7: Actin Microfilaments
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actin is usually found – because it has
beneath the PM because it has mainly cortical function for cell shape and movement of organelles beneath PM
actin responds to
extracellular signals
Actin based structures include:
• epithelial cells: microvili, cell cortex, adherens belt • migrating cells: filopodia, lamellipodia, stress fibers • muscle and non-muscle functions: phagocytosis, moving endocytic vesicles, contractile ring
actins can form
networks and bundles
actin structure includes
• monomers ie G-actin/globular actin with 4 domains and ATP binding cleft (polar) • alpha actin in muscle • beta actin in cortex • gamma actin in stress fibers
actin polmer is
• F-actin (filamenous) = microfilaments with ATP binding cleft at minus end • double helix structure 36nm for g-actin to end in same spot • polarity
myosin is
actin motor protein that binds to actin with arrow pattern where pointed end is (-) end and moves towards + end stabilizes actin = can’t depolymerize can use myosin coated actin as nucleus
which in is faster for polymerization of actin
(+) end has faster polymerization because of increased polymerization speed and the critical concentration is 0.12µM (-) end Cc is 0.60µM
for actin polymerization, there are 3 requirements?
• nucleus + above critical G-actin concentration • all actin in ATP form • uncapped
depolymerization occurs when
G-actin below critical conc
if the g-actin critical conc is greater than 0.6
polymerize (+) and (-) end
if g-actin critical conc is less than 0.12µM
depolymerization at both ends
if g-actin critical conc is between 0.12µM and 0.6µM
treadmilling occurs: growth @ plus end and depoly @ minus end
what can treadmilling do?
can push something in front or pull something from behind
minus end of microtubule is different from actin because
MT: (-) end capp = not usually poly/depoly actin: (-) end poly and depoly slower bc slow rate + high Cc
cellular conc of G-actin is — so we need –
400µM so we need regulation of actin poly. so that we don’t always poly
What is thymosin?
sequesters most available actin by binding to it and releases g-actin when needed
What is profilin?
protein that promotes actin polymerization by charging G-ADP into G-ATP actin (no profilin activation = regulate excess g-actin)
What is cofilin?
protein enhancing depolymerization
actin regulating proteins?
• thymosin • profilin • cofilin • capZ • tropomodulin • formins • Arp2/3
What is CapZ?
actin capping protein for (+) end to stop poly and depoly
What is Tropomodulin?
actin capping protein for (-) end to stop poly and depoly
Actin-disrupting drugs?
cytochalasin: depoly actin microfilaments phalloidin: stabilizes actin microfilaments for no depoly
What are formins?
• nucleating proteins that regulate polymerization at + end for unbranched filaments (- end grows away) • activity is regulated by Rho-GTP at the PM • if Rho is in GDP form = no formin activity also still need above Cc, G-actin in ATP form and profilin activation