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Endo‐ and exocytic rate constants for spontaneous and protein kinase C‐activated T cell receptor cycling

To determine the rate constants of spontaneous and activated TCR cycling, we examined TCR endo‐ and exocytosis in the human T cell line Jurkat by three different methods. Using a simple kinetic model for TCR cycling and non‐linear regression analyses, we found that the spontaneous endocytic rate constant of the TCR was low (≈ 0.012 min‐1) whereas the spontaneous exocytic rate constant was similar to that of other cycling receptors (≈ 0.055 min‐1). Following protein kinase C activation (PKC) the endocytic rate constant was increased tenfold (to ≈ 0.128 min‐1) whereas the exocytic rate constant was unaffected. Thus, the TCR becomes a rapidly cycling receptor with kinetics similar to classical cycling receptors subsequent to PKC activation. This results in a reduction of the half‐life of cell surface expressed TCR from ≈ 58 to 6 min and allows rapid redistribution of the TCR during T cell activation.