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ZAP-70 is a protein tyrosine kinase (PTK) that associates with the ζ subunit of the T cell antigen receptor (TCR) and undergoes tyrosine phosphorylation following TCR stimulation. ZAP-70 contains two SH2-like domains with the PTK domain being at the C-terminus. It appears that both ZAP-70 and Syk are recruited to the phosphorylated CD3 and ζ subunits after TCR stimulation. The src-family PTKs, lck, and fyn, seem to play a role upstream of ZAP-70 and Syk where they phosphorylate CD3 and ζ. Thus both Src- and ZAP-70/Syk-family PTKs are critical for efficient induction of tyrosine phosphoproteins in heterologous cells. The significance of ZAP-70 in mediating TCR signal transduction has been confirmed by showing that ZAP-70 activity is absent in an autosomal recessive form of severe combined immunodeficiency (SCID). This is due to mutations affecting the ZAP-70 kinase domain which affect the stability of the protein. The levels of lck and fyn appear to be normal in the same cells. Results of the ZAP-70 mutations in these patients include an unusual absence of peripheral CD8 T cells and an abundance of CD4 T cells in peripheral lymphoid organs. These results indicate that these two populations of T cells develop through utilization of different intracellular signaling pathways.Immunofluorescence, Immunohistochemistry, Immunoprecipitation, Western Blotting
