Biochemical and genetic analyses of the Tec kinases Itk and Rlk/Txk.
The Tec kinases have been implicated as important components of signalling pathways downstream of lymphocyte antigen receptors. Activation of these kinases requires two steps: (i) phosphorylation by Src family kinases and (ii) plasma membrane localization, which is mediated by interaction between the pleckstrin homology (PH) domains of Tec kinases and the products of phosphoinositide-3 kinase (PI-3K). Itk and Rlk/Txk are Tec kinases expressed in T-lymphocytes. Despite similarity to other Tec kinases, Rlk/Txk lacks a PH domain and instead possesses a palmitoylated cysteine-string motif. We have found that both Rlk/Txk and Itk are phosphorylated in response to T-cell receptor stimulation and can be activated by phosphorylation by Src family kinases. However, consistent with its lack of PH domain, Rlk/Txk is phosphorylated independent of PI-3K activity. Furthermore, we demonstrated that like Itk, Rlk/Txk is associated with lipid RAFTs (detergent-insoluble, cholesterol-rich regions of the membrane), but unlike Itk, Rlk/Txk's RAFT association is independent of PI-3K activity. Despite these differences, Rlk/Txk partially compensates for loss of Itk in gene-targeted animals, suggesting overlapping functions for these kinases.
Czar
M J
MJ
National Human Genome Research Institute, National Institutes of Health, Bethesda, MD 20814, USA.
Debnath
J
J
Schaeffer
E M
EM
Lewis
C M
CM
Schwartzberg
P L
PL
D000818:Animals; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D021962:Membrane Microdomains / Q000378:metabolism; D009154:Mutation; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011505:Protein-Tyrosine Kinases / Q000737:chemistry* / Q000235:genetics*
C089848:Tec protein-tyrosine kinase; D011505:Protein-Tyrosine Kinases; C080813:emt protein-tyrosine kinase