Human T lymphocyte cAMP‐dependent protein kinase: subcellular distributions and activity ranges of type I and type II isozymes

The role of the type I and type II protein kinase A isozymes in the regulation of human T lymphocyte immune effector functions has not been ascertained. To approach this question, we first characterized the distribution and enzyme activities of the type I and type II protein kinase A (PKA) isozymes in normal, human T lymphocytes. T cells possess both type I and type II isozymes with an activity ratio of 5.0:1 ± 0.71 (mean ± SD). The type I isozyme associates predominately with the plasma membrane whereas the type II isozyme localizes primarily to the cytosol. Analyses of isozyme activities demonstrated that T cells from approximately one‐third of 16 healthy donors exhibited significantly higher type II isozyme activities (higher type II, type II H ) than the remaining donors (lower type II, type II L ) (mean = 605 ± 75 pmol · min –1 · mg protein –1 , P < 0.001). Scatchard analyses of [ 3 H]cAMP binding in the cytosolic fraction demonstrated similar K d values (type II H , 1.1 × 10 –7 M; type II L , 9.0 × 10 –8 M); however, the B max (maximal binding) of the type II H was 400 fmol/mg protein compared to the B max of the type II L of 126 fmol/mg protein. Scatchard analysis of [ 3 H]cAMP binding to the type I isozyme associated with membrane fragments had a K d of 5.6 × 10 –8 M and a B max of 283 fmol/mg protein. Eadie‐Hofstee plots of type II H and type II L gave a K m and V max of 2.3 mg/ml and 1.5 nmol · mg –1 · min –1 , and 2.1 mg/ml and 1.6 nmol · mg –1 · min –1 , respectively. The 3.2‐fold higher maximal binding of the type II isozyme in one‐third of healthy donors may reflect a greater amount of isozyme protein. The compartmentalization of type I PKA isozyme to the plasma membrane and type II PKA isozyme to the cytosol may serve to localize the isozymes to their respective substrates in T lymphocytes.— Hasler, P.; Moore, J. J.; Kammer, G. M. Human T lymphocyte cAMP‐dependent protein kinase: subcellular distribution and activity ranges of type I and type II isozymes. FASEB J. 6: 2735‐2741; 1992.