Specific activation of Src family kinases by T cell costimulatory molecules

T cell activation is initiated by the activation of Src family kinases Lck and Fyn. Recent crystal structures of Src and Hck demonstrate that engagement of the SH3 domain is involved in kinase regulation. As SH3 domains bind PXXP motifs, we examined whether such sequences might be present in the cytoplasmic tails of membrane proteins known to be involved in T cell activation. The T cell membrane proteins, CD2, CD28 and CD3e all contain potential SH3 binding sites. Peptide analogs to the proline regions of CD2, CD28, and CD3e were generated and tested for their ability to activate Lck and Fyn in an in vitro kinase assay. A CD28 peptide corresponding to the C-terminal 20 residues was capable of activating Lck while a C terminal proline peptide of CD2 activated Fyn. These observations suggest a model by which the proximal events of T cell receptor signalling involve clustering of proline rich transmembrane molecules to activate tyrosine kinases.