Proline residues in CD28 and the Src homology (SH)3 domain of Lck are required for T cell costimulation

The Src family tyrosine kinases Lck and Fyn are critical for signaling via the T cell receptor. However, the exact mechanism of their activation is un known. Recent crystal structures of Src kinases suggest that an important m echanism of kinase activation is via engagement of the Src homology (SH)3 d omain by proline-containing sequences. To test this hypothesis, we identifi ed several T cell membrane proteins that contain potential SH3 ligands. Her e we demonstrate that Lck and Fyn can be activated by proline motifs in the CD28 and CD2 proteins, respectively. Supporting a role for Lck in CD28 sig naling, we demonstrate that CD28 signaling in both transformed and primary T cells requires Lck as well as proline residues in CD28. These data sugges t that Lck plays an essential role in CD28 costimulation.