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J. Cell Biol.,
Volume 145, Number 3, May 3, 1999 457-468
MRC Laboratory for Molecular Cell Biology and Department of Biochemistry, University College London, London WC1E
6BT, United Kingdom
The Src-related tyrosine kinase p56lck (Lck)
is primarily expressed in T lymphocytes where it localizes to the cytosolic side of the plasma membrane and
associates with the T cell coreceptors CD4 and CD8. As
a model for acylated proteins, we studied how this localization of Lck is achieved. We followed newly synthesized Lck by pulse-chase analysis and found that
membrane association of Lck starts soon after synthesis, but is not complete until at least 30-45 min later.
Membrane-binding kinetics are similar in CD4/CD8-positive and CD4/CD8-negative cells. In CD4-positive T cells, the interaction with CD4 rapidly follows membrane association of Lck. Studying the route via which
Lck travels from its site of synthesis to the plasma
membrane, we found that: CD4 associates with Lck
within 10 min of synthesis, long before CD4 has
reached the plasma membrane; Lck associates with intracellular CD4 early after synthesis and with cell surface CD4 at later times; and transport of CD4-bound
Lck to the plasma membrane is inhibited by Brefeldin
A. These data indicate that the initial association of
newly synthesized Lck with CD4, and therefore with
membranes, occurs on intracellular membranes of the
exocytic pathway. From this location Lck is transported
to the plasma membrane.
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