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The cytoplasmic domain of the platelet glycoprotein Ibalpha is phosphorylated at serine 609
R J Bodnar1, M Gu, Z Li
1Department of Pharmacology, College of Medicine, University of Illinois, Chicago, Illinois 60612, USA.
Insights
Platelet glycoprotein Ib-alpha (GPIbalpha) is phosphorylated at Ser(609), a modification crucial for binding the signaling molecule 14-3-3zeta. This phosphorylation regulates GPIb-IX interaction, impacting platelet adhesion and spreading.
Area of Science:
- Cell Biology
- Molecular Biology
- Hematology
Background:
- The alpha chain of platelet glycoprotein Ib (GPIbalpha), a von Willebrand factor receptor, was not previously known to be phosphorylated.
- Understanding post-translational modifications of GPIbalpha is crucial for elucidating platelet function.
Purpose of the Study:
- To investigate the phosphorylation status of GPIbalpha's cytoplasmic domain.
- To determine the functional significance of GPIbalpha phosphorylation in platelet signaling and adhesion.
Main Methods:
- Immunoblotting using a novel anti-phosphopeptide antibody (anti-pS609) specific for phosphorylated Ser(609) of GPIbalpha.
- Immunoabsorption assays to quantify GPIbalpha phosphorylation.
- Inhibition studies using phosphopeptides and dephosphorylation with potato acid phosphatase to assess binding interactions.
- Confocal microscopy to visualize GPIbalpha phosphorylation patterns in spreading platelets.
Main Results:
- The cytoplasmic domain of GPIbalpha is phosphorylated at Ser(609), with a high proportion of platelet GPIbalpha existing in this phosphorylated state.
- Phosphorylation at Ser(609) is essential for the binding of GPIb-IX to the intracellular signaling molecule 14-3-3zeta.
- Dephosphorylation of GPIb-IX disrupts both anti-pS609 and 14-3-3zeta binding.
- Confocal microscopy revealed reduced anti-pS609 staining in specific regions of spreading platelets, indicating localized dephosphorylation.
Conclusions:
- Phosphorylation of GPIbalpha at Ser(609) is a key regulatory event for GPIb-IX interaction with 14-3-3zeta.
- Dynamic phosphorylation and dephosphorylation at Ser(609) likely play significant roles in platelet adhesion and spreading processes.
Abstract:
The alpha chain of the platelet von Willebrand factor receptor, glycoprotein (GP) Ib, is not known to be phosphorylated. Here, we report that the cytoplasmic domain of GPIbalpha is phosphorylated at Ser(609); this was detected by immunoblotting with an anti-phosphopeptide antibody, anti-pS609, that specifically recognizes the GPIbalpha C-terminal sequence S(606)GHSL(610) only when Ser(609) is phosphorylated. Immunoabsorption with anti-pS609 removed almost all of the GPIbalpha from platelet lysates, indicating a high proportion of GPIbalpha phosphorylation. Anti-pS609 inhibited GPIb-IX binding to the intracellular signaling molecule, 14-3-3zeta. Dephosphorylation of GPIb-IX with potato acid phosphatase inhibited anti-pS609 binding and also 14-3-3zeta binding. A synthetic phosphopeptide corresponding to the GPIbalpha C-terminal sequence (SIRYSGHpSL), but not a nonphosphorylated identical peptide, abolished GPIb-IX binding to 14-3-3zeta. Thus, phosphorylation at Ser(609) of GPIbalpha is important for 14-3-3zeta binding to GPIb-IX. In certain regions of spreading platelets, particularly at the periphery, there was a reduction in GPIbalpha staining by anti-pS609 as observed under a confocal microscope, indicating that a subpopulation of GPIbalpha molecules in these regions is dephosphorylated. These data suggest that phosphorylation and dephosphorylation at Ser(609) of GPIbalpha regulates GPIb-IX interaction with 14-3-3 and may play important roles in the process of platelet adhesion and spreading.