The structure of the Ach receptor from the electric organ of Torpedo has been 
elucidated by Nigel Unwin. His three-dimentional electron chrystallographic analysis at 
9-A resolution revealed that the five subunits of the closed form of the receptor are 
arranged regularly around its central axis. The structure has approximate fivefold 
symmetry, in harmony with the similarity of its five constituent subunits. The receptor 
is cylindrical with a mean diameter of about 65 A. All five rod-shaped subunits span 
the membrane. The receptor protrudes about 60 A on the synaptic side of the 
membrane and  about 20 A on the cytosolic side. The pore of the channel is along its 
symmetry axis. The two Ach binding sites are at the synaptic end of the a subunits, far 
from each other and from the pore.
      The mouth of the channel on the synaptic surface of the receptor is very wide 
(22A). At the level of the outer phospholipid head groups, the lumen abruptly narrows 
to less than 10 A. It widens again to about 20 A at the level tof the inner phospholipid 
head groups. Thus, the channel is tripartite. It contains an extracellular entrance 
domain, a transmembrane domain arranged around a narrow pore, and a cytosolic 
entrance domain. The pore is lined by five a helices, one from each subunits.

       

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