Optimization of an Escherichia coli system for cell-free synthesis of selectively N-labelled proteins for rapid analysis by NMR spectroscopy.
Cell-free protein synthesis offers rapid access to proteins that are selectively labelled with [15N]amino acids and suitable for analysis by NMR spectroscopy without chromatographic purification. A system based on an Escherichia coli cell extract was optimized with regard to protein yield and minimal usage of 15N-labelled amino acid, and examined for the presence of metabolic by-products which could interfere with the NMR analysis. Yields of up to 1.8 mg of human cyclophilin A per mL of reaction medium were obtained by expression of a synthetic gene. Equivalent yields were obtained using transcription directed by either T7 or tandem phage lambdapR and pL promoters, when the reactions were supplemented with purified phage T7 or E. coli RNA polymerase. Nineteen samples, each selectively labelled with a different 15N-enriched amino acid, were produced and analysed directly by NMR spectroscopy after ultracentrifugation. Cross-peaks from metabolic by-products were evident in the 15N-HSQC spectra of 13 of the samples. All metabolites were found to be small molecules that could be separated readily from the labelled proteins by dialysis. No significant transamination activity was observed except for [15N]Asp, where an enzyme in the cell extract efficiently converted Asp-->Asn. This activity was suppressed by replacing the normally high levels of potassium glutamate in the reaction mixture with ammonium or potassium acetate. In addition, the activity of peptide deformylase appeared to be generally reduced in the cell-free expression system.
Ozawa
Kiyoshi
K
Research School of Chemistry, Australian National University, Canberra, Australia.
Headlam
Madeleine J
MJ
Schaeffer
Patrick M
PM
Henderson
Blair R
BR
Dixon
Nicholas E
NE
Otting
Gottfried
G
D000596:Amino Acids / Q000737:chemistry / Q000378:metabolism*; D000604:Amino Acyl-tRNA Synthetases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D000818:Animals; D001412:Bacillus subtilis / Q000201:enzymology; D017123:Bacteriophage T7 / Q000235:genetics; D010582:Bacteriophage lambda / Q000235:genetics; D002417:Cattle; D002474:Cell-Free System; D021984:Cyclophilin A / Q000032:analysis / Q000096:biosynthesis / Q000737:chemistry / Q000235:genetics; D021983:Cyclophilins / Q000032:analysis / Q000096:biosynthesis / Q000737:chemistry / Q000235:genetics; D012321:DNA-Directed RNA Polymerases / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology* / Q000235:genetics*; D029968:Escherichia coli Proteins / Q000032:analysis / Q000096:biosynthesis / Q000737:chemistry; D006801:Humans; D007553:Isotope Labeling / Q000379:methods*; D007700:Kinetics; D031297:Lupinus / Q000201:enzymology / Q000235:genetics; D009587:Nitrogen Isotopes; D019906:Nuclear Magnetic Resonance, Biomolecular / Q000379:methods*; D010231:Paracoccus denitrificans / Q000201:enzymology; D019696:Peptidylprolyl Isomerase; D011401:Promoter Regions, Genetic; D014176:Protein Biosynthesis / Q000235:genetics*; D012440:Saccharomyces / Q000201:enzymology / Q000235:genetics; D014764:Viral Proteins
D000596:Amino Acids; D029968:Escherichia coli Proteins; D009587:Nitrogen Isotopes; D014764:Viral Proteins; C081786:cyclophilin B; C062210:bacteriophage T7 RNA polymerase; D012321:DNA-Directed RNA Polymerases; D021984:Cyclophilin A; D021983:Cyclophilins; D019696:Peptidylprolyl Isomerase; D000604:Amino Acyl-tRNA Synthetases