Actin Microfilaments Flashcards
Actin Based Strucutres
Networks and Bundles
- Epithelial Cells (microvilli, cell cortex, adherens belt)
- Migrating Cells (filopodia, lamellipodia, stress fibers)
- Muscle Functions (phagocytosis, contractile ring)
Actin Structure
- Vertebrate isoforms
- G-actin polymerizes into F-actin
F-actin Structure
- turn at 36nm
- non-cleft end = (+) end
- cleft end = (-) end — polymerize slower
Actin Polarity
- myosin S1 – motor protein for actin
- “arrow heads”
- points to (-) end
Actin Polymerization
- myosin S1 stabilizes actin
- add at (+) end
- need to be bound to ATP and above critical concentration
Actin Assembly
actin with Cc > 0.60μm —> both ends polymerize
actin with Cc < 0.12μm —> both ends depolymerize
actin with Cc btwn 0.12μm-0.60μm —> treadmilling (polymerize + depolymerize)
Regulate Actin Polymerization
Thymosin — reservoir for actin (promote polymerization)
Profilin — charge G-ADP to G-ATP (promote polymerization)
Cofilin — promote depolymerization
Actin Capping Proteins
Block assembly + disassembly (regulate growth)
- CapZ = bind to (+) end
- Tropomodulin = bind to (-) end
Actin-Disrupting Drugs
Cytochalasin – depolymerizes actin filaments
Phalloidin – stabilizes actin filaments
Formins
- regulate assembly of UNBRANCHED filaments
- found at (+) end
- regulated by RhoGTP
Arp2/3
- mediates filament BRANCHING
- Cdc42 activates WASp to activate Arp2/3 cause branching
- Rac activates WAVE to activate Arp2/3 cause branching
Uses of Arp2/3
Listeria = activated Arp2/3 --> rapid branching Endocytosis = Arp2/3 pulls membrane inwards Phagocytosis = Arp2/3 push membrane outwards
Actin-Binding Proteins and Cellular Structures
Fimbrin = form and hold bundles of actin (microvilli) α-actinin = form and hold bundles of actin (stress fibers) Spectrin = form networks of actin (cell cortex) Filamin = form networks of actin (leading edge)
Actin and Red Blood Cells
Ankyrin
- anchors actin and spectrin to plasma membrane
Actin and Microvilli
Ezrin
- anchors actin to plasma membrane
Actin and Muscles
Dystrophin
- anchors actin network to plasma membrane
Myosin II
- heavy and light chains
- head = ATPase activity
- neck binds light chain
- tail binds cargo
Myosin Classes
I – no heavy chain (bind to membranes)
II – myosin btwn parallel actin structures (contraction)
V – tail binds to vesicle + ATP moves to (+) end (organelle transport)
Sliding Filament Assay
- myosin head fragments bound to glass slide
- BSA washes unbound myosin
- add labelled actin filaments — bind to myosin
- ATP = myosin walks toward (+) end of actin
- longer myosin neck = faster they move
Myosin Movement (5)
1) rigor state — myosin bound to actin w/o ATP
2) myosin bind ATP –> actin released
3) hydrolysis of ATP –> myosin head rotates + bind new actin
4) release phosphate = power stroke (actin to (-) end b/c sliding)
5) ADP released
Skeletal Muscle Sarcomere Contraction
- addition of ATP and Ca2+ causes contraction
- A band (myosin II filaments) = no size change
- Z disks come closer
- I bands (btwn neighbouring myosin) = decrease in size
Skeletal Muscle Scaffolding Proteins
Titin = centers myosin in sarcomere CapZ = stabilizes actin at (+) end Tropomodulin = stabilizes actin at (-) end Nebulin = stabilize actin
Sarcoplasmic Reticulum
- modified ER in muscle cells
- stores + regulates Ca2+
Transverse Tubules
- projections from PM into cell
- contact with sarcoplasmic reticulum
