Migrating cells possess intracellular gradients of Rho GTPases, but it is
unknown whether these shallow gradients themselves can induce motility. Here we
describe a new method to present cells with induced linear gradients of active,
endogenous Rac without receptor activation. Gradients as low as 15% were
sufficient to not only trigger cell migration up the synthetic gradient, but
also to induce both cell polarization and repolarization. Response kinetics
were inversely proportional to Rac gradient values, in agreement with a new
mathematical model, suggesting a role for natural input gradient amplification
upstream of Rac. Increases in Rac levels beyond a well-defined threshold
dramatically augmented polarization and decreased sensitivity to the gradient
value. The threshold was governed by initial cell polarity and PI3K activity,
supporting a role for both in defining responsiveness to natural or synthetic
Rac activation. Our methodology suggests a general way to investigate processes
regulated by intracellular signaling gradients