The mechanism(s) by which extracelullar stimuli are transmitted from the sensory domains of transmembrane receptors to the effector domains are still substantially under debate. Whereas the most widespread model for propagation of the stimulus is the piston model, in which the axial displacement of one receptor subunit relative to the other activates the effector domain, we have put forward the cogwheel model, in which axial rotation of the helices between two coiled-coil packing modes leads to the activation of the effector domain by a constrain-and-release mechanism. In this mechanism, the coiled-coil backbone of the receptor sequesters the catalytic effector domains in an inactive conformation, until axial rotation of its helices releases the effector domains to assume a catalytically productive conformation. We have recently shown that this model applies not only to histidine kinases, but also to adenylyl cyclases