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 .