Membrane-integral pyrophosphatase subfamily capable of translocating both Na+ and H+статья
Статья опубликована в высокорейтинговом журнале
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Дата последнего поиска статьи во внешних источниках: 19 июля 2013 г.
Аннотация:One of the strategies used by organisms to adapt to life under conditions
of short energy supply is to use the by-product pyrophosphate
to support cation gradients in membranes. Transport reactions
are catalyzed by membrane-integral pyrophosphatases
(PPases), which are classified into two homologous subfamilies:
H+-transporting (found in prokaryotes, protists, and plants) and
Na+-transporting (found in prokaryotes). Transport activities have
been believed to require specific machinery for each ion, in accordance
with the prevailing paradigm in membrane transport.
However, experiments using a fluorescent pH probe and 22Na+
measurements in the current study revealed that five bacterial
PPases expressed in Escherichia coli have the ability to simultaneously
translocate H+ and Na+ into inverted membrane vesicles
under physiological conditions. Consistent with data from phylogenetic
analyses, our results support the existence of a third, dualspecificity
bacterial Na+,H+-PPase subfamily, which apparently
evolved from Na+-PPases. Interestingly, genes for Na+,H+-PPase
have been found in the major microbes colonizing the human
gastrointestinal tract. The Na+,H+-PPases require Na+ for hydrolytic
and transport activities and are further activated by K+. Based
on ionophore effects, we conclude that the Na+ and H+ transport
reactions are electrogenic and do not result from secondary antiport
effects. Sequence comparisons further disclosed four Na+,H+-PPase
signature residues located outside the ion conductance channel
identified earlier in PPases using X-ray crystallography. Our results
collectively support the emerging paradigm that both Na+ and H+
can be transported via the same mechanism, with switching between
Na+ and H+ specificities requiring only subtle changes in
the transporter structure.