The Molecular History of Eukaryotic Life  Ion Channels 


David Nelson  Dec. 13, 2000

	Ion channels include neurotransmitter activated channels like the acetylcholine 
receptor (ligand gated), voltage gated channels and mechanosensitive channels that 
respond to changes in membrane properties that could be altered by mechanical forces 
caused by high or low osmolarity conditions or shear forces.  The ligand gated channels 
like the acetylcholine receptor are found at synapses and these are nerve specific.  They 
are not found outside animals.  

	The mechanosensitive channels are found widely in nature in bacteria, 
archaebacteria and eukaryotes.  These are the simplest channels, since thay do not need a 
ligand binding site or a voltage sensor.  They do have to exist in at least two states 
(open and closed) with possibilities for additional substates.  The nature of channels is 
determined by patch clamp techniques, where the size of the channel is estimated from the 
current it passes and the duration of open and closed states can be measured.  A 
mechanosensitive channel has been purified from the archaeon Haloferax volcanii.  It was 
only 37kDa, much smaller than vertebrate ion channels that can be about 2000 amino acids 
and 200kDa.  

	Voltage gated ion channels include the porins of bacteria, mitochondria and 
archaea.  However, porins are quite different from the nerve based ion channels for Na, K 
and Ca. These channels have alpha helical transmembrane segments, while porins have a 
beta barrel structure.  The two protein superfamilies must have separate evolutionary 
origins.  Among the K, Na and Ca channels, the K channel appears to be the oldest.  It is 
a tetramer of identical subunits, whereas the Na and Ca channels have four domains, each 
similar to one K channel.  K channels have been found in bacteria, in 1998 a 
crystal structure was determined of the Streptomyces lividans K channel.  It is 
tetrameric, with each identical subunit contributing two transmembrane segments.  The 
pore is in the center.  It made the cover of Science (Vol. 280, 69-77 April 3 issue).  
Some eukaryotic K channels have six transmembrane segments rather than two, but the first 
four seem to be extra.  The last two transmembrane segments are very similar to the 
bacterial K channel.  The evolution of Na and Ca channels probably started from a K 
channel.  Recently, a Na channel has been cloned from a jellyfish (Cnidarian). These are 
the most primitive organisms with nerves.

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