The Immune Receptor Family

Immune receptors are a unique family of receptor complexes typically made up of integral membrane proteins of the immunoglobulin (Ig) superfamily that are involved in ligand recognition, associated with other transmembrane signaling proteins that have unique tyrosine-containing motifs in their cytoplasmic tails. Whereas the signaling components are generally separate proteins from those involved in ligand recognition, in a few members of the family, the receptor consists of a single chain in which the extracellular domain is involved in ligand recognition and the cytoplasmic tail contains tyrosine residues that contribute to signaling. The signaling proteins of the immune receptor family are often positioned close to non-receptor tyrosine kinases of the Src family. The latter also possess N-terminal lipid anchors that tether them to the inner leaflet of the plasma membrane. The cytoplasmic tyrosine-containing motifs on the signaling proteins of the immune receptor family are generally one of two different types. ITAMs (immunoreceptor tyrosine-based activating motifs) are found on receptors involved in cell activation and have the sequence YxxL/I(x)6-8YxxL/I, where Y represents a tyrosine residue, L represents leucine, I represents isoleucine, and x refers to any amino acid. ITAM motifs can be phosphorylated on both tyrosine residues that are present in this motif by Src family kinases when immune receptors are activated. Tyrosine-phosphorylated ITAMs recruit a distinct tyrosine kinase of the Syk/ZAP-70 family, which contains tandem SH2 domains that each bind to one of the two phosphorylated YxxL/I motifs of the ITAM. Binding of Syk (or ZAP-70) to

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