Platelet glycoprotein Ibβ is phosphorylated on serine 166 by cyclic AMP-dependent protein kinase

Platelet responses are inhibited by agents such as prostaglandin E1 that increase the cytoplasmic concentration of cyclic AMP. Inhibition is thought to result from phosphorylation of specific proteins. One protein that becomes phosphorylated is glycoprotein (GP) Ibβ, a component of the GP Ib.IX complex. We have suggested that phosphorylation of GP Ibβ inhibits the collagen-induced polymerization of actin. The aim of the present study was to identify the amino acid(s) in GP Ibβ that is phosphorylated. Purified GP Ib.IX complex was phosphorylated by the catalytic subunit of purified bovine cyclic AMP-dependent protein kinase in the presence of [γ-32P]ATP. Phosphoamino acid analysis showed that in GP Ibβ, [32P]phosphate was incorporated only into serine and was in a single tryptic peptide. Amino acid sequencing showed that this peptide was from the cytoplasmic domain of GP Ibβ and encompassed residues 161–175. A single serine residue, serine 166, contained the radiolabel. To determine whether the same residue was phosphorylated in intact platelets, GP Ibβ was isolated from 32P-labeled platelets before or after their exposure to prostaglandin E1. In both cases, radiolabel was present in phosphoserine and was in a single tryptic peptide. This peptide was the same as that which was phosphorylated in the purified GP Ib.IX complex, as shown by its identical mobility on two-dimensional tryptic maps, the presence of a positively charged residue in the fourth position, and the presence of the radiolabel in the sixth position of the peptide. This study shows that when cyclic AMP concentrations rise in platelets, the cytoplasmic domain of GP Ibβ is phosphorylated on serine 166, probably by cyclic AMP-dependent protein kinase. We suggest that phosphorylation of this residue may contribute to the inhibitory actions of cyclic AMP by inhibiting collagen-induced polymerization of actin.