The distinct capacity of Fyn and Lck to phosphorylate Sam68 in T cells is essentially governed by SH3/SH2-catalytic domain linker interactions.
Sam68 phosphorylation correlates with Fyn but not Lck expression in T cells. This substrate has been used here to explore the possible basis of the specificity of Fyn versus Lck. We show that this specificity is not based on a spatial segregation of the two kinases, since a chimeric Lck molecule containing the membrane anchoring domain of Fyn does not phosphorylate Sam68. Moreover, a Sam68 molecule targeted to the plasma membrane by the farnesylation signal of c-Ha-Ras remains poorly phosphorylated by Lck. In T cells, Fyn appears to be the active Src kinase in rafts, but Sam68 is not expressed in rafts, and its distinct phosphorylation by Fyn and Lck is not affected by raft dispersion. The Fyn/Lck specificity does not reflect a higher kinase activity of Fyn in general, as both Fyn and Lck are similarly recognized by an anti-active Src antibody. Both also strongly phosphorylate another Src substrate in vivo. Mainly, Lck phosphorylates Sam68 when the interaction between the SH3 domain and the SH2-catalytic domain linker is altered in heterologous Src molecules or after mutating key residues in the linker that increase the accessibility of the SH3 domain. Thus, the distinct potential of Fyn and Lck to phosphorylate Sam68 is likely controlled by the interaction of the kinase SH3 domain with the linker and Sam68, possibly on a competitive binding basis.
Feuillet
Vincent
V
Département de Biologie Cellulaire, INSERM U.567, CNRS UMR 8104, Université René Descartes, Institut Cochin, 75014 Paris, France.
Semichon
Monique
M
Restouin
Audrey
A
Harriague
Julie
J
Janzen
Julia
J
Magee
Anthony
A
Collette
Yves
Y
Bismuth
Georges
G
D048868:Adaptor Proteins, Signal Transducing; D020134:Catalytic Domain; D004268:DNA-Binding Proteins; D006801:Humans; D019169:Jurkat Cells; D019860:Lymphocyte Specific Protein Tyrosine Kinase p56(lck) / Q000378:metabolism*; D010766:Phosphorylation; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D051076:Proto-Oncogene Proteins c-fyn; D016601:RNA-Binding Proteins / Q000378:metabolism*; D013379:Substrate Specificity; D013601:T-Lymphocytes / Q000378:metabolism*; D018909:src Homology Domains
D048868:Adaptor Proteins, Signal Transducing; D004268:DNA-Binding Proteins; C489559:KHDRBS1 protein, human; D011518:Proto-Oncogene Proteins; D016601:RNA-Binding Proteins; C494957:FYN protein, human; D019860:Lymphocyte Specific Protein Tyrosine Kinase p56(lck); D051076:Proto-Oncogene Proteins c-fyn