Back in 2008 Noha Mesbha published her excellent PhD thesis
ANAEROBIC HALOPHILIC ALKALITHERMOPHILES: DIVERSITY AND PHYSIOLOGICAL ADAPTATIONS TO MULTIPLE EXTREME CONDITIONS
which introduced the world, via this paper
The halophilic alkalithermophile Natranaerobius thermophilus adapts to multiple environmental extremes using a large repertoire of Na+(K+)/H+ antiporters
to Natranaerobius thermophilus and its antiporter nt-Nha. Which gives every impression of being a stand alone Na/H+ antiporter very closely related to the invariably operon controlled MrpA protein (named shaA) of Clostridium tetani. As she says
"Gene nt-Nha had 35% identity to the shaA (mrpA) gene of Clostridium tetani. The Mrp proteins belong to the monovalent cation/proton antiporter-3 protein family. ...Sequence analysis of the regions surrounding gene nt-Nha, however, did not show that it was part of an operon. This indicates that gene nt-Nha does not encode a subunit of an Mrp system, but rather a mono-subunit antiporter".
All well and good.
Then in 2010 Morino et al published
Single Site Mutations in the Hetero-oligomeric Mrp Antiporter from Alkaliphilic Bacillus pseudofirmus OF4 That Affect Na+/H+ Antiport Activity, Sodium Exclusion, Individual Mrp Protein Levels, or Mrp Complex Formation
Although the whole paper was about B subtilis (and how none of its Mrp subunits worked in any incomplete combination to antiport anything) they did make this throw-away comment:
"A MrpA/MrpD homologue encoded by a “stand alone” gene from polyextremophilic Natranaerobius thermophilus was recently reported to exhibit Na+/H+ and K+/H+antiport activity in anaerobically grown E. coli KNabc (24)"
where (24) is Mesbha's PhD paper. Notice that at this stage Mesbha's
nt-Nha ~ shaA, very closely related at 35% conserved gene sequence,
has been changed by Morino in to
nt-Nha ~ An "MrpA/MrpD homologue".
This is a just about acceptable per se because MrpA and MrpD are homologous to each other and nt-Nha is closely related to MrpA (shaA) of C tetani. But Mesbha herself never mentions MrpD in her 2009 paper or in her PhD thesis. And "MrpA/MrpD" is open to mis-interpretation. So we have "definition-creep" here, where nt-Nha could be accidentally seen as some sort of composite of MrpA in combination with MrpD. Ouch.
So next we have the 2017 offering by Jasso-Chávez et al
Functional Role of MrpA in the MrpABCDEFG Na+/H+ Antiporter Complex from the Archaeon Methanosarcina acetivorans
where we have this bizarre statement
"On the other hand, a fused MrpA-MrpD homolog in the alkaliphilic Natranaerobius thermophilus displayed Na+/H+ antiport activity when produced in E. coli strain KNabc (5, 28)"
Ref (5) is Morino's paper on B subtilis Mrp, in which one rather misleading citation suggests that nt-Nha is an "MrpA/MrpD homologue". This has developed to the extent that nt-Nha has now "become" a fusion of two genes to give a rather mythical monster.
Ref (28) is just Mesbha's PhD paper in which nothing of the sort was suggested.
So the Jasso-Chávez paper is utterly flawed due to misinterpreting a poorly phrased statement and adding an erroneous modification so as to grossly mis-represent an initial very solid finding by Mesbha. The Jasso-Chávez discussion of nt-Nha can be distilled as:
"Send three and fourpence, we're going to a dance".
The chance of their understanding how nt-Nha or their very own archaeal MrpA subunit work as a stand-alone antiporter appears to be approximately zero.
BTW The folks who worked from the actual gene to model the nt-Nha protein structure suggest that
"The final model presents 13 transmembrane α-helices organized in a similar arrangement as the NuoL subunit".
You know the picture but here it is again 'cos I think it's lovely