Specificity of signal transduction through CD16, TCR-CD3 and BCR receptor chains containing the tyrosine-associated activation motif.

FcR gamma III (CD16) is a member of a family of receptor proteins that bind to Fc regions of IgG molecules. Ligation of CD16 transduces signal and alters tyrosine phosphorylation of proteins in natural killer, B and T cells. However, the identity of protein tyrosine kinases (PTKs) involved in CD16 signaling and the mode of their interaction with the receptor are not completely understood. Here, we show that the transmembrane form of CD16 transfected in a CD3- Jurkat cell line associates with p56lck and ZAP-70 PTKs. Using chimeric proteins consisting of the extracellular domain of CD16 and the cytoplasmic portions of the zeta chain, we find that these PTKs interact with the 17-residue activation motif present in the zeta chain. Substitution of tyrosine residues in the motif with phenylalanine blocks the interaction of PTKs and abrogates signal transduction. Comparative studies demonstrate that similar to zeta, MB1 protein of the IgM receptor also binds to p56lck and ZAP-70 in T cells and induces strong activation responses, whereas CD3 gamma, delta and epsilon chains do not interact with ZAP-70 and transduce weak signals. Together, these results demonstrate that: (i) PTKs bound to CD16 mediate signal transduction, (ii) ZAP-70 plays a crucial role in signaling and (iii) apart from tyrosine residues in the activation motif, other structural constraints also regulate the interaction of PTKs with the receptor molecule.