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Adult and Fetal Torpedo nAChR

Nicolas Le Novere lenov at pasteur.fr
Mon Jan 15 14:29:14 EST 1996

In article <drg-120196130551 at ache.pharm.pitt.edu>, drg at prophet.pharm.pitt.edu (Duke Groebe) writes:
|> I've been mulling over an issue that I thought I'd present here.  In
|> nicotinic acetylcholine receptors (nAChR) from embryonic muscle, the
|> subunits are alpha, beta, gamma, and delta.  In nAChR from adult muscle,
|> the gamma is replaced by the epsilon subunit.
|> Traditionally, nicotinic receptors from Torpedo electric organ are
|> characterized as having alpha, beta, gamma, and delta subunits.  So my
|> question is, are these the fetal subunits or the adult subunits? 
|> Furthermore, if one were to characterize the nicotinic receptors from
|> actual Torpedo *muscle*, would one find that the gamma subunit had been
|> replaced by an epsilon subunit?  Do embryonic Torpedo nicotinic receptors
|> have the same four subunits as found in the electric organ or is there yet
|> a different combination of subunits?
|> Or is it just a matter of nomenclature?
|> Duke Groebe
|> ******************************************************************************
|> The opinions expressed here are my own.  MY very own and NOBODY else's!  I
|> THOUGHT OF THEM FIRST, HA HA, AND YOU DIDN'T!!  SEE!?  There's my name up
|> there!  And the date - Ooohh, the DATE!  That's important!  HEE HEE, I'M
|> GONNA BE RICH!  DO YOU HEAR ME!?  RICH!!  HA HA!!  They're mine...heh, heh,
|> all mine....  GOD!  I'm so smart!

At present nobody knows if there is an "epsilon" subunit in Torpedo.
In Xenopus, there is a gamma and an epsilon subunit. It seems that the
epsilon and gamma subunit diverged at about the same time than osteichthies
and elasmobranchia (see my paper in the 1995 march issue of Journal of 
Molecular Evolution). It is difficult to say if the Torpedo "gamma" subunit
is a gamma or an epsilon subunit. The electrophysiology of Torpedo a1b1gd
seems to be "epsilon"-like. I think there is two ways to answer your question:

1- To clone the Torpedo subunits with the modern methods (PCR ...) in order
to verify the existence of only 4 muscle subunits. 

2- To clone the ancestor subunit, for instance the Lamprey one, and a "modern"
subunit, let's say the Goldfish or zebrafish one.

Then you have to perform an extremely cautious phylogenetic analysis.

If you got:

			|	|______Zeps
		|	|
		|	|______________T
	|	|
	|	|		_______Xgam
	|	|_______________|
	|			|
	|			|______Zgam

The Torpedo subunit is an epsilon. And so on (you can reconstruct yourself 
every possible topologies and infer the conclusions).
About the terminology, I think gamma is better by default because it is
the lower (in the chain alpha, beta, gamma, delta, epsilon ...)

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