Mechanical switching and coupling between two dissociation pathways in a P-selectin adhesion bond

doi:10.1073/pnas.0401870101

Mechanical switching and coupling between two dissociation pathways in a P-selectin adhesion bond

Departments of ^*Biomedical Engineering and ^†Physics, Boston University, Boston, MA 02215; Departments of ^‡Physics and ^§Pathology, University of British Columbia, Vancouver, BC, Canada V6T 1Z1; and ^∥Schools of Mechanical Engineering and Biomedical Engineering, Georgia Institute of Technology, Atlanta, GA 30332

Edited by James A. Spudich, Stanford University School of Medicine, Stanford, CA, and approved June 15, 2004 (received for review March 17, 2004)

Abstract

Many biomolecular bonds exhibit a mechanical strength that increases in proportion to the logarithm of the rate of force application. Consistent with exponential decrease in bond lifetime under rising force, this kinetically limited failure reflects dissociation along a single thermodynamic pathway impeded by a sharp free energy barrier. Using a sensitive force probe to test the leukocyte adhesion bond P-selectin glycoprotein ligand 1 (PSGL-1)–P-selectin, we observed a linear increase of bond strength with each 10-fold increase in the rate of force application from 300 to 30,000 pN/sec, implying a single pathway for failure. However, the strength and lifetime of PSGL-1–P-selectin bonds dropped anomalously when loaded below 300 pN/sec, demonstrating unexpectedly faster dissociation and a possible second pathway for failure. Remarkably, if first loaded by a “jump” in force to 20–30 pN, the bonds became strong when subjected to a force ramp as slow as 30 pN/sec and exhibited the same single-pathway kinetics under all force rates. Applied in this way, a new “jump/ramp” mode of force spectroscopy was used to show that the PSGL-1–P-selectin bond behaves as a mechanochemical switch where force history selects between two dissociation pathways with markedly different properties. Furthermore, replacing PSGL-1 by variants of its 19-aa N terminus and by the crucial tetrasaccharide sialyl Lewis^X produces dramatic changes in the failure kinetics, suggesting a structural basis for the two pathways. The two-pathway switch seems to provide a mechanism for the “catch bond” response observed recently with PSGL-1–P-selectin bonds subjected to small-constant forces.

Footnotes

↵ ¶ To whom correspondence should be addressed. E-mail: evanse{at}bu.edu.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: PSGL-1, P-selectin glycoprotein ligand 1; sLe^x, the tetrasaccharide sialyl Lewis^X; b-sLe^x, biotinylated form of sLe^X; PEG, polyethylene glycol; BFP, biomembrane force probe.