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Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710
Several receptor-mediated signal transduction pathways, including EGF and IgE receptor pathways, have been proposed to be spatially restricted to
plasma membrane microdomains. However, the experimental evidence for signaling events in these microdomains is largely based on biochemical fractionation
and immunocytochemical studies and only little is
known about their spatial dynamics in living cells. Here
we constructed green fluorescent protein-tagged SH2
domains to investigate where and when IgE receptor
(Fc
RI)-mediated tyrosine phosphorylation occurs in
living tumor mast cells. Strikingly, within minutes after
antigen addition, tandem SH2 domains from Syk or
PLC-
1 translocated from a uniform cytosolic distribution to punctuate plasma membrane microdomains.
Colocalization experiments showed that the microdomains where tyrosine phosphorylation occurred were indistinguishable from those stained by cholera
toxin B, a marker for glycosphingolipids. Competitive
binding studies with coelectroporated unlabeled Syk,
PLC-
1, and other SH2 domains selectively suppressed
the induction of IgE receptor-mediated calcium signals
as well as the binding of the fluorescent SH2 domains.
This supports the hypothesis that PLC-
1 and Syk SH2 domains selectively bind to Syk and IgE receptors, respectively. Unlike the predicted prelocalization of EGF
receptors to caveolae microdomains, fluorescently labeled IgE receptors were found to be uniformly distributed in the plasma membrane of unstimulated cells and
only transiently translocated to glycosphingolipid rich microdomains after antigen addition. Thus, these in
vivo studies support a plasma membrane signaling
mechanism by which IgE receptors transiently associate with microdomains and induce the spatially restricted activation of Syk and PLC-
1.
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