Unit 6 Lecture 3 - Actin Filaments

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