Al. Depass et al., Inositol 1,4,5 trisphosphate is inactivated by a 5-phosphatase in stamen hair cells of Tradescantia, PLANTA, 213(4), 2001, pp. 518-524
Inositol 1,4,5 trisphosphate [Ins(1,4,5)P-3] is produced from the hydrolysi
s of phosphatidylinositol 4,5 bisphosphate, and as part of a second-messeng
er signal transduction mechanism, induces release of Ca2+ from internal sto
res in both plant and animal systems. It is less well established how the a
ctive Ins(1,4,5)P-3 is inactivated. Studies in animal cells have demonstrat
ed two separate metabolic pathways. Ins(1,4,5)P-3 can be hydrolyzed by a 5-
phosphatase or phosphorylated by a 3-kinase, resulting in the formation of
Ins(1,4)P-2 and Ins(1,3,4,5)P-4, respectively, neither of which is able to
mobilize intracellular Ca2+. Plant cell extracts have been reported to have
hydrolytic and kinase activities that produce Ins(1,4)P-2, and Ins(4,5)P-2
and Ins(1,4,5,6)P-4 from Ins(1,4,5)P-3. These results offer little insight
into the enzyme activities in the intact plant cell since the observed act
ivities might be confined to intracellular compartments that have little if
any impact on the signaling events within the cytosol that require Ins(1,4
,5)P-3. To resolve the mechanism of Ins(1,4,5)P-3 inactivation, we microinj
ected stamen hair cells of Tradescantia virginiana L. with nonhydrolysable
analogs of Ins(1,4,5)P-3 that have been previously shown to cause Ca2+ rele
ase from intracellular stores. Our results indicate a sustained cytosolic [
Ca2+] increase when cells were injected with the 5-phosphatase-insensitive
5-monophosphorothioate derivative of Ins(1,4,5)P-3, in contrast to a brief
transient when injected with the 3-kinase-insensitive 3-fluoro-3-deoxy Ins(
1,4,5)P-3 analog. We conclude that the 5-phosphatase pathway is the preferr
ed pathway for Ins(1,4,5)P-3 inactivation in the stamen hair cells of Trade
scantia.