Party Favors Are in My Blood

You may be familiar with the finger trap, a small woven tube commonly sold as a party favor.  I can amuse myself for hours with one of these things.  Basically, I insert a finger into each end of the tube and pull outwards.  The weave tightens as the tube is stretched, and my fingers become trapped.  The harder I pull, the stronger the trap becomes.  In addition to growing longer, the tube becomes narrower in response to pulling.  This could be considered a conformational change of the tube.  Once my fingers are good and trapped, there are two methods for successfully removing them.  One option is to push my fingers back together, allowing the trap to regain its original shape, and then gently pull one finger out at a time.  The other option is to pull very hard and rip the tube, causing the weave to separate and release my fingers.  This conformational change in response to a change in applied forces is an analogy for the protein-ligand complex that I study.

My daughter Samantha demonstrates cutting-edge science. (photo: Kim Gunnerson)

Many biological functions are dependent on a series of adhesions between proteins and ligands.  The inflammatory response provides an interesting example of this behavior that is characterized by an immune cascade; a series of biochemical events that lead to the removal of an invading species (like a virus or bacteria) from infected tissue.  The selectin family of proteins plays a role in this multi-step process by plucking leukocytes (white blood cells) from the blood so they can be delivered to the site of infection. 

bolobs

Figure 1: P-selectin/PSGL-1 ligand complex created using VMD graphical representation of x-ray crystal structure. (W. Somers, et. al. Cell 103, 467 (2000))

I Love Tether and Roll

Blood vessels are lined with endothelial cells.  When inflammatory signals are released by the surrounding tissue, these cells produce p-selectin proteins on their surfaces (a process called exocytosis).  p-Selectin then binds with an oligosaccharide called PSGL-1 on the surface of a passing leukocyte.  This tethering interaction experiences stress due to increasing hemodynamic forces as more blood flows to the area of inflammation.  The attraction between p-selectin and PSGL-1 must be strong enough to grab the leukocyte, but also weak enough to allow subsequent unbinding and binding with adjacent selectins, rolling the leukocyte along the vessel wall towards the inflammation. 

Techniques such as atomic force microscopy and flow chamber analysis have been used to study the binding lifetimes of the p-selectin/PSGL-1 complex.  Results of these analyses reveal the unusual manner in which this system responds to forces.  When force is first applied and slowly increased, the binding lifetime between p-selectin and PSGL-1 actually increases, similar to how the finger trap becomes tighter as I pull my fingers harder.  Eventually a critical force threshold is reached beyond which the binding lifetimes decrease.  This has been identified as a catch-slip bond transition.  This ability to produce a catch bond may be what allows the proper rolling of the leukocyte.

Figure 2: Another view of the complex detailing the placement of the PSGL-1 ligand (blue) in the binding site of the P-selectin protein (red).

Dynamic Description of Deformation

The purpose of using steered molecular dynamics to study the p-selectin/PSGL-1 complex is to determine if there are conformational changes in the complex in response to an applied force.  Our hypothesis is that as the hydrodynamic force increases, the receptor and ligand become deformed in such a way that the binding energy increases, resulting in a longer bond lifetime.  This is the “catch-bond” regime.  The deformation eventually reaches a critical point, after which any additional force causes “slip-bond” behavior to dominate.
 
 
 
 

Research: Steered MD Study of Protein-Receptor Binding