Functional association of phosphoinositide-3-kinase with platelet glycoprotein Ibalpha, the major ligand-binding subunit of the glycoprotein Ib-IX-V complex.

BACKGROUND

The adhesion receptor glycoprotein (GP)Ib-IX-V, which binds von Willebrand factor (VWF) and other ligands, initiates platelet activation and thrombus formation at arterial shear rates, and may control other vascular processes, such as coagulation, inflammation, and platelet-mediated tumor metastasis. The cytoplasmic C-terminal domain of the ligand-binding GPIbalpha subunit contains binding sites for filamin (residues 561-572, critically Phe568/Trp570), 14-3-3zeta (involving phosphorylation sites Ser587/590 and Ser609), and the phosphoinositide-3-kinase (PI3-kinase) regulatory subunit, p85.

OBJECTIVES

We previously showed that, as compared with wild-type receptor, deleting the contiguous sequence 580-590 or 591-610, but not upstream sequences, of GPIbalpha expressed as a GPIb-IX complex in Chinese hamster ovary cells inhibited VWF-dependent Akt phosphorylation, which is used as a read-out for PI3-kinase activity. Pulldown experiments using glutathione-S-transferase (GST)-p85 or GST-14-3-3zeta constructs, and competitive inhibitors of 14-3-3zeta binding, suggested an independent association of 14-3-3zeta and PI3-kinase with GPIbalpha. The objective of this study was to analyze a further panel of GPIbalpha deletion mutations within residues 580-610.

RESULTS

We identified a novel deletion mutant, Delta591-595, that uniquely disrupts 14-3-3zeta binding but retains the functional p85/PI3-kinase association. Deletion of other sequences within the 580-610 region were less discriminatory, and either partially affected p85/PI3-kinase and 14-3-3zeta binding (Delta580-585, Delta586-590, Delta596-600, Delta601-605), or strongly inhibited binding of both proteins (Delta606-610).

CONCLUSIONS

Together, these findings have significant implications for interpreting the functional role of p85 and/or 14-3-3zeta in GPIb-dependent signaling or platelet functional studies involving truncation of the C-terminal residues in cell-based assays and mouse models. The Delta591-595 mutation provides another strategy for determining the function of GPIbalpha-associated 14-3-3zeta by selective disruption of 14-3-3zeta but not p85/PI3-kinase binding.