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Department of Human Microbiology, Sackler School of Medicine, Tel-Aviv University, Tel-Aviv, Israel
The significance of the glycosylphosphatidylinositol (GPI) anchor
is unknown. Since GPI-anchored proteins mediate signaling, it has been
suggested that the GPI structure serves as a signal-transducing
element. However, the division of signaling functions between
transmembrane and GPI-anchored proteins is unclear. Studies of distinct
membrane-anchored forms of the same protein may resolve this issue. The
adhesion molecule CD58 is expressed on the cell surface in both a
transmembrane and a GPI-anchored form and hence provides a useful
model. We studied CD58 in the human B lymphoblastoid cell line JY. In
addition to mediating adhesion, CD58 is involved in signal
transduction. Incubation of JY cells with immobilized anti-CD58 Abs
results in extensive tyrosine phosphorylation and in secretion of
TNF-
. We demonstrate that CD58 is associated with protein kinase(s)
and with several kinase substrates. We further demonstrate that both
CD58 isoforms are involved. CD58 in JY variant cells, which express
only the transmembrane form, as well as CD58 in JY variant cells, which
express only the GPI-anchored form, are associated with kinase
activity. This association results in a phosphorylation pattern that is
common to the variant and to wild-type JY cells. Thus, these findings
suggest that the capacity of GPI-anchored proteins to interact with
kinases is not always dependent on the GPI anchor itself.
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