Protein tyrosine kinases in the initiation of antigen receptor signaling.

Intracellular responses to antigen receptor engagement involve the activation of protein tyrosine kinases and the tyrosine phosphorylation of cellular proteins, including components of the antigen receptor. Phosphorylation of two tyrosine residues within an 18 amino acid segment of the cytoplasmic domain of antigen receptor subunits, and the subsequent association of either the Syk or Zap protein tyrosine kinase, has recently been shown to be required for successful antigen receptor signal propagation. The recent finding that distinct primary human immunodeficiencies result from mutations in genes encoding two non-transmembrane protein tyrosine kinases underscores the importance of this class of enzyme in antigen receptor signal transduction.