O-glycosylation of platelet receptors, fibronectin, and fibrinogen. (A) Platelet receptors. Platelet receptors are critical mediators of platelet activation and adherence, and include the well-described GPIB-V-IX complex, the collagen binding GPVI, and multiple integrin receptors.50 GPIB α is part of the GPIB-V-IX (VWF receptor) complex and contains a mucin-like macroglycopeptide stem region, which is known to be O-glycosylated; however, the specific sites are poorly described, with only a single probable site at Thr308 (indicated by and asterisk).51 In this study, 7 glycosites were identified in this region. Novel O-glycosylation was identified on all other members of the complex in regions flanking the leucine-rich repeats. An additional novel O-glycosite was also detected on the collagen receptor GPVI. Integrin receptors (glycoproteins IIb/IIIa and Ia/IIa) were also found to be O-glycosylated at multiple sites. In particular, novel glycosylation was identified in the VWFA domains of β integrins (β1 and β3), and a novel glycosite was identified juxtaposed to the transmembrane region of integrin α2. Major ligands for each of the receptors are indicated above the receptors. (B) Fibronectin. Four sites of O-glycosylation have been described for fibronectin, 3 sites located in the variable region (T2024, T2064, T2065) and a fourth site in an N-terminal linker region (T279). All sites except T2024 were identified in this study. Moreover, in total, hemostatic fibronectin was found to carry 31 unambiguous glycosites and 14 additional ambiguous sites of glycosylation. In total, 71 unambiguous sites have been identified on fibronectin from all sources. (C) Fibrinogen. Fibrinogen α was found to be extensively glycosylated, with 45 glycosites identified in hemostatic samples and 68 sites identified across all sources. The glycosites were distributed across the protein, with the majority located within the coiled-coiled domain. Fibrinogen β and γ were much less glycosylated and were found to carry just 2 and 1 sites in this study, respectively. In total, across all studies, 4 sites have been detected on fibrinogen β and 7 on fibrinogen γ. Because of the large number of identified sites, no distinction is made between native (in vivo) and in vitro glycosylation on fibronectin and fibrinogen. Fibronectin (fn).
