Figure 1.

Principles of TM signaling. (A) Two TM helices can form transient interactions in a membrane (translational motion), which results in interactions of receptor intra- and extracellular domains. (B) Movements of interacting TM helices perpendicular to the membrane plane (piston motion) result in structural changes in the contacts between intra- and extracellular domains. (C) A change in the crossing angle of two TM helices (pivot motion) also leads to a reorientation of water soluble receptor domains. (D) Rotations around the helix axis of two interacting TM helices lead to a rotation of the outer-membrane regions and positions the soluble domains differently. All these motions of TM helices would enable the transfer of a signal from one to the other surface of a membrane.