Previous Article | Next Article ![]()
Mol. Cell. Biol., 09 1996, 4996-5003, Vol 16, No. 9
U D'Oro, K Sakaguchi, E Appella and JD Ashwell
The CD45 tyrosine phosphatase has been reported to activate the src family
tyrosine kinases Lck and Fyn by dephosphorylating regulatory COOH-terminal
tyrosine residues 505 and 528, respectively. However, recent studies with
CD45- T-cell lines have found that despite the fact that Lck and Fyn were
constitutively hyperphosphorylated, the tyrosine kinase activity of both
enzymes was actually increased. In the present study, phosphoamino acid
analysis revealed that the increased phosphorylation of Lck in CD45- YAC-1
T cells was restricted to tyrosine residues. To understand the relationship
between tyrosine phosphorylation and Lck kinase activity, CD45- YAC-1 cells
were transfected with forms of Lck in which tyrosines whose phosphorylation
is thought to regulate enzyme activity (Tyr-192, Tyr-394, Tyr-505, or both
Tyr-394 and Tyr-505) were replaced with phenylalanine. While the Y- to-F
mutation at position 192 (192-Y-->F) had little effect, the 505-Y--
>F mutation increased enzymatic activity. In contrast, the 394-Y-->F
mutation decreased the kinase activity to very low levels, an effect that
the double mutation, 394-Y-->F and 505Y-->F, could not reverse.
Phosphopeptide analysis of tryptic digests of Lck from CD45- YAC-1 cells
revealed that it is hyperphosphorylated on two tyrosine residues, Tyr-505
and, to a lesser extent, Tyr-394. The purified and enzymatically active
intracellular portion of CD45 dephosphorylated Lck Tyr-394 in vitro. These
results demonstrate that in addition to Tyr- 505, CD45 can dephosphorylate
Tyr-394, and that in the absence of CD45 the hyperphosphorylation of
Tyr-394 can cause an increase in the kinase activity of Lck despite the
inhibitory hyperphosphorylation of Tyr-505. Therefore, Lck kinase activity
is determined by the balance of activating and inhibitory tyrosine
phosphorylations that are, in turn, regulated by CD45.
Copyright © 1996, American Society for Microbiology
Mutational analysis of Lck in CD45-negative T cells: dominant role of tyrosine 394 phosphorylation in kinase activity
Laboratory of Immune Cell Biology, National Cancer Institute, Bethesda, Maryland 20892, USA.
This article has been cited by other articles:
Copyright © 2009 by the American Society for Microbiology. For an alternate route to Journals.ASM.org, visit: http://intl-journals.asm.org | More Info»