Role of GPIbα in the clearance of ST3Gal-IV^−/− platelets. (A) A 140-kDa glycoprotein recognized by anti-GPIbα antibody contained increased amounts of βGal, as determined by ECA lectin blotting. The 240-kDa glycoprotein, also recognized on the ECA blot, was identified as VWF by anti-VWF antibody. (B) Surface expression of GPIbα and GPIX was not altered in ST3Gal-IV^−/− platelets. Glycoprotein expression was determined with antibodies by flow cytometry. (C) Removal of the extracellular N-terminal portion of GPIbα with O-sialoglycoprotein endopeptidase enhanced the survival of transfused ST3Gal-IV^−/− platelets (triangles; P < .001). Circulation of WT platelets is also shown (squares). Open symbols represent platelets treated with O-sialoglycoprotein endopeptidase; closed symbols, untreated platelets. (D) Ingestion of ST3Gal-IV^−/− platelets by HepG2 cells in vitro was significantly reduced by removal of the extracellular N-terminal portion of GPIbα with O-sialoglycoprotein endopeptidase (). The data are mean ± SD of 2 independent experiments done in triplicate. Significance: **P < .01; NS, P ≥ .05.