Protein Kinase C δ-specific Activity Reporter Reveals Agonist-evoked Nuclear Activity Controlled by Src Family of Kinases

Conventional and novel protein kinase C (PKC) isozymes transduce the abundance of signals mediated by phospholipid hydrolysis; however redundancy in regulatory mechanisms confounds dissecting the unique signaling properties of each of the eight isozymes constituting these two subgroups. Previously, we created a genetically encoded reporter (C kinase activity reporter (CKAR)) to visualize the rate, amplitude, and duration of agonist-evoked PKC signaling at specific locations within the cell. Here we designed a reporter, delta CKAR, that specifically measures the activation signature of one PKC isozyme, PKC delta, in cells, revealing unique spatial and regulatory properties of this isozyme. Specifically, we show two mechanisms of activation: 1) agonist-stimulated activation at the plasma membrane (the site of most robust PKC delta signaling), Golgi, and mitochondria that is independent of Src and can be triggered by phorbol esters and 2) agonist-stimulated activation in the nucleus that requires Src kinase activation and cannot be triggered by phorbol esters. Translocation studies reveal that the G-protein-coupled receptor agonist UTP induces the translocation of PKC delta into the nucleus by a mechanism that depends on the C2 domain and requires Src kinase activity. However, translocation from the cytosol into the nucleus is not required for the Src-dependent regulation of nuclear activity; a construct of PKC delta prelocalized to the nucleus continues to be activated by UTP by a mechanism dependent on Src kinase activity. These data identify the nucleus as a signaling hub for PKC delta that is driven by receptor-mediated signaling pathways (but not phorbol esters) and differs from signaling at plasma membrane and Golgi in that it is controlled by Src family kinases.