We report a general strategy for creating protein kinases in mammalian cells that are poised for very rapid activation by light. By photoactivating a caged version of MEK1, we demonstrate the specific, rapid, and receptor independent activation of an artificial subnetwork within the Raf/MEK/ERK pathway. Time-lapse microscopy allowed us to precisely characterize the kinetics of elementary steps in the signaling cascade and provided insight into adaptive feedback and rate-determining processes in the pathway.