J. Biol. Chem., Vol. 259, Issue 22, 13652-13655, 11, 1984
Inositol 1,4,5-trisphosphate stimulates phosphorylation of a 62,000- dalton protein in monkey fibroblast and bovine brain cell lysates
MR Whitman, J Epstein and L Cantley
Inositol 1,4,5-trisphosphate (IP3) is produced in cells as a breakdown
product of the diphosphorylated form of phosphatidylinositol,
phosphatidylinositol 4,5-bisphosphate. Stimulated breakdown of
phosphoinositides has been correlated with a wide variety of hormonal
stimuli which mobilize intracellular calcium, and IP3 has recently been
found to cause calcium release from intracellular stores, thus implicating
it as a second messenger in hormonal stimulation. In this paper we have
examined the effect of IP3 on protein phosphorylation, and have found that
IP3 stimulates phosphorylation of a 62-kDa protein in cell lysates made
from cultured monkey fibroblasts and from bovine brain. Fifty per cent
maximal stimulation of phosphorylation of this protein occurred at 2.5 X
10(-7) M IP3. Other inositol phosphates (inositol 1,4-bisphosphate,
inositol 1-phosphate, inositol hexaphosphate, and myo-inositol) had no
effect on protein phosphorylation at 10(-6)M, although inositol
1,4-bisphosphate at higher concentrations enhanced phosphorylation of the
62-kDa protein in brain lysates. The IP3-stimulated phosphorylation was
calcium- independent and did not appear to result from inhibition of an
endogenous protein phosphatase. We suggest that IP3, like other second
messengers, acts as a protein kinase regulator.
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