The protein kinase C (PKC) family of serine and threonine kinases, consisting of three distinctly regulated subfamilies, has been established as critical for various cellular functions. However, how PKC enzymes are regulated at different subcellular locations, particularly at emerging signaling hubs, is unclear. Here we present a sensitive excitation ratiometric C kinase activity reporter (ExRai-CKAR2) that enables the detection of minute changes (equivalent to 0.2% of maximum stimulation) in subcellular PKC activity. Using ExRai-CKAR2 with an enhanced diacylglycerol (DAG) biosensor, we uncover that G-protein-coupled receptor stimulation triggers sustained PKC activity at the endoplasmic reticulum and lysosomes, differentially mediated by Ca(2+)-sensitive conventional PKC and DAG-sensitive novel PKC, respectively. The high sensitivity of ExRai-CKAR2, targeted to either the cytosol or partitioning defective complexes, further enabled us to detect previously inaccessible endogenous atypical PKC activity in three-dimensional organoids. Taken together, ExRai-CKAR2 is a powerful tool for interrogating PKC regulation in response to physiological stimuli.