Table 1. Parameter table for simulations.

Parameter	Name	Keratocyte Value	XTC Value	Reference/Justification
$v_{pol}$	Polymerization rate at leading edge	150 sub s-1	38 sub s-1	Matches observed protrusion rate
$v_{depol}$	Pointed end depolymerization rate	5 sub s-1	5 sub s-1	Wioland et al., 2017; Johnston et al., 2015
$k_{cap}$	Capping rate	0.6 s-1	0.2 s-1	Estimated (Materials and methods)
$k_{uncap}$	Uncapping rate	1.0 s-1	1.0 s-1	Miyoshi et al., 2006
$k_{br}$	Branching rate	150 s-1µm-1	30 s-1µm-1	Estimated (Materials and methods)
$k_{debr}$	Debranching rate	0.1 s-1	0*	Narrower distribution of Arp2/3 complex compared to F-actin (Lai et al., 2008, Miyoshi et al., 2006, Ryan et al., 2012)
$v_{net}$	Network velocity with respect to leading edge	0.2µm s-1	0.05µm s-1
$k_{unif}^{sev}$	Uniform severing rate	Varied	Varied
$k_{end}^{sev}$	Severing rate near barbed end	Varied	Varied
$k_{anneal}$	Annealing rate constant	60µM-1s-1	60µM-1s-1	Close to Popp et al., 2007
$l_{max}^{olig}$	Maximum oligomer size	Varied	Varied
$D_{olig}$	Oligomer diffusion coefficient	0.25µm2 s-1	0.25µm2 s-1	Estimated

*Since severing and depolymerization contributed to debranching in XTC cells, we did not include a separate debranching rate constant.