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Motor proteins allow
polarized movement along microtubules
Molecular motors bind ATP in
two motor domains
ATP hydrolysis causes
conformational change to drive movement of motor
Kinesin
A large family of motor proteins that uses the energy of ATP hydrolysis to move toward the plus end of a microtubule.
Most important motor protein in the maintenance of the structure of the ER
kinesin
Most important motor protein in the maintenance of the structure of the golgi
dynein
Microtubules are composed of
alpha and beta tubulin
Microtubule function
organellar position, moving vesicles, spindle fibre formation
Mechanism of microtubule assembly
dynamic instability
Dynamic Instability
The rapid switching between growth and shrinkage shown by microtubules.
G-actin
globular actin
F-actin
filamentous actin
New monomers are added to [+/-] end of microtubules
+
Phases of microtubule assembly
nucleation (lag) phase, elongation (growth phase), steady state (equilibrium)
Lamellipodia
flat, sheetlike extensions from the core of growth cones, located between the filopodia
Filopodia
very fine, tubular outgrowths from the growth cone
In the cell the concentration of free actin is 50-200 micrometers. The critical concentration is less than 1 micrometer. What can account for this difference?
Proteins bind to free actin subunits
APBs
actin binding proteins
ABPs control
behaviour of F-actin polymers in cells
ARP
Actin related protein
ARP 2/3 complex
nucleates branched actin arrays
Actin arrangements
stress fibers, lamellipodium, filopodium
Stress fibers
contractile bundles of actin
Lamellipodium
branched actin network
Filopodium
parallel bundles of actin
Formation of actin network at plasma membrane
pushes membrane forward and allows cells to move
Actin polymerization in cell cortex results in
cell movement
Parallel bundles structure
f-actin closely spaced and arranged in parallel, linked by fimbrin
Parallel bundles are seen in
filopodia and microvilli
Contractile bundles structure
f-actin are arranged in anti-parallel, linked by alpha actinin and other proteins
Contractile bundles are seen in
stress fibers, contractile ring
Cross-linked gel structure
random orientation of f-actin, linkedin by filamin
Cross-linked gel is seen in
forms part of the cell cortex, important in amoeboid motion and cytoplasmic streaming
Myosins
actin-dependent motor proteins
Attached arrangement of actin and myosin
ADP-bound myosin head is attached to actin filament
Release and shift of actin and myosin
ADP switched for ATP, affinity for actin reduced, myosin head can slide along filament
Powerstroke
ATP hydrolysis, myosin head binds to new site on actin, inorganic phosphate is released, head binds tightly and moves back to original conformation, dragging actin with it
Actin microfilaments are conposed of
globular actin subunits, 2 filaments twisted together
General function of actin microfilaments
musce contraction, movement of cells, cytokinesis and cell contractility
Mechanism of actin microfilament assembly
dynamic instability, monomers added to + end preferentially
