Design and Escherichia coli Expression of a Natively Folded Multi-Disulfide Bonded Influenza H1N1-PR8 Receptor-Binding Domain (RBD).

Refolding multi-disulfide bonded proteins expressed in E. coli into their native structure is challenging. Nevertheless, because of its cost-effectiveness, handiness, and versatility, the E. coli expression of viral envelope proteins, such as the RBD (Receptor-Binding Domain) of the influenza Hemagglutinin protein, could significantly advance research on viral infections. Here, we show that H1N1-PR8-RBD (27 kDa, containing four cysteines forming two disulfide bonds) expressed in E. coli and was purified with nickel affinity chromatography, and reversed-phase HPLC was successfully refolded into its native structure, as assessed with several biophysical and biochemical techniques. Analytical ultracentrifugation indicated that H1N1-PR8-RBD was monomeric with a hydrodynamic radius of 2.5 nm. Thermal denaturation, monitored with DSC and CD at a wavelength of 222 nm, was cooperative with a midpoint temperature around 55 °C, strongly indicating a natively folded protein. In addition, the 15N-HSQC NMR spectrum exhibited several 1H-15N resonances indicative of a beta-sheeted protein. Our results indicate that a significant amount (40 mg/L) of pure and native H1N1-PR8-RBD can be produced using an E. coli expression system with our refolding procedure, offering potential insights into the molecular characterization of influenza virus infection.

Bacterial expression and re-engineering of Gaussia princeps luciferase and its use as a reporter protein.

Bioluminescence, the generation of visible light in a living organism, is widely observed in nature, and a large variety of bioluminescent proteins have been discovered and characterized. Luciferase is a generic term for bioluminescent enzymes that catalyze the emission of light through the oxidization of a luciferin (also a generic term). Luciferase are not necessarily evolutionary related and do not share sequence or structural similarities. Some luciferases, such as those from fireflies and Renilla, have been thoroughly characterized and are being used in a wide range of applications in bio-imaging. Gaussia luciferase (GLuc) from the marine copepod Gaussia princeps is the smallest known luciferase, and it is attracting much attention as a potential reporter protein. GLuc identification is relatively recent, and its structure and its biophysical properties remain to be fully characterized. Here, we review the bacterial production of natively folded GLuc with special emphasis on its disulfide bond formation and the re-engineering of its bioluminescence properties. We also compare the bioluminescent properties under a strictly controlled in vitro condition of selected GLuc's variants using extensively purified proteins with native disulfide bonds. Furthermore, we discuss and predict the domain structure and location of the catalytic core based on literature and on bioinformatics analysis. Finally, we review some examples of GLuc's emerging use in biomolecular imaging and biochemical assay systems.

Solubilization and folding of a fully active recombinant Gaussia luciferase with native disulfide bonds by using a SEP-Tag.

Gaussia luciferase (GLuc) is the smallest known bioluminescent protein and is attracting much attention as a potential reporter protein. However, its 10 disulfide bond forming cysteines have hampered the efficient production of recombinant GLuc and thus limited its use in bio-imaging application. Here, we demonstrate that the addition of a short solubility enhancement peptide tag (SEP-Tag) to the C-terminus of GLuc (GLuc-C9D) significantly increased the fraction of soluble protein at a standard expression temperature. The expression time was much shorter, and the final yield of GLuc-C9D was significantly higher than with our previous pCold expression system. Reversed phase HPLC indicated that the GLuc-C9D variant folded with a single disulfide bond pattern after proper oxidization. Further, the thermal denaturation of GLuc-C9D was completely reversible, and its secondary structure content remained unchanged until 40°C as assessed by CD spectroscopy. The (1)H-NMR spectrum of GLuc indicated sharp well dispersed peaks typical for natively folded proteins. GLuc-C9D bioluminescence activity was strong and fully retained even after incubation at high temperatures. These results suggest that solubilization using SEP-Tags can be useful for producing large quantities of proteins containing multiple disulfide bonds.

Biophysical characterization of highly active recombinant Gaussia luciferase expressed in Escherichia coli.

Recently, the smallest bioluminescent protein (MW: 19.9 kDa), Gaussia luciferase (GLuc), has been isolated from the marine copepod Gaussia princeps and has attracted much attention as a reporter protein. However, preparation of large quantities of homogeneous natively folded recombinant GLuc appears to be difficult due to its ten cysteines. Here, we report the biophysical characterization of recombinant GLuc expressed using a novel Escherichia coli expression system based on a cold induced expression vector (pCold). Using this system, a large fraction of the protein was expressed in the soluble fraction. GLuc, purified exclusively from the supernatant using nickel affinity chromatography, yielded a large amount of pure GLuc with a native disulfide bond pattern (Soluble-GLuc). Soluble-GLuc had a strong bioluminescence activity and it retained 65% of its activity after 30 min incubation at 95 degrees C. Soluble-GLuc remained fully folded until 40 degrees C, as assessed by circular dichroism; and the thermal denaturation curve was S-shaped, indicating a cooperative transition, with a midpoint temperature of 56 degrees C. These results indicate that both the structure and bioluminescence activity of GLuc remain stable at high temperatures, and they strongly suggest GLuc's potential as a reporter protein.