microfilaments Flashcards
what is the structure of microfilaments like?
they are the smallest fibres in the the cytoskeleton (6-8nm)
- made of actin protein
- flexible, inextensible helical filament
- can generate tension as a contractile element
- important for movement within cell and of the cell itself
what are the five microfilament functions
- cell shape - cortex = thin region below PM
- cell migration
- transport of vesicles and organelles - especially in plants
- cytokinesis
- muscle contraction
what is the structure of globular actin like?
= monomer
has lobes - each lobe has two domains, ATP binds in the cleft
what is the structure like in a filamentous actin?
= polymer
in a mature filament, two f-actins wrap around each other to form a helical structure
when they assemble into f-actin, all monomers…
point in the same direction, so the final filament has polarity
- the way actin monomers assembles provide polarity to the filament
two bound actin molecules are … but the addition of a third actin monomer makes the complex …
weak…more stable
what happens at each end of actins
- at minus end the ATP-binding pocket is exposed
- at PLUS end the ATP-binding pocket is buried in the filament
filament growth occurs where
at the PLUS end (barbed end)
- minus = pointed end
myosin
the molecular motor of actin
- there is myosin II and myosin I or myosin V
myosin II (conventional) vs myosin I or myosin V (unconventional)
II:
- first ones discovered
- two heads and long tail
- no cargo, they twist with each other
- form bipolar filaments
I/V:
- smaller
- myosin I = single head
- myosin V = two heads
- no filament formation
- tail binds vesicles and membranes
thick filament of skeletal muscle …
- have hundreds of myosin II molecules
- they twist with each other
form bipolar filaments
myosin heads in muscle contraction…
relaxed = myosin heads not interacting with actin microfilaments
contracted = myosin heads have pulled the actin closer
thin vs thick filaments
thin = actin
thick = myosin
how does myosin move?
through ATP dependent processes
- like kinesin and dynein, use chemical energy to do work
the cyclic process of muscle contraction involving actin and myosin filaments =
CCPPAA
CCPPAA =
C - cocked myosin bc of ATP hydrolysis
C - cross-bridge
P - pi released
P - power stroke
A - ADP released
A - ATP binds and myosin detaches
what is the organization of actin?
there are two types, both found in cell cortex.
- bundles = parallel fibers
- networks = can be 2D or 3D
microfilament organization is mediated by …
actin-binding proteins
actin-binding proteins regulate …
polymerization and length of filaments
- nucleating proteins
forms a nucleating center by mimicking the shape of actin subunits so G-actins will start to add
ex. Arp2/3
- monomer sequestering
- controls amount of g-actin available for polymerization
ex, thymosin beta4
- end blocking (capping)
ex. capZ caps ‘plus’ end (filament will shrink
ex 2. tropomodulin caps ‘minus’ end (filament will grow)
capping proteins prevent …
g- actin addition and loss
- monomer polymerizing
increases actin filament growth rates
promotes g - actin addition to filament plus ends
ex. profilin competes with thymosin beta4 for g-actin monomers
