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Department of Physiology, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75235-9040
Gelsolin and CapG are actin regulatory proteins that remodel the cytoskeleton in response to
phosphatidylinositol 4,5-bisphosphate (PIP2) and Ca2+
during agonist stimulation. A physiologically relevant
rise in Ca2+ increases their affinity for PIP2 and can
promote significant interactions with PIP2 in activated
cells. This may impact divergent PIP2- dependent signaling processes at the level of substrate availability.
We found that CapG overexpression enhances PDGF-stimulated phospholipase C
(PLC
) activity (Sun, H.-q.,
K. Kwiatkowska, D.C. Wooten, and H.L. Yin. 1995. J. Cell Biol. 129:147-156). In this paper, we examined the
ability of gelsolin and CapG to compete with another
PLC for PIP2 in live cells, in semiintact cells, and in
vitro. We found that CapG and gelsolin overexpression
profoundly inhibited bradykinin-stimulated PLC
. Inhibition occurred at or after the G protein activation
step because overexpression also reduced the response
to direct G protein activation with NaF. Bradykinin responsiveness was restored after cytosolic proteins, including gelsolin, leaked out of the overexpressing cells.
Conversely, exogenous gelsolin added to permeabilized
cells inhibited response in a dose-dependent manner.
The washout and addback experiments clearly establish
that excess gelsolin is the primary cause of PLC inhibition in cells. In vitro experiments showed that gelsolin and CapG stimulated as well as inhibited PLC
, and
only gelsolin domains containing PIP2-binding sites
were effective. Inhibition was mitigated by increasing
PIP2 concentration in a manner consistent with competition between gelsolin and PLC
for PIP2. Gelsolin and
CapG also had biphasic effects on tyrosine kinase-
phosphorylated PLC
, although they inhibited PLC
less than PLC
. Our findings indicate that as PIP2 level
and availability change during signaling, cross talk between PIP2-regulated proteins provides a selective
mechanism for positive as well as negative regulation of
the signal transduction cascade.
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