= Solution
\b[The useful product is a correctly identified, active and reproducible protein preparation, not merely a large expression band.] I would first examine the <coding sequence>, <protein sequence> and available biological information to identify the likely mature chain, <protein domains>, <signal peptides>, transmembrane segments, <disulfide bonds> and required <post-translational modifications>. A predicted <protein structure> can guide construct boundaries, but cannot establish that the expressed product will fold or function. A <eukaryotic> genomic sequence containing <introns> generally needs replacement by an intron-free <Complementary DNA> or synthetic coding region for expression in <Escherichia coli>. I would verify the chosen isoform and full sequence, since a database annotation can be wrong.
The expression host follows from the biochemical requirements. <Escherichia coli> is inexpensive and convenient for many soluble <proteins>; yeast, insect or mammalian cells may be preferable for secreted <proteins> or essential <glycosylation>. A <membrane protein> also requires a suitable membrane environment during expression and an appropriate detergent or membrane mimic after extraction. I would preserve biologically necessary domains and modifications. Removing a flexible region to improve yield creates a different construct whose suitability must be justified by the intended analysis.
For a bacterial preparation, I would clone the coding region into an <expression vector> with a controllable <promoter (genetics)>, a suitable <ribosome-binding site>, a selectable marker and a removable <affinity tag>. A terminal polyhistidine tag is a practical initial choice, provided the terminus is compatible with activity. Correct reading frame, both junctions and the complete insert must be checked by <DNA sequencing>. The <T7 expression system> offers strong inducible expression through a phage <RNA polymerase>; the original work established selective expression of cloned genes, but strong transcription alone does not guarantee soluble, active product.<Use of bacteriophage T7 RNA polymerase to direct selective high-level expression of cloned genes by F. William Studier and others>[Abstract and pp. 113–130, inducible selective expression]{ref}
I would screen small cultures before scale-up, varying induction strength, induction duration and <temperature>, and comparing total lysate with soluble and insoluble fractions by <SDS-PAGE>. Empty-vector and uninduced controls identify host bands and background expression; an <immunoblot> against the tag can help identify the product. Expression at lower <temperature> or a weaker rate can improve <protein folding>; persistent insolubility may require changing the construct, host or folding environment. An <inclusion body> is evidence of abundant insoluble material, not of correctly folded <protein>. Refolding may be possible, but its yield and recovered activity need independent validation.
Once a small-scale condition produces active soluble material, I would expand the culture or fermentation while monitoring growth and retaining the validated induction conditions. After harvesting, controlled lysis at low <temperature>, an appropriate <buffer solution> and protection against <proteolysis> preserve the product. <Centrifugation> removes debris; a membrane-associated target instead requires isolation and solubilization of the relevant fraction. I would use <immobilized metal affinity chromatography> for a polyhistidine-tagged construct, wash away nonspecific binders, and elute under conditions compatible with activity. Tag cleavage, a second affinity step to remove tag and tagged protease, and an orthogonal separation such as <ion-exchange chromatography> can improve purity. A final <size-exclusion chromatography> step removes aggregates and separates different assemblies. No single chromatographic peak proves identity or homogeneity.
At each stage I would record <protein concentration>, total recovered amount and a suitable activity or binding readout, rather than optimizing yield alone. Final checks would include <SDS-PAGE>, intact or peptide <mass spectrometry>, a native-size measurement and the expected biological activity. Buffer-only and host-purification controls test whether apparent activity comes from contaminants; comparing tagged and cleaved material tests tag interference. Small frozen aliquots should be used only after checking stability and freeze–thaw effects. \b[Scale-up is justified by recovery of the desired molecular species with retained function], with any differences from the native <protein> documented.
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