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The p85 and p110 Subunits of Phosphatidylinositol 3-Kinase- Are
Substrates, In Vitro, for a Constitutively Associated Protein
Tyrosine Kinase in Platelets
Norman R. Geltz and
James A. Augustine
From Blood Research Institute, Milwaukee, WI.
Phosphatidylinositol 3-kinase (PI3K) is a heterodimer lipid kinase
consisting of an 85-kD subunit bound to a 110-kD catalytic subunit that also possesses intrinsic, Mn2+-dependent
protein serine kinase activity capable of phosphorylating the 85-kD
subunit. Here, we examine the Mn2+-dependent protein
kinase activity of PI3K immunoprecipitated from normal
resting or thrombin-stimulated platelets, and characterize p85/p110
phosphorylation, in vitro. Phosphoamino acid analysis of phosphorylated
PI3K showed p85 and p110 were phosphorylated on serine, but in
contrast to previous results, were also phosphorylated on threonine and
tyrosine. Wortmannin and LY294002 inhibited p85 phosphorylation;
however, p110 phosphorylation was also inhibited suggesting p110
autophosphorylation on serine/threonine. The protein tyrosine kinase
inhibitor, erbstatin analog, partially inhibited p85 and p110
phosphorylation but did not appear to affect PI3K lipid kinase
activity. The in vitro phosphorylation of p85 or p110 derived
from thrombin-stimulated platelets was no different than that of
resting platelets, but we confirm that in thrombin receptor-stimulated
platelets enhanced levels of p85 and PI3K lipid kinase activity were
recovered in antiphosphotyrosine antibody immunoprecipitates. These
results suggest PI3K can autophosphorylate on serine and threonine,
and both p85 and p110 are substrates for a
constitutively-associated protein tyrosine kinase in platelets.
Blood, Vol. 91 No. 3 (February 1), 1998:
pp. 930-939
© 1998 by The American Society of Hematology.
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