3 research outputs found
Probing the Extracellular Access Channel of the Na,K-ATPase
When the Na,K-ATPase pumps at each
turnover two K<sup>+</sup> ions
into the cytoplasm, this translocation consists of several reaction
steps. First, the ions diffuse consecutively from the extracellular
phase through an access pathway to the binding sites where they are
coordinated. In the next step, the enzyme is dephosphorylated and
the ions are occluded inside the membrane domain. The subsequent transition
to the E<sub>1</sub> conformation produces a deocclusion of the binding
sites to the cytoplasmic side of the membrane and allows in the last
steps ion dissociation and diffusion to the aqueous phase. The interaction
and competition of K<sup>+</sup> with various quaternary organic ammonium
ions have been used to gain insight into the molecular mechanism of
the ion binding process from the extracellular side in the P-E<sub>2</sub> conformation of the enzyme. Using the electrochromic styryl
dye RH421, evidence has been obtained that the access pathway consists
of a wide and water-filled funnel-like part that is accessible also
for bulky cations such as the benzyltriethylammonium ion, and a narrow
part that permits passage only of small cations such as K<sup>+</sup> and NH<sub>4</sub><sup>+</sup> in a distinct electrogenic way. Benzyltriethylammonium
ions inhibit K<sup>+</sup> binding in a competitive manner that can
be explained by a stopper-like function at the interface between the
wide and narrow parts of the access pathway. In contrast to other
quaternary organic ammonium ions, benzyltriethylammonium ions show
a specific binding to the ion pump in a position inside the access
pathway where it blocks effectively the access to the binding sites
Modulation of the Na,K-ATPase by Magnesium Ions
Since the beginning
of investigations of the Na,K-ATPase, it has
been well-known that Mg<sup>2+</sup> is an essential cofactor for
activation of enzymatic ATP hydrolysis without being transported through
the cell membrane. Moreover, experimental evidence has been collected
through the years that shows that Mg<sup>2+</sup> ions have a regulatory
effect on ion transport by interacting with the cytoplasmic side of
the ion pump. Our experiments allowed us to reveal the underlying
mechanism. Mg<sup>2+</sup> is able to bind to a site outside the membrane
domain of the protein’s α subunit, close to the entrance
of the access channel to the ion-binding sites, thus modifying the
local concentration of the ions in the electrolyte, of which Na<sup>+</sup>, K<sup>+</sup>, and H<sup>+</sup> are of physiological interest.
The decrease in the concentration of these cations can be explained
by electrostatic interaction and estimated by the Debye–Hückel
theory. This effect provokes the observed apparent reduction of the
binding affinity of the binding sites of the Na,K-ATPase in the presence
of various Mg<sup>2+</sup> concentrations. The presence of the bound
Mg<sup>2+</sup>, however, does not affect the reaction kinetics of
the transport function of the ion pump. Therefore, stopped-flow experiments
could be performed to gain the first insight into the Na<sup>+</sup> binding kinetics on the cytoplasmic side by Mg<sup>2+</sup> concentration
jump experiments
TtOmp85, a β‑Barrel Assembly Protein, Functions by Barrel Augmentation
Outer
membrane proteins are vital for Gram-negative bacteria and
organisms that inherited organelles from them. Proteins from the Omp85/BamA
family conduct the insertion of membrane proteins into the outer membrane.
We show that an eight-stranded outer membrane β-barrel protein,
TtoA, is inserted and folded into liposomes by an Omp85 homologue.
Furthermore, we recorded the channel conductance of this Omp85 protein
in black lipid membranes, alone and in the presence of peptides comprising
the sequence of the
two N-terminal and the two C-terminal β-strands of TtoA. Only
with the latter could a long-living compound channel that exhibits
conductance levels higher than those of the Omp85 protein alone be
observed. These data
support a model in which unfolded outer membrane protein after docking
with its C-terminus penetrates into the transmembrane β-barrel
of the Omp85 protein and augments its β-sheet at the first strand.
Augmentation with successive β-strands leads to a compound,
dilated barrel of both proteins