Cs. Abrams et al., PHOSPHOPLECKSTRIN INHIBITS G-BETA-GAMMA-ACTIVABLE PLATELET PHOSPHATIDYLINOSITOL-4,5-BISPHOSPHATE 3-KINASE, The Journal of biological chemistry, 271(41), 1996, pp. 25192-25197
Pleckstrin, the prototypic protein containing two copies of the plecks
trin homology domain, is a prominent substrate of protein kinase C in
platelets and neutrophils. Both cell types have p85 subunit-containing
phosphoinositide 3-kinase (p85/PI3K) and non-p85-containing PI3K (PI3
K gamma) that is activated by beta gamma subunits of heterotrimeric GT
P-binding proteins. We have shown that a PI3K product, phosphatidylino
sitol (PI) 3,4,5-trisphosphate, promotes pleckstrin phosphorylation in
platelets. Since pleckstrin homology domains are thought to interact
with G beta gamma heterodimers and/or PI(4,5)P-2, we have examined the
effects of recombinant pleckstrins on platelet PI3K gamma and p85/PI3
K activities. Depending upon its phosphorylation/charged state, plecks
trin inhibits PI3K gamma, but not p85/PI3K. Pleckstrin-mediated inhibi
tion of PI3K gamma is overcome by excess G beta gamma and is restricte
d to PI(4,5)P-2 as substrate, i.e. pleckstrin does not inhibit phospho
rylation of PI(4)P or PI. Consistent with this, activation of protein
kinase C by exposure of platelets to beta-phorbol diester (to increase
endogenous pleckstrin phosphorylation) prior to platelet lysis causes
inhibition of G beta gamma-stimulatable PI3K activity only with respe
ct to PI(4,5)P-2 substrate. This phosphopleckstrin-mediated inhibition
is overcome by increasing concentrations of G beta gamma. We propose
that phosphorylation of pleckstrin may constitute an important inhibit
ory mechanism for PI3K gamma-mediated cell signaling.