Differential intrinsic enzymatic activity of Syk and Zap-70 protein-tyrosine kinases.
Syk and Zap-70 are related protein-tyrosine kinases implicated in antigen and Fc receptor signaling. While Zap-70 is restricted to T-cells and natural killer cells, Syk accumulates in B-cells, mast cells, platelets, and immature T-cells. In addition, we found that an isoform of Syk (SykB), which carries a 23-amino acid deletion in the "linker" region, is prominently expressed in bone marrow. To better understand the relative impact of Syk, SykB, and Zap-70 on signal transduction, we compared their intrinsic enzymatic properties in transiently transfected COS-1 cells and in hemopoietic cells. Using modified versions of these enzymes bearing a common Myc epitope at the amino terminus, we determined that the ability of Syk and SykB to undergo autophosphorylation and to phosphorylate erythrocyte band 3 in immune complex kinase reactions was at least 100-fold greater than that of Zap-70. Similarly, Syk and SykB, but not Zap-70, caused prominent tyrosine phosphorylation of p120(c-)cbl in COS-1 cells. A similar pattern of activity was also noted for endogenous Syk and Zap-70 from hemopoietic cells. To understand the structural basis for these characteristics, we also created and analyzed a series of chimeras between Syk and Zap-70. These studies indicated that the catalytic domain of Syk and Zap-70, but not their SH2 domains, linker region or carboxyl-terminal tail, was responsible for their respective activity. Taken together, these data demonstrated that the intrinsic enzymatic activity of Syk and SykB is superior to that of Zap-70 and that such a distinction relates to structural variations in the catalytic domain.
Latour
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McGill Cancer Centre, McGill University, Montréal, Canada H3G 1Y6.
Chow
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LM
Veillette
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D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001853:Bone Marrow / Q000201:enzymology; D002460:Cell Line; D004792:Enzyme Precursors / Q000378:metabolism*; D006412:Hematopoietic Stem Cells / Q000201:enzymology; D047908:Intracellular Signaling Peptides and Proteins; D007527:Isoenzymes / Q000378:metabolism; D051379:Mice; D008969:Molecular Sequence Data; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D000072377:Syk Kinase; D051746:ZAP-70 Protein-Tyrosine Kinase
D004792:Enzyme Precursors; D047908:Intracellular Signaling Peptides and Proteins; D007527:Isoenzymes; D011993:Recombinant Fusion Proteins; D011505:Protein-Tyrosine Kinases; D000072377:Syk Kinase; C000606897:Syk protein, mouse; D051746:ZAP-70 Protein-Tyrosine Kinase; C497262:Zap70 protein, mouse