Figure 7.

Crowd surfing of vesicles. A, Schematic of tether simulations. Vesicles (yellow, 40 nm diameter) could interact with tethers (light blue, 30 nm in length) within a 20 nm binding region offset 10 nm from the surface of the ribbon (gray). Vesicles that were unbound to tethers and diffused outside of the binding region were considered lost and were used to calculate the rate of vesicle loss from the ribbon. B, Example view of a simulation. Vesicles were created in the center (black dot) of an array of tethers, and the distance versus time of the vesicle was tracked for each trial until the vesicle had diffused 125 nm (black ring). C, Example diffusion coefficient calculation for a binding:unbinding ratio of 4.0 and a tether density of 2704 tethers/μm². Trace represents the average of 100 trials; dashed curve represents fit; dotted lines indicate the number of time steps (1350) to diffuse 50 nm. Using the diffusion equation r² = 4Dt, D = (50 nm)²/4(.135 s) = 4630 nm² /s. D, The diffusion coefficient of a vesicle crowd surfing along tethers was dependent on the density of tethers and their binding:unbinding ratio. Relatively low binding ratios (e.g., ∼3) were capable of maintaining low vesicle off rates (equivalent to ∼2 vesicles per ribbon per second) while still allowing sufficiently fast diffusion (D_ribbon/D = ∼0.5).