Actin

Actin Polymerization Rates

Prototypical Actin Polymerization Rates

-	k +   µM-1s-1	k −   s-1	Cc   µM
barbed (+) end	10	1.6	0.16
pointed (-) end	1	0.6	0.6
Overall Keq free actin: 0.2 µM

• Note that only the on-rate (Ka) is concentration dependent.
• The critical concentration (Cc) is calculated by dividing the off-rate by the on-rate.

Actin rate dynamics is well characterized, both experimentally and via modeling. Simple Ka/Kd on-rate and off-rate values for prototypical actin filament polymerization are given above. From this we can see that actin will polymerize extremely quickly when the free actin concentration is above the critical concentration for polymerization. For example, if 125 µM actin was suddenly made available in a prototypical dendritic spine, at the barbed (+) end the initial polymerization rate would be:

125 µM * 10 p/µM*s = 1250 p/s

Within 30 seconds, the 125 µM free actin would have already dropped to under 1 µM, with 19850 (of 20000) nm filament added to the network, while losing a nominal 30 monomers back into the free-actin pool.

STEADY-STATE

vol. conversions

• 1 µm³ = 1 femtoliter (fL)
• 1 fL = 1e-15 L
• 1 µM = 1e-16 mol/L

The actin filament network will achieve steady-state when free Ga actin levels are at the Cc. Given that the majority of (-)ends are capped (by capping protein or Arp2/3), we can see that the critical concentration is ~0.1 µM (the Cc for (+)end polymerization). Thus, at steady-state, in a prototypical spine volume of 0.1 µm³, the expected number of free Ga monomers will be:

0.1e-6 mol/L * 6e23 p/mol * 0.1e-15 L = 6 particles