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Original Article |
Brdi
kaa,
Dagmar Pavli
továa,
Albrecht Leob,
Eddy Bruynsb,
Vladimír Ko
íneka,
Pavla Angelisováa,
Jeanette Schererb,
Andrej Shevchenkoc,
Anna Shevchenkoc,
Ivan Hilgerta,
Jan
ern
a,d,
Karel Drbala,
Yasuhiro Kuramitsub,
Birgit Kornackerb,
Václav Ho
ej
ía,d, and
Burkhart Schravenb
Correspondence to: Burkhart Schraven, Institute for Immunology, Immunomodulation Laboratory, Ruprecht-Karls University Heidelberg, Im Neuenheimer Feld 305, 69120 Heidelberg, Germany. Tel:6221-56-4059 Fax:6221-56-5541 E-mail:m53{at}popix.urz.uni-heidelberg.de.
According to a recently proposed hypothesis, initiation of signal transduction via immunoreceptors depends on interactions of the engaged immunoreceptor with glycosphingolipid-enriched membrane microdomains (GEMs). In this study, we describe a novel GEM-associated transmembrane adaptor protein, termed phosphoprotein associated with GEMs (PAG). PAG comprises a short extracellular domain of 16 amino acids and a 397-amino acid cytoplasmic tail containing ten tyrosine residues that are likely phosphorylated by Src family kinases. In lymphoid cell lines and in resting peripheral blood
/ß T cells, PAG is expressed as a constitutively tyrosine-phosphorylated protein and binds the major negative regulator of Src kinases, the tyrosine kinase Csk. After activation of peripheral blood
/ß T cells, PAG becomes rapidly dephosphorylated and dissociates from Csk. Expression of PAG in COS cells results in recruitment of endogenous Csk, altered Src kinase activity, and impaired phosphorylation of Src-specific substrates. Moreover, overexpression of PAG in Jurkat cells downregulates T cell receptormediated activation of the transcription factor nuclear factor of activated T cells. These findings collectively suggest that in the absence of external stimuli, the PAGCsk complex transmits negative regulatory signals and thus may help to keep resting T cells in a quiescent state.
Key Words: signal transduction, lymphocytes, Src family kinases, phosphorylation, Csk
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