Figure 3. Integrin-FN clutch model of force transmission.

(a) Myosin motors pull on actin filaments, which move with rearward speed v_a. Integrins of two different types connect to the actin flow through adaptor proteins, and compete for binding to FN with effective binding rates k_on1 and k_on2 given by true binding rates (k_ont1, k_ont2) multiplied by integrin densities on the membrane (d_int1, d_int2). FN molecules are in turn connected to a compliant substrate, represented as a linear elastic spring of varying rigidity. (b) Flow of events (from top to bottom). (i) The model considers a given number of FN molecules (triangles) attached to the substrate, to which integrins can bind. (ii) Orange integrins, with higher k_on, will bind faster. (iii) Once they bind, actin rearward movement applies a force on all integrins and the substrate. Because orange integrins also have a higher k_off, they will detach sooner. (iv) Eventually, blue integrins will detach as well, and if before detaching the force transmitted through them reaches a threshold value, a reinforcement mechanosensing event will result in increased integrin density. (v) Once all integrins detach, force on the substrate is released and the cycle begins again. (c-e) Cell traction forces (c), integrin densities (d), and actin speeds (e) predicted by the model, the three regimes (1-3) are discussed in the main text and in Supplementary Note 4.