Selectin - Bonding Mechanisms

Bonding Mechanisms

Selectins have hinge domains, allowing them to undergo rapid conformational changes in the nanosecond range between ‘open’ and ‘closed’ conformations. Shear stress on the selectin molecule causes it to favor the ‘open’ conformation.

In leukocyte rolling, the ‘open’ conformation of the selectin allows it to bind to inward sialyl Lewis molecules farther up along the PSGL-1 chain, increasing overall binding affinity—if the selectin-sialyl Lewis bond breaks, it can slide and form new bonds with the other sialyl Lewis molecules down the chain. In the ‘closed’ conformation, however, the selectin is only able to bind to one sialyl Lewis molecule, and thus has greatly reduced binding affinity.

The result of such is that selectins exhibit catch and slip bond behavior—under low shear stresses, their bonding affinities are actually increased by an increase in tensile force applied to the bond because of more selectins preferring the ‘open’ conformation. At high stresses, the binding affinities are still reduced because the selectin-ligand bond is still a normal slip bond. It is thought that this shear stress threshold helps select for the right diameter of blood vessels to initiate leukocyte extravasion, and may also help prevent inappropriate leukocyte aggregation during vascular stasis.