Serine Phosphorylation of a 67-kDa Protein in Human T Lymphocytes Represents an Accessory Receptor-mediated Signaling Event

Phosphorylation on serine residues of a 67-kDa cytoplasmic protein (p67) occurs as an early signaling event after stimulation of resting human T cells via CD2 but not via TCR/CD3. Phosphorylation of p67 is, however, not restricted to CD2 stimulation because it can also be induced by costimulation through CD4 and CD8 coreceptors when approximated to the TCR-CD3 complex, suggesting a common signaling mechanism for these accessory receptors of the Ig superfamily. Because late functional responses of T cell activation like IL-2 production and DNA synthesis correlate with phosphorylation of p67, this intracellular event may represent an accessory receptor-mediated costimulatory second signal for human T cell activation. The biochemical characteristics (m.w. and isoelectric point) of p67 are identical with those of the actin binding protein L-plastin (Fimbrin), a cytoplasmic protein that contains two Ca2+ binding sites and a calmodulin binding domain. Moreover, p67 reacts specifically with an L-Plastin (Fimbrin) antiserum.