Initial rolling of circulating neutrophils on a blood vessel wall prior to adhesion and transmigration to damaged tissue is dependent upon P-selectin expressed on endothelial cells and its specific neutrophil receptor, the P- selectin glycoprotein ligand-1 (PSGL-1). Pretreatment of neutrophils, HL60 cells, or a recombinant fucosylated soluble form of PSGL-1 (sPSGL-1.T7) with the cobra venom metalloproteinase, mocarhagin, completely abolished binding to purified P-selectin in a time-dependent and EDTA- and diisopropyl fluorophosphate-inhibitable manner consistent with mocarhagin selectively cleaving PSGL-1. A polyclonal antibody against the N-terminal peptide Gln-1- Glu-15 of mature PSGL-1 immunoprecipitated sPSGL-1.T7 but not sPSGL-1.T7 treated with mocarhagin, indicating that the mocarbagin cleavage site was near the N terminus. A single mocarhagin cleavage site between Tyr-10 and Asp-11 of mature PSGL-1 was determined by N-terminal sequencing of mocarhagin fragments of sPSGL-1.T7 and is within a highly negatively charged amino acid sequence 1-QATEYEYLDY ↓ DFLPETEPPE, containing three tyrosine residues that are consensus sulfation sites. Consistent with a functional role of this region of PSGL-1 in binding P-selectin, an affinity-purified polyclonal antibody against residues Gln-1-Glu-15 of PSGL-1 strongly inhibited P- selectin binding to neutrophils, whereas an antibody against residues Asp-9- Arg-23 was noninhibitory. These combined data strongly suggest that the N- terminal anionic/sulfated tyrosine motif of PSGL-1 as well as downstream sialylated carbohydrate is essential for binding of P-selectin by neutrophils.