lecture 17

G-actin

The monomeric form of actin, a globular protein that binds ATP or ADP.

F-actin

The polymerized, helical form of actin made of G-actin subunits, exhibiting polarity with a plus (barbed) and a minus (pointed) end.

Critical concentration (Cc)

The minimum concentration of ATP-actin required for polymerization; lower at the plus end than the minus end.

Profilin

A protein that binds to G-actin, promotes the exchange of ADP for ATP, and delivers actin monomers to formins.

Formin

A protein that nucleates and promotes elongation of unbranched actin filaments by binding to the barbed end.

Cofilin

A protein that binds to ADP-actin regions in F-actin, promoting disassembly and severing of older filaments.

Tropomyosin

A protein that binds along the length of F-actin filaments, stabilizing them and protecting against severing.

Spontaneous nucleation

Occurs when three G-actin monomers come together to form a stable nucleus.

Arp2/3 complex

A protein complex that nucleates new filaments by binding to an existing filament and providing a scaffold for polymerization.

WASp (Wiskott-Aldrich Syndrome Protein)

Activates the Arp2/3 complex, enhancing filament branching and nucleation.

Listeria monocytogenes

A bacterial pathogen that hijacks the host's actin machinery to promote actin polymerization.

Plus end (barbed end)

The end of F-actin where growth is more rapid and polymerization occurs if ATP-actin concentration is above Cc.

Minus end (pointed end)

The end of F-actin where dissociation is more likely and polymerization occurs but is less common.