LECTURE 1 - ACTIN Flashcards
Describe the structure and role of microfilaments
Actin filaments (microfilaments) are composed of monomeric actin protein subunits assembled into a twisted, two-stranded polymer.
Actin filaments provide structural support, particularly to the plasma membrane, and are important for certain types of cell motility.
Structure and Polymerisation of Actin Filaments
G-actin = Globular actin (monomeric form).
F-actin = Filamentous actin (polymerised form).
Assembly is reversible: G-actin ↔ F-actin, depending on cellular conditions.
Polarity of F-actin:
The filament has a “+” (plus) end and a “–” (minus) end.
The ATP-binding site is exposed at the – end.
Polarity means the two ends behave differently during assembly/disassembly.
ATP Dynamics in Actin Filaments
ATP-G-actin adds to both ends of the filament, especially the + end.
Once incorporated, ATP is hydrolyzed to ADP + Pi.
Therefore, most of a stable filament consists of ADP-actin in the middle, with newer ATP-actin subunits at the growing ends.
If ATP-actin keeps adding, the filament grows.
If ADP-actin is exposed at the ends (i.e., no ATP-actin is added), the filament shrinks from those ends
Effects of Mutations in Actin
If actin can’t bind ATP → no polymerisation → filaments won’t form.
If actin can’t hydrolyze ATP → filaments form, but won’t disassemble normally.
Critical Concentration (Cc) and treadmilling
Cc = the concentration of ATP-G-actin at which polymerisation = depolymerisation at a filament end.
Above Cc → net growth.
Below Cc → net shrinkage.
Cc is different for each end:
+ end: low Cc (~0.12 μM) → grows easily.
– end: high Cc (~0.6 μM) → slower to grow, quicker to shrink
Formin Activation via Rho-GTP
Formins are kept inactive by interaction between their N- and C-terminal domains (autoinhibition).
When the Rho GTPase is in its active (GTP-bound) state:
It binds to formin and causes a conformational change.
This activates formin, allowing it to nucleate and grow new actin filaments
How are actin filaments organised
organised into specific structures by cross-linking proteins.
These proteins physically link actin filaments together to give shape and mechanical strength to different cell regions.
Examples of actin cross-linking proteins
Fimbrin
Structure organised: Microvilli
Function: Tight bundles that increase surface area (e.g. intestines)
Spectrin
Structure organised: Cell cortex
Function: Mesh-like network under plasma membrane; maintains cell shape
Filamin
Structure Organised: Filopodia
Function: Cross-links actin into a flexible, 3D network for protrusive structures
What Is Dystrophin?
Dystrophin is a large actin-binding adapter protein.
It’s located at the muscle cell cortex, just under the cell membrane.
It links the actin cytoskeleton to a transmembrane protein called dystroglycan, which anchors into the extracellular matrix
What are the actin binding proteins
Actin structures are organised by cross-linking proteins
Profilin
Cofilin
Thymosin b4
Capping proteins
Formins
Role of profilin
Promotes ADP → ATP exchange on G-actin → recharges actin for polymerisation.
Role of cofilin
Binds to ADP-actin in F-actin, mostly at the – end, and enhances disassembly.
Role of thymosin b4
Binds to G-actin and holds it in reserve (prevents premature polymerisation).
Acts like a buffer system.
Role of capping proteins
Bind to filament ends (either + or –).
Prevent both assembly and disassembly → stabilise filaments.
Role of formins
Nucleate unbranched filaments (help start the growth of long, straight filaments).
Remain attached at the + end and help elongate the filament
