Microfilaments and actin Flashcards
What are the 8 different arrangements of microfilaments?
Microvilli Cell cortex Adherens belt Flipodia Lamellipodium Cell cortex Stress fibres Contractile ring
Microvilli
Microfilaments which exist as a tight bundle of filaments
Cell cortex
Microfilaments found less ordered beneath the plasma membrane where they provide support and organisation
Adherens belt
In epithelial cells microfilaments form a contractile band around the cell to provide strength to the epithelium
Lamellipodium
In migrating cells a network of microfilaments are at the front of the cell from which bundles of filaments may protrude
When do contractile rings form?
In cytokinesis to separate the cell to two daughter cells
What is actin required for?
Cell movement Cell division Vesicle transport Phagocytosis Movement of organelles
What 3 groups are actin isoforms classified into?
alpha-actin (associated with contractile structures)
beta-actin (found in cell cortex)
gamma-actin (filaments in stress fibres)
Why is actin described as dynamic?
Length and organisation can change
What is myosin?
molecular motor proteins that work with actin filaments
G actin
Globular monomer that is the building block of actin
G actin polypeptide folding
Actin polypeptide folds into 4 subdomains that generate two lobes separated by a cleft. At the base of the cleft there is an ATPase fold that binds ATP complexed with a Mg2+ ion.
The floor of the cleft acts as a hinge that allows the lobes to flex relative to each other. Binding of ATP induces a shape change.
G actin and F actin
Actin exists as a glubular monomer (G actin) and as a filamentous polymer (F actin) that is a long chain of G-actin subunits
Why is actin an ATPase?
It will hydrolyse ATP to ADP and Pi
What happens when cations are added to a solution of G actin?
Polymerisation of G actin to F actin filaments.
Reversible process
How are subunits arranged in actin filaments?
Helical structure and can be considered as two strands wound around each other
F actin- polarity
Two strands of monomers where all subunits are oriented the same way so the whole filament exhibits polarity. One end of the element is favoured for the addition of actin subunits (+ end) and the other is favoured for subunit dissociation (-end)
What is in contact with the + and - end of the F actin monomer strands?
The + end is in contact with neighbouring subunits
The - end is exposed to the surrounding solution
F actin
arrow head
Arrow head points to the - end so the - end is called the pointed end
The + end is called the barbed end and is where the actin filaments grow
What 3 steps are needed in actin polymerisation?
Nucleation phase
Elongation phase
Steady state phase
Nucleation phase
Marked by a lag period in which G actin subunits combine into an oligomer of two or three subunits
Elongation phase
Short oligomer quickly increases in length by addition of actin monomers to both ends. As F actin filaments grow the concentration of G actin monomers decrease until equilibrium is reached between the filament end and monomers.
Steady state phase
G actin monomers exchange with subunits at the filament end, no net change in total length of filament
Critical concentration
The concentration of free G-actin at which growth at one end is balanced by loss at the same end.
