Binding sites are close and flexible enough that the presence of other streptavidin molecules on the surface eliminates paths which an adsorbing streptavidin molecule can take to a free binding site.

The biotin-streptavidin bond is one of the strongest bonds found in nature, and can be used to cross-link moving microtubules. Here, microtubules are functionalized with biotin, which attaches to exposed lysine sites on the tubulin. Streptavidin has four biotin binding sites, which allows it to connect one microtubule to another.

In the past, typical assays involving cross-linking of microtubules assumed that a 5 minute adsorption time was sufficient for all of the streptavidin in the flow cell to adsorb onto the microtubule due to its diffusion constant of ~120 μm² s^-1 (a typical experimental flow cell has a height of about 100 um) and on-rate of about 10⁶ M^-1 s^-1 (K_d ≈ 10^-14 M). However, experiments performed by our group showed that was not the case; after 5 minutes, the streptavidin concentration on the microtubules still had not plateaued, and the adsorption process was still well in the kinetic regime.

Binding sites are close and flexible enough that the presence of other streptavidin molecules on the surface eliminates paths which an adsorbing streptavidin molecule can take to a free binding site.