ABSTRACT
Fc γ RII cross-linking and anti-CD9 mAbs incuded tyrosine phosphorylation of Fc γ
RII, Syk, and Lyn associated with Fc γ RII in Fc γ RII cross-linking but not in anti-CD9
mAb-induced platelet activation. We prepared various GST fusion proteins expressing
one or two SH2 domains of Syk and evaluated the association between these GST fusion
proteins with Fc γ RII. Based on the results obtained from these experiments, we suggest
that only one tyrosine residue in ITAM of Fc γ RII is phosphorylated with anti-CD9
mAb and that both are phosphorylated with Fc γ RII cross-linking. Platelet activation
mediated by GPIb, the receptor for vWF, is also related with tyrosine phosphorylation.
Botrocetin and vWF induced Syk activation. Shc was also rapidly and heavily tyrosine
phosphorylated. Sre and Lyn, a 54-kDa tyrosine kinase, was associated with cytoskeletal
proteins. When GPIb was immunoprecipitated with nonfunctional anti-GPIb mAbs after
platelets were activated with vWF and botrocetin, an in vitro kinase assay revealed
the transient association of a kinase activity with GPIb after platelet activation.
Phosphoamino acid analysis of phosphorylated proteins in this assay demonstrated that
only tyrosine residues but not serine or threonine were phosphorylated, suggesting
that the kinase was indeed a tyrosine kinase.
KEYWORD
Tyrosine kinases - platelet activation - GPIb - botrocetin - Fc γ - RII
