Dendritic actin filament nucleation causes traveling waves and patches

Abstract

The polymerization of actin via branching at a cell membrane containing nucleation-promoting factors is simulated using a stochastic-growth methodology. The polymerized-actin distribution displays three types of behavior: (a) traveling waves, (b) moving patches, and (c) random fluctuations. Increasing actin concentration causes a transition from patches to waves. The waves and patches move by a treadmilling mechanism not involving myosin II. The effects of downregulation of key proteins on actin wave behavior are evaluated.

FIG. 1

3D network structure (left) and simulated fluorescence (right) for actin wave (red) propagating across three-micron square area of membrane (green). Time difference between frames is 6 sec. Parameters are: k_att = 0.025s⁻¹, k_det = 0.009μM⁻¹s⁻¹, E_b = 2.8k_BT, k_nuc = 0.003μM⁻¹s⁻¹, [A] = 22.5μM, and [CP] = 0.25μM. See Supplemental Video 1 for complete time evolution.

FIG. 2

Simulated fluorescence of patches obtained with parameters of Fig. 1 but with [A] reduced to 15μM (top row) or [CP] decreased to 0.10μM (bottom row). Images are 12 sec (top row) and 18 sec (bottom row) apart. See Supplemental Videos 2 and 3 for complete time evolution.

FIG. 3

Closeup of edge of actin patch at top of Fig. 2 moving to right. Frames are 0.2 sec apart.

FIG. 4

Total number of polymerized subunits N_p vs time: a) Baseline parameters as in Fig. 1; b) [CP] decreased to 0.10μM.; c) k_nuc increased to 0.03μM⁻¹s⁻¹; d) E_b increased to 4.5k_BT; e) k_att increased to 0.05s⁻¹