Mechanism of activation for Zap-70 catalytic activity.
There is a growing body of evidence, including data from human genetic and T-cell receptor function studies, which implicate a zeta-associated protein of M(r) 70,000 (Zap-70) as a critical protein tyrosine kinase in T-cell activation and development. During T-cell activation, Zap-70 becomes associated via its src homology type 2 (SH2) domains with tyrosine-phosphorylated immune-receptor tyrosine activating motif (ITAM) sequences in the cytoplasmic zeta chain of the T-cell receptor. An intriguing conundrum is how Zap-70 is catalytically activated for downstream phosphorylation events. To address this question, we have used purified Zap-70, tyrosine phosphorylated glutathione S-transferase (GST)-Zeta, and GST-Zeta-1 cytoplasmic domains, and various forms of ITAM-containing peptides to see what effect binding of zeta had upon Zap-70 tyrosine kinase activity. The catalytic activity of Zap-70 with respect to autophosphorylation increased approximately 5-fold in the presence of 125 nM phosphorylated GST-Zeta or GST-Zeta-1 cytoplasmic domain. A 20-fold activity increase was observed for phosphorylation of an exogenous substrate. Both activity increases showed a GST-Zeta concentration dependence. The increase in activity was not produced with nonphosphorylated GST-Zeta, phosphorylated zeta, or phosphorylated ITAM-containing peptides. The increase in Zap-70 activity was SH2 mediated and was inhibited by phenylphosphate, Zap-70 SH2, and an antibody specific for Zap-70 SH2 domains. Since GST-Zeta and GST-Zeta-1 exist as dimers, the data suggest Zap-70 is activated upon binding a dimeric form of phosphorylated zeta and not by peptide fragments containing a single phosphorylated ITAM. Taken together, these data indicate that the catalytic activity of Zap-70 is most likely activated by a trans-phosphorylation mechanism.
LoGrasso
P V
PV
Department of Molecular Design and Diversity, Merck Research Laboratories, Rahway, NJ 07065, USA. lograsso@merck.com
Hawkins
J
J
Frank
L J
LJ
Wisniewski
D
D
Marcy
A
A
D000595:Amino Acid Sequence; D015153:Blotting, Western; D004591:Electrophoresis, Polyacrylamide Gel; D004789:Enzyme Activation; D006801:Humans; D007700:Kinetics; D008969:Molecular Sequence Data; D010766:Phosphorylation; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D011948:Receptors, Antigen, T-Cell / Q000378:metabolism*; D051746:ZAP-70 Protein-Tyrosine Kinase
D011948:Receptors, Antigen, T-Cell; D011505:Protein-Tyrosine Kinases; D051746:ZAP-70 Protein-Tyrosine Kinase; C497261:ZAP70 protein, human