SOLUTION STRUCTURE OF THE C-TERMINAL SH2 DOMAIN OF THE HUMAN TYROSINEKINASE SYK COMPLEXED WITH A PHOSPHOTYROSINE PENTAPEPTIDE

Background: Recruitment of the intracellular tyrosine kinase Syk to ac tivated immune-response receptors is a critical early step in intracel lular signaling. In mast cells, Syk specifically associates with doubl y phosphorylated immunoreceptor tyrosine-based activation motifs (ITAM s) that are found within the IgE receptor. The mechanism by which Syk recognizes these motifs is not fully understood. Both Syk SH2 (Src hom ology 2) domains are required for high-affinity binding to these motif s, but the C-terminal SH2 domain (Syk-C) can function independently an d can bind, in isolation, to the tyrosine-phosphorylated IgE receptor in vitro. In order to improve understanding of the cellular function o f Syk, we have determined the solution structure of Syk-C complexed wi th a phosphotyrosine peptide derived from the gamma subunit of the IgE receptor. Results: The Syk-C:peptide structure is compared with ligan ded structures of both the SH2 domain of Src and the C-terminal SH2 do main of ZAP-70 (the 70 kDa zeta-associated protein). The topologies of these domains are similar, although significant differences occur in the loop regions. In the Syk-C structure, the phosphotyrosine and leuc ine residues of the peptide ligand interact with pockets on the protei n, and the intervening residues are extended. Conclusions: Syk-C resem bles other SH2 domains in its peptide-binding interactions and overall topology, a result that is consistent with its ability to function as an independent SH2 domain in vitro. This result suggests that Syk-C p lays a unique role in the intact Syk protein. The determinants of the binding affinity and selectivity of Syk-C may reside in the least-cons erved structural elements that comprise the phosphotyrosine- and leuci ne-binding sites. These structural features can be exploited for the d esign of Syk-selective SH2 antagonists for the treatment of allergic d isorders and asthma.