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I am reviewing the paper "Glycophorin A Dimerization Is Driven by Specific Interactions between Transmembrane Alpha-Helices." There is a statement in the abstract which I don't understand:

"The transmembrane alpha-helical domain of interest is fused to the C-terminus of staphylococcal nuclease. The resulting chimera can be expressed at high levels in Escherichia coli and is readily purified."

As I understand it, this describes that an alpha-helix within Glycophorin A is fused to straphylococcal nuclease? Then, how does Escherichia coli come into the discussion and what is meant by being readily purified?

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Basically, they engineered a vector which, when transfected to E. coli, produces transcripts of -and thus proteins to- a fusion gene which produces these transmembrane α helices conjugated to the staphylococcal nuclease. In other words, the E. coli are just factories for producing proteins:

Expression, Extraction, and Purification of SN/GpA - For high levels of SN/GpA production, pT7SN/GpA was transformed into E. coli MGT7 (kindly provided by D. LeMaster), containing the plasmid pLYS-S.

And the reasoning therein was proposed in the discussion,

We report here the use of a chimeric protein to show that the presence of just the transmembrane domain of GpA, fused to a normally monomeric soluble protein, is sufficient to mediate the dimerization of this artificial membrane protein -in SDS

What they mean by readily purified is that the protein can be extracted relatively easily, and in the experimental method they explain some commonplace methods of purification:

One-step purification of milligram quantities of SN/GpA from the extract could be achieved by reversed-phase HPLC utilizing an acetonitrile/isopropyl alcohol/water gradient on a semipreparative Vydac C4 column. Alternatively, purification of larger quantities was achieved using two rounds of cation-exchange chromatography...

...The chimera SN/GpA131, but not truncated forms, could alterNaCl, 50 mM Tris-HC1, 5 mM EDTA, 1 mM PMSF, 0.025% NaNa, natively be extracted by sonication of the pellet after cell lysis in 1 M pH 7.9, containing no detergent. This extraction was of similar efficiency to that with Lubrol, and was utilized in the preparation of material for generation of transmembrane peptide by trypsin treatment.

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