Overview of Cell Biology/Actin Structure and Dynamics

Readings

• Study guide:
• Wikipedia article: Actin

Actin Structure

Actin is separated into lobes by an ATP-Mg²⁺ binding site called an ATPase fold. G-actin is the monomeric unit of actin. F-actin is the polymeric form which is held by noncovalent interactions and is polar. Adding subfragment-1 of myosin II will distinguish the negatively charged end (pointed end) and the positively charged end (barbed end). Each end of the polymer has different functional properties.

Actin Polymerization

Actin polymerization can be initiated by adding salts. This process is reversible, if the salts are removed, the filament will depolymerize into its monomer form. Actin polymerization can be measured by a wide variety of techniques including lightscattering, a fluorescent tag on the momer, electron microscopy, and centrifugation.

The C_c of actin is 0.2 μM and is also the concentration at which steady state is achieved.

The polymerization rate of the two ends is different

C_c (+) end = 0.1 μM

C_c (-) end = 0.8 μM

If the free actin concentration is < 0.1 μM, both ends shrink. If the free actin concentration is > 0.8 μM, both ends grow. If the free actin concentration is between 0.1 μM and 0.8 μM, the filament will grow at the (+) end and shrink at the (-) end.

Drugs affecting polymerization

Phalloidin binds to the interface between subunits in the filaments and prevents filament depolymerization

Cytochalasin binds to the (+) end of actin and prevents elongation. The C_c shifts to that of the (-) end. Cytochalasin is membrane permeable.

Latrunculin binds to monomers, prevents disassembly and also causes polymer disassembly. Latrunculin is membrane permable.