Wednesday, March 06, 2019

Life (23) Antiporting: Choose your ion well

Sorry to go on about Yu et al's paper

Structure of an Ancient Respiratory System

but there's an awful lot in it. Here again is their working model of the MBH from Pyrococcus furiosus:

The Na+ channel on the right hand end shown as MbhC was looked at in great detail in the paper and is very convincingly a Na+ channel. The group never looked at the actual structure of the unpowered antiporter Mrp so they are working with other people's assumptions, some of which are quite likely better than others as regards the nature of the ion favoured by a particular channel. So here is the membrane arm of MBH they ended up with, labelled to represent the unpowered original Mrp antiporter. Obviously the hydrogenase has been replaced by the MrpA N-terminal:

Now the problem with this, apart from the highly electrogenic charge exchange of three protons in for one Na+ out, is that the left hand MrpA module is actually a Na+ channel. There's lots of pretty convincing evidence for this but I won't waffle on any more about it. So lets set up the Mrp antiporter as a pure H+/Na+ exchanger like this:

Two protons inwards, two Na+ antiported outwards, what could be nicer? This is the structure favoured by group in Sweden comparing MrpA and MrpD with the antiporter-like subunits of complex I.

Functional Differentiation of Antiporter-Like Polypeptides in Complex I; a Site-Directed Mutagenesis Study of Residues Conserved in MrpA and NuoL but Not in MrpD, NuoM, and NuoN

 I think it might be completely wrong.

I like this paper but I'm still left with a niggling thought that having two antiporters in one complex, apparently both doing the same thing, is just a little wasteful. The normal bacterial approach is one where the jettisoning of genes is developed to a fine art.

So Yu et al and Sperling et al have rather different ideas about the function of various channels in the Mrp complex. No one is really talking about why there should be four channels...

The modern Mrp antiporter is very interesting in it's own right. It needs a little post of its own.


No comments: