The glycoprotein Ib-V-IX (GPIb-V-IX) complex interacts with subendothelial von Willebrand factor (VWF) to ensure recruitment of platelets at sites of vascular injury, a process that culminates in integrin ␣ IIb  3 -dependent stable adhesion and spreading. Interaction of the 14-3-3 adaptor protein with the C-terminal 606-610 phosphoserine motif of the GPIb␣ subunit has been implicated in the control of ␣ IIb  3 activation and cell spreading. In this study, we have examined potentially novel 14-3-3 binding sites by expressing mutant forms of GPIb␣ in Chinese-hamster-ovary (CHO) cells. Analysis of a series of neighboring 11-12 residue deletions identified a critical role for the 580-LVAGRRPSALS-590 sequence in promoting GPIb␣-14-3-3 interaction. Development of a phosphospecific antibody demonstrated high levels of phosphorylation of the Ser587 and Ser590 residues in resting platelets (which became dephosphorylated during platelet spreading on VWF), and peptides containing these phosphorylated residues effectively displaced 14-3-3 from GPIb␣. Analysis of single and double alanine substitutions of Ser587 and Ser590 demonstrated a major role for these residues in promoting GPIb␣-14-3-3 binding. Moreover, these cell lines exhibited a defect in cell spreading on immobilized VWF. These studies demonstrate the existence of a second major 14-3-3 binding site within the cytoplasmic tail of GPIb␣ that has an important functional role in regulating integrin-dependent cell spreading. IntroductionThe 14-3-3 protein family consists of ubiquitous homodimeric or heterodimeric intracellular adaptor proteins that take part in the signaling pathways of numerous biologic responses. 1 There have been 5 isoforms identified in human platelets, the ⑀ and isoforms being weakly expressed relative to the more abundant ␥, , and  isoforms. 2 The crystal structure of 14-3-3 revealed the association of 2 monomers, each composed of a bundle of 9 antiparallel helices, to form a large negatively charged groove that could be implicated in interactions with other proteins. 3 The 14-3-3 proteins bind to specific phosphoserine-containing motifs, 4 and their dimeric nature allows them to act as intramolecular and intermolecular phosphorylation-dependent bridges. 5 The functions of the different isoforms in platelets are, however, still poorly understood.In a seminal study, it was reported that 14-3-3 interacted with the platelet von Willebrand factor (VWF) receptor, the glycoprotein Ib-V-IX (GPIb-V-IX) complex. 6 A binding site for 14-3-3 was found at the C-terminus of GPIb␣ within a serine-rich 606-SGHSL-610 sequence bearing similarities to phosphoserine containing 14-3-3 binding motifs. 1,7 Further work showed that the serine at position 609 was predominantly phosphorylated in resting platelets and that phosphorylation was required for 14-3-3 binding. 8 Glutathione S-transferase (GST)-14-3-3 pull-down and immunoprecipitation studies of GPIb-IX-transfected cells pointed to the existence of a separate binding or regulatory site in the GPIb␣ 570-590 ...
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