Figure 2: Molecular architecture of gap junctions. Gap junctions are grouped in plaques at the cell plasma membrane surface and are composed of twelve connexin proteins, organized as two hexameric hemichannels of two apposed cells. The connexin protein as such is organized as four membrane-spanning domains (TM1-4), two extracellular loops (EL1-2), one cytoplasmic loop (CL), one cytoplasmic amino tail (NT) and one cytoplasmic carboxy tail (CT).