= Proteoliposome
A \b[proteoliposome] is a <liposome> containing one or more reconstituted <membrane proteins> in its <lipid bilayer>. It combines a controlled membrane boundary with a chosen molecular machine. A solution containing detergent-solubilized <protein> is not automatically a <proteoliposome>: a closed vesicle and a membrane-incorporated <protein> must actually be formed.
The artificial compartment is useful because material can be placed on one side of a <lipid bilayer> and transport measured across it. In an originating experiment on the <NhaA> sodium/proton <antiporter>, purified protein was reconstituted into sodium-loaded <proteoliposomes>. Parallel sodium and proton flux measurements, together with an independent membrane-potential equilibrium method, supported a stoichiometry of two protons per sodium ion over the tested pH range 7.2–8.1.<Proton-sodium stoichiometry of NhaA, an electrogenic antiporter from Escherichia coli by D. Taglicht and others>[JBC 268,5382–5387(1993), abstract: purified NhaA, sodium-loaded vesicles, two methods and tested pH range.]{ref}
For a bookkeeping example,100 sodium ions moved outward through a perfectly coupled $2\mathrm{H^+}/\mathrm{Na^+}$ exchanger would accompany 200 protons moving inward. The net movement is 100 positive elementary charges inward, which explains the word electrogenic. These invented counts illustrate coupling, not a measured single-vesicle trace.
Interpreting a <proteoliposome> assay still requires knowing which way the <membrane protein> faces, whether the vesicles leak, and how much protein is incorporated. An otherwise matched empty <liposome> helps estimate membrane background. Reconstitution simplifies a biological system; it does not automatically eliminate every alternative source of the measured flux.
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