a b s t r a c tWe report an unexpected finding of common structural principles in two unrelated signaling systems: the FAS death domain transformation that initializes the extrinsic apoptotic pathway and signaling by calmodulin bending. The location and design of the hinge is postulated to be a general principle for creating potential signaling event. We suggest that already existing tool can predict the existence of such a hinge and formulate the hypothesis that the internal instabilities designed into the hinge sequences are necessary devices in effective signaling events. Ó 2012 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.FAS receptor initiates signaling of an extrinsic apoptotic response pathway that culminates with the formation of the death inducing signaling complex (DISC) and the subsequent cell death in Type 1 cells. In our recent letter to Nature [1] we reported an X-ray crystal model for the interaction between the death domain of the FAS receptor and death effector domain (DED) of the FADD protein. This observation shed new light onto important apoptotic signaling events and enhanced our understanding of how the individual pieces of the apoptotic machinery work. However, the main question regarding a protein switch (conformational change) was not addressed in the paper. This communication is designed to fill this gap.In a subsequent study, the conformational changes of all proteins in the PDB were reviewed [2]. Our analysis showed that, despite no sequence similarity, the FAS death domain shares a significant structural similarity with calmodulin [3]. Fig. 1 shows the helical fragments of both molecules that superpose particularly well, with RMSD 2.3 Å along 33 residues. As a result, a significant similarity is found between FAS-FADD and calmodulin based on the common conformational transition.Calmodulin has been thoroughly investigated with particular emphasis on the mobility of the protein [4,5]. In calmodulin, a long, stem helix bends during a signaling event and brings two globular helical domains together, closing on a target [3]. The FAS death domain undergoes a transition from the closed to the open state [1] in a similar manner as calmodulin. The significant difference between these systems is which conformation is adopted in the resting state. While the resting conformation of calmodulin is an open state, the resting conformation of FAS death domain is closed.