Figure 1

A cartoon of the model used: a roughly 1-to-1 matching of integrins and ligands is assumed to be in thermal equilibrium, so that the number of molecules binding and unbinding per time unit is equal. The FA plaque is assumed to be a perfect force transducer, distributing a constant force evenly between bound proteins. Ligands are assumed to be evenly spaced in a hexagonal pattern: differences in ligand spacing are obtained by varying the distance $d$ between gold particles.Reuse & Permissions

Figure 2

(a) The fraction of bound integrin as a function of ligand spacing. For higher ligand spacings, an increased force per bond and a decreased chance of rebinding causes a decreased bound fraction of integrin. This leads to a critical distance beyond which, at a fixed stress, no stable adhesions can form. The black curves represent physically relevant values, other values are shown in gray. Three stability regimes (stable, unstable and transient) can be inferred from this graph: at spacings below 76 nm integrin binding is allowed even at the high stress of 9 kPa. At spacings between 76 and 93 nm (shaded area) binding is allowed for the average stress of 5.5 kPa, but forbidden in some part of the interval 5.5–9.0 kPa. For spacings above 93 nm, integrin binding is forbidden at the average stress of 5.5 kPa. The error bars indicate the range over which these stability regimes vary when the parameter $h$ varies in the range 0.4–0.5 nm. (b) Fraction of bound integrin as a function of stress at fixed ligand spacing. The interval between 5.5 and 9.0 kPa is marked by the shaded area. At low ligand spacings (58 nm), adhesions are stable throughout the stress interval. This is not the case for 80 spacings, where adhesions become unstable at stress exceeding 8 kPa. For even higher spacings (110 nm), adhesions are unstable throughout the stress interval and therefore forbidden.Reuse & Permissions

Figure 3

Time-lapse series taken at two minute intervals for substrates with lateral ligand spacings of (top to bottom) 58, 80 and 110 nm. The topmost row shows FAs which follow a regular cycle of formation, elongation and turnover. An increase in the lateral ligand spacing to 80 nm disrupts this cycle: clear adhesion deformation is visible. Occasionally, an adhesion is disrupted when the core part of an adhesion (arrow) is dragged at higher speeds than its surroundings. At 110 nm spacing, attachment formation is limited to transient structures that appear and disappear at rates faster than the time-lapse interval.Reuse & Permissions

Figure 4

(Color online) Distribution of FA movement speeds for different ligand spacings (left). A monotonous increase in the average speed of the centers of FAs is observed for increasing ligand spacing. Kymographs along FAs at different spacings (right) show the differences in movement between stable adhesion at low spacings (58 nm, top) and the more erratic behavior at higher spacings (80 nm, bottom). At high spacings, adhesions slide inward faster, while the cell follows a faster cycle of extension and retraction.Reuse & Permissions