Microfilaments Flashcards
components of the cytoskeleton
microfilaments, intermediate filaments, microtubules
microfilament
smallest component of cytoskeleton; dynamic structure involved in movement
intermediate filament
midsized component, static structure involved in stabilization
microtubules
largest component, dynamic structure involved in movement
epithelial cell polarity
apical domain w/ microvilli and cell junctions; basolateral domain with cortical band & attached to basal lamina
motile cell polarity
leading edge w/ filopodium; lagging edge
cellular structures containing actin
microvilli, cell cortex, circumferential belt, leading edge, filopodia, contractile ring, stress fibers
types of actins
α actin, ß actin, gamma actin
α actin
involved in muscle contraction (circus. belt and contractile ring)
ß actin
part of the leading edge/lamellipodium
gamma actin
makes up stress fibers
4 states of actin
G-Actin ATP, G-Actin ADP, F-Actin ATP, F-Actin ADP
most common states of actin
G-Actin ATP and F-Actin ADP
actin polymerization
nucleation, elongation, steady state
How do the plus and minus ends of microfilaments differ?
ATP binding site of a G-Actin align to the - end of the filament; Monomers add preferentially to the + end of the microfilament.
critical concentration
Concentrations of monomers needed to add to the +/- end of the microfilament; Higher conc. needed to add to - end of microfilament.
treadmilling
Monomers add preferentially to one end and dissociate at the other.
Accelerated by profilin and cofilin.
Cofilin
severing factor
Profilin
exchange factor; removes ADP and adds ATP to encourage polymerization
Thymosin ß4
provides a reservoir of ATP G-actin by binding and preventing its addition to the microfilament
CapZ
binds + end of microfilament to prevent subunit addition/loss
Tropomodulin
binds - end of microfilament to prevent subunit addition/loss
Formins
create a dimer at the end of the filament and guides the addition of G-actin to prevent branching
Arp 2/3
(protein machine) binds to the side of the actin filament and binds a new nucleus to form branch at 70 degree angle
regulatory proteins
profilin, thymosin ß4, cofilin
capping molecules
capZ, tropomodulin
molecules influencing microfilament structure
formins, arp2/3
How do actin cross linking proteins stabilize actin structures?
They have 2 actin binding domains that bind to the microfilament and stabilize in various arrangements.
The type of cross linking depends on the distance between the actin binding domains.
actin crosslinking proteins
fimbrin, α actinin, spectrin, filamin
fimbrin
2 actin binding domains with short tail; stabilizes microvilli, filopodia, and focal adhesions
α actinin
actin binding domain with short tail, arranged head to tail; stabilizes stress fibers, filopodia, & muscle Z band
spectrin
actin binding domain with longer tail, arranged head to tail; stabilizes cell cortex
filamin
actin binding domain with medium tail, connected at the tail end; stabilizes leading edge, stress fibers, and filopodia
myosin structure
globular (ATP binding) head, long coiled tail, flexible neck; myosin I is a monomer, others are dimers
How does myosin II move actin filaments?
Binds ATP, head released from actin.
Hydrolysis of ATP to ADP + P, myosin head rotates into “cocked” state.
Myosin head binds actin filament.
“Power stroke” - release of P and elastic energy straightens myosin & moves actin filament toward (-) end.
ADP released; ATP bound; head released from actin
How does myosin V move vesicles?
Binds microfilament with head.
Tail interacts with receptor or molecule in the membrane of a vesicle/organelle.
directional movement of myosin
all myosin EXCEPT myosin VI move toward the positive end
actin-containing contractile structures in nonmuscle cells
polarized epithelial cells: circumferential belt (structural)
motile cells: stress fibers, focal adhesions w/ integrins
cleavage furrow: contractile ring
describe how cells move
Extension - sensing the environment and reaching out with the leading edge (signalling activates Cdc42 and Rac).
Adhesion - formation of new focal adhesion (integrin binds to ECM and interacts with internal microfilaments); gives leverage to start moving forward.
Translocation - nucleus will orient near front of the cell; mass of the cell will move forward toward leading edge as actin filaments rearrange.
De-adhesion - rear focal adhesion releases to allow cell to propel forward (in some cells, Rho promotes de-adhesion).
members of the Rho family of GTPases
Rho, Rac, and Cdc42
Rho
found in stress fibers; activates actin and myosin activity
Rac
found in the lamellipodia; activates WAVE > Arp 2/3
Cdc42
found in filopodia; activates WASP > Arp2/3 and sets polarity
How do intermediate filaments differ from microfilaments and microtubules?
do not hydrolyze ATP or GTP, more rigid, rod-like structure, made of lamins and keratins
lamins
give nuclear structure and strength (found in all cells but red blood cells)
keratins
found outside of the nucleus; give structure to cell and the placement of the nucleus
