Glycan Bound to the Selectin Low Affinity State Engages Glu-88 to Stabilize the High Affinity State under Force.

Selectin interactions with fucosylated glycan ligands mediate leukocyte rolling in the vasculature under shear forces. Crystal structures of P- and E-selectin suggest a two-state model in which ligand binding to the lectin domain closes loop 83-89 around the Ca2+ coordination site, enabling Glu-88 to engage Ca2+ and fucose. This triggers further allostery that opens the lectin/EGF domain hinge. The model posits that force accelerates transition from the bent (low affinity) to the extended (high affinity) state. However, transition intermediates have not been described, and the role of Glu-88 in force-assisted allostery has not been examined. Here we report the structure of the lectin and EGF domains of L-selectin bound to a fucose mimetic; that is, a terminal mannose on an N-glycan attached to a symmetry-related molecule. The structure is a transition intermediate where loop 83-89 closes to engage Ca2+ and mannose without triggering allostery that opens the lectin/EGF domain hinge. We used three complementary assays to compare ligand binding to WT selectins and to E88D selectins that replaced Glu-88 with Asp. Soluble P-selectinE88D bound with an ∼9-fold lower affinity to PSGL-1, a physiological ligand, due to faster dissociation. Adhesion frequency experiments with a biomembrane force probe could not detect interactions of P-selectinE88D with PSGL-1. Cells expressing transmembrane P-selectinE88D or L-selectinE88D detached from immobilized ligands immediately after initiating flow. Cells expressing E-selectinE88D rolled but detached faster. Our data support a two-state model for selectins in which Glu-88 must engage ligand to trigger allostery that stabilizes the high affinity state under force.