How is Glycophorin A and straphylococcal related to Escherichia coli and what does readily purified mean in this context?
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.