Intercellular communication via gap junction membrane channels cannot occur until two apposing hemichannels (connexons) meet and dock to form a sealed cell-cell conduit. In particular, an important question is how does the structure at the extracellular surface influence the molecular recognition of the two connexons. In this study, cryoelectron microscopy and computer modeling provide evidence that the formation of the gap junction intercellular channel requires a 30 degree rotation between hemichannels for proper docking. With this amount of rotation, the peaks (protrusions) on one connexon fit into the valleys of the apposed connexon in the 3-D model, which would make for an ionically tight interface necessary for a functional cell-cell channel. Docking appears to be governed by a "lock and key" mechanism via a simple interdigitation of the six protrusions from each connexon. This interdigitation increases significantly the contact surface area and potential number of hydrogen bonds or hydrophobic interactions and/or other attractive interactions. Having a larger surface area than if the surfaces were flat would explain the biochemical requirements for conditions characterized previously for splitting of channels into hemichannels. The docked connexons were computationally fitted into two gap junction structures, which further confirmed the interdigitated manner of docking.