Regulates myosin-actin interactions in sarcomeres, controlled by troponin.
What does Troponin do?
Actin-associated protein: Regulates myosin-actin interactions in sarcomeres via tropomyosin (Ca2++ dependent)
Describe ADF/Cofilin.
- Three highly conserved and highly (70-82%) identical genes: cofilin 1, cofilin 2, and destrin.
- Ubiquitous actin-binding factor required for the reorganization of actin filaments.
- Recycles “older” ADP-F-actin.
- Helps cells to maintain ATP-G-actin pool for sustained motility.
- Involved in ruffling membranes and at the leading edge of mobile cells.
- Changes the average twist of filaments
F-actin affinity is regulated by what?
pH, phosphorylation, and phosphoinositides.
Where is cofilin 1 found?
Non-muscle (n-cofilin)
Where is cofilin 2 found?
In muscle (m-cofilin)
What is destrin known as?
ADF = actin depolymerizing factor
Describe Profilin.
- Binds to monomeric actin at the barbed end (+ end) (preference for ATP-G-actin) **
- Regulates the direction of filament growth.
- Controls spatial and temporal growth of F-actin.
- Sequesters actin from the pool of polymerizable actin monomers. **
- Involved in the dynamic turnover and restructuring of the actin cytoskeleton (essential for organ development, wound healing, and hunting down of infectious intruders).
- Over 50 binding partners
- Profilin is the major allergen present in birch, grass, and other pollen. **
Describe Gelsolin/Villin.
- Actin can be cross-linked into a gel by actin cross-linking proteins. Gelsolin can turn this gel into a sol (colloidal suspension)
- Key regulator of actin filament assembly and disassembly (produces fresh uncapped barbed ends)
- Highly efficient actin filament severing factor. **
- Can be found in the cytosol and mitochondria as well as in blood plasma. **
- Ca++ regulated, binds to the barbed end. **
- Key player in podosomes. **
- 82 kDa protein complex with six subunits.
What is fimbrin?
Actin bundling protein
Describe ARP2/3 complex.
- Key factor for actin branching, e.g. in lamellipodia. **
- Simultaneously controls nucleation of actin polymerization and branching of filaments.
- Typically found in cellular regions characterized by dynamic actin filament activity e.g. lamellipodia, motile cells **
- ARP2/3 stimulates actin polymerization by creating a new nucleation core (unlike creating a free barbed-end such as gelsolin or cofillin do)
- Activated by WASp (Wiscott Aldrich Syndrome Protein. WAS = X-linked immune dysregulation) **
- Seven-subunit complex: Two of its subunits, the Actin-Related Proteins ARP2 and ARP3 closely resemble the structure of monomeric actin and serve as nucleation sites for new actin filaments. **
Describe the Spectrin Family (e.g. alpha-Actinin).
- Actin filament crosslinkers and linkers to the plasma membrane. **
- Spcetrin cleavage causes the membrane to form blebs and ultimately to be degraded, usually leading to the seat of the cell.
- Extended spectrin cytoskeleton in Erythrocytes.
- alpha-actinin (non-muscle): cross linking and binding actin filaments to the plasma membrane. **
- alpha-actinin (muscle): localized to the Z-disc, involved in anchoring actin filaments to the Z-disc. **
Which of the following statements are wrong?
A. The ARP2/3 complex is involved in actin filament branching.
B. Branched actin filaments grow with the pointed end at their outer tips.
C. The ARP2/3 complex inserts fully into the “mother” actin filament and triggers branching.
D. The F-actin branching by ARP2/3 is triggered by two actin-like domains.
E. B and C are both wrong.
E
