A modified luciferase protein which is a sensor for molecules including cAMP, cGMP, calcium, chelators thereof, kinases, or phosphatases is provided. Also provided is a circularly permuted anthozoan luciferase protein and a decapod crustacean luciferase protein, optionally containing one or more heterologous amino acid sequences, at least one of which directly or indirectly interacts with a molecule of interest. Further provided is a modified anthozoan luciferase protein and a decapod crustacean luciferase protein containing an insertion of one or more heterologous amino acid sequences, at least one of which directly or indirectly interacts with a molecule of interest.