PKA Activity Compartmentation Requires Slow Nuclear Transport Kinetics in Cardiac Myocytes

Abstract

Segregated cytosolic and nuclear cAMP-dependent protein kinase (PKA) activity is important for specifying contractile vs. transcriptional phenotypes in cardiac health and disease. However, the molecular mechanisms selecting for activity in these different compartments remain uncertain. Here, we develop a biochemically mechanistic computational model of cytosolic and nuclear PKA activity to test competing hypotheses for regulating important cytosolic (PLB) and nuclear (CREB) PKA substrates in cardiac myocytes. Our model predicts a sustained 30 min exposure to isoproterenol fully activates PKA in both the cytosol and nucleus, while a transient 2 min exposure fully activates PKA in the cytosol but only achieves 44% max activation in the nucleus. Comparing experimentally measured (28 min) vs. hypothetically fast (2.8 min) PKA diffusion time constants, we show that nuclear PKA activation is rate-limited by translocation of PKA catalytic subunit (but not catalytic enzyme kinetics). In contrast, by blocking PKI-mediated nuclear export, we show that nuclear PKA activity termination is primarily determined by PKA deactivation (and not active nuclear transport). These simulated responses provide quantitative support for the hypothesis that slow nuclear transport kinetics are required for segregated PKA signaling in the cytosol and nucleus.