Proline Residues in Cd28 and the Src Homology (Sh)3 Domain of Lck Are Required for T Cell Costimulation

Abstract

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 unknown. Recent crystal structures of Src kinases suggest that an important mechanism of kinase activation is via engagement of the Src homology (SH)3 domain by proline-containing sequences. To test this hypothesis, we identified several T cell membrane proteins that contain potential SH3 ligands. Here 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 signaling, we demonstrate that CD28 signaling in both transformed and primary T cells requires Lck as well as proline residues in CD28. These data suggest that Lck plays an essential role in CD28 costimulation.

Genetically Encoded Förster Resonance Energy Transfer Sensors for the Conformation of the Src Family Kinase Lck

“…Knockout mice for TSAd exhibit severely impaired proximal TCR signaling due to lower Lck kinase activity (47). Also, the classical costimulatory molecule CD28 carries proline motifs in the cytoplasmic domain that were demonstrated to activate Lck (48) and recruit it to the immunological synapse (49). A similar costimulatory capacity of Lck activity was shown for integrins.…”

The coreceptor CD2 uses plasma membrane microdomains to transduce signals in T cells

“…The proline-rich cytoplasmic domain of CD2 might be a direct Lck activator (by binding the autoinhibitory SH3 domain in Lck; Bell et al, 1996), but high local concentrations may be required to compete with this tethered intramolecular interaction. A similar model could apply to activation of Fyn (Holdorf et al, 1999). High local concentrations allows CD58-mediated microdomain formation to occur.…”

CD28 co‐signaling in the adaptive immune response

“…At the same time, a distal proline-rich P 187 YAP region has been implicated. This proline-rich site has been reported to bind to the SH3-domain of src kinase p56 lck , Grb2 and filamin A (FLNa) 61,76,94,95 (Fig. 4).…”