SH2 domain-mediated targeting, but not localization, of Syk in the plasma membrane is critical for Fc epsilon RI signaling

Aggregation of the high-affinity IgE receptor induces the tyrosine phosphor ylation of subunits of the receptor and the subsequent association with the receptor of the cytosolic protein tyrosine kinase Syk, The current experim ents examined the functional importance of membrane association of Syk and the role of the SH2 domain in receptor-mediated signal transduction. Wild-t ype Syk and chimeric Syk molecules with the c-Src myristylation sequence at the amino terminus were expressed in a Syk-negative mast cell line. Chimer ic Syk with the myristylation sequence was membrane associated, and a small fraction was constitutively colocalized with Fc epsilon RI, Lyn, and LAT ( linker for T-cell activation) in the glycolipid-enriched microdomains or ra fts. However, even under these conditions, the tyrosine phosphorylation of Syk and the downstream propagation of signals required FceRI aggregation. T his chimeric Syk was less active than wild-type Syk in Fc epsilon RI-mediat ed signal transduction. In contrast, a truncated membrane associated form o f Syk that lacked the SH2 domains was not tyrosine phosphorylated by recept or aggregation and failed to transduce intracellular signals. These finding s suggest that SH2 domain-mediated membrane translocation of Syk is essenti al for the FceRI-mediated activation of Syk for downstream signaling events leading to histamine release. Furthermore, the localization of Syk in glyc olipid-enriched microdomains by itself is not enough to generate or enhance signaling events.(Blood. 2001;97:1352-1359) (C) 2001 by The American Socie ty of Hematology.