Phosphorylation of T cell membrane proteins by activators of protein kinase C

Activation of the enzyme protein kinase C (PKC) plays an important role in T cell activation. We investigated the phosphorylation of CD2, CD3, CD4, CD5, CD7, CD8, CD28 (Tp44), CD43 (sialophorin, gp115), and LFA-1 after incubation of human PBMC with the (PKC) activator PMA. These proteins were chosen for their role in transmembrane signal transduction (CD2, CD3, CD5, CD28, CD43), cell-cell interaction and adhesion (CD2, CD4, CD8, and LFA-1), or involvement in immunodeficiency states (CD43, CD7). CD5, CD7, CD43, and the alpha-chain of LFA-1 were found to be constitutively phosphorylated. PMA induced rapid hyperphosphorylation of CD5, CD7, and CD43, but not of the LFA-1 alpha-chain, and induced the phosphorylation of CD3, CD4, CD8 and of the LFA-1 beta-chain. PMA did not cause the phosphorylation of CD2 and CD28. PMA-induced phosphorylation was partially inhibited by the PKC inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine dihydrochloride. Finally, the T cell activator Con A, which binds to the CD3/TCR complex was shown to induce a profile of protein phosphorylation similar to that observed with PMA. We conclude that PKC-mediated phosphorylation of T cell Ag may represent an important regulatory mechanism that governs the process of T cell activation.