Endotoxin-free purification of recombinant membrane scaffold protein expressed in Escherichia coli

Improved Pendant-Bearing Glucose-Neopentyl Glycols for Membrane Protein Stability

Membrane proteins are biologically and pharmaceutically significant, and determining their 3D structures requires a membrane-mimetic system to maintain protein stability. Detergent micelles are widely used as membrane mimetics; however, their dynamic structures often lead to the denaturation and aggregation of encapsulated membrane proteins. To address the limitations of classical detergents in stabilizing membrane proteins, we previously reported a class of glucose-neopentyl glycols (GNGs) and their pendant-bearing versions (P-GNGs), several of which proved more effective than DDM in stabilizing membrane proteins. In this study, we synthesized additional GNG derivatives by varying the lengths of the pendant (P-GNGs), and by introducing hemifluorinated pendants to the GNG scaffold (fluorinated pendant-bearing GNGs or FP-GNGs). The synthetic flexibility of the GNG chemical architecture allowed us to create a diverse range of derivatives, essential for the effective optimization of detergent properties. When tested with two model membrane proteins (a transporter and a G-protein coupled receptor (GPCR)), most of the new (F)P-GNGs demonstrated superior stabilization of these membrane proteins compared to DDM, the original GNG (OGNG)), and a previously developed P-GNG (i.e., GNG-3,14). Notably, several P-GNGs synthesized in this study were as effective as or even better than lauryl maltose neopentyl glycol (LMNG) in stabilizing a human GPCR, beta2 adrenergic receptor (β2AR). Enhanced protein stability was particularly observed for the P-GNGs with a butyl (C4) or pentyl (C5) pendant, indicating that these pendant sizes are optimal for membrane protein stability. The volumes of these pendants appear to minimize the empty spaces in the micelle interiors, thereby enhancing detergent-detergent interactions in micelles complexed with the membrane proteins. Additionally, we identified one FP-GNG that was more efficient at extracting the transporter and more effective at stabilizing the GPCR than DDM. Thus, the current study demonstrates that both chain length and number of fluorine atoms in the pendants of the P-GNGs were crucial determinants for membrane protein stability. We not only developed a few (F)P-GNGs that are significantly more effective than maltoside detergents (LMNG/DDM) for protein extraction and stability but we also provided an effective strategy for detergent design through the utilization of partially fluorinated pendants of varying length.

Purification of the full-length, membrane-associated form of the antiviral enzyme viperin utilizing nanodiscs

Viperin is a radical S-adenosylmethionine enzyme that catalyzes the formation of the antiviral ribonucleotide, 3'-deoxy-3',4'-didehydroCTP. The enzyme is conserved across all kingdoms of life, and in higher animals viperin is localized to the ER-membrane and lipid droplets through an N-terminal extension that forms an amphipathic helix. Evidence suggests that the N-terminal extension plays an important role in viperin's interactions with other membrane proteins. These interactions serve to modulate the activity of various other enzymes that are important for viral replication and constitute another facet of viperin's antiviral properties, distinct from its catalytic activity. However, the full-length form of the enzyme, which has proved refractory to expression in E. coli, has not been previously purified. Here we report the purification of the full-length form of viperin from HEK293T cells transfected with viperin. The purification method utilizes nanodiscs to maintain the protein in its membrane-bound state. Unexpectedly, the enzyme exhibits significantly lower catalytic activity once purified, suggesting that interactions with other ER-membrane components may be important to maintain viperin's activity.

ClearColi as a platform for untagged pneumococcal surface protein A production: cultivation strategy, bioreactor culture, and purification

Abstract

Several studies have searched for new antigens to produce pneumococcal vaccines that are more effective and could provide broader coverage, given the great number of serotypes causing pneumococcal diseases. One of the promising subunit vaccine candidates is untagged recombinant pneumococcal surface protein A (PspA4Pro), obtainable in high quantities using recombinant Escherichia coli as a microbial factory. However, lipopolysaccharides (LPS) present in E. coli cell extracts must be removed, in order to obtain the target protein at the required purity, which makes the downstream process more complex and expensive. Endotoxin-free E. coli strains, which synthesize a nontoxic mutant LPS, may offer a cost-effective alternative way to produce recombinant proteins for application as therapeutics. This paper presents an investigation of PspA4Pro production employing the endotoxin-free recombinant strain ClearColi® BL21(DE3) with different media (defined, auto-induction, and other complex media), temperatures (27, 32, and 37 °C), and inducers. In comparison to conventional E. coli cells in a defined medium, ClearColi presented similar PspA4Pro yields, with lower productivities. Complex medium formulations supplemented with salts favored PspA4Pro yields, titers, and ClearColi growth rates. Induction with isopropyl-β-d-thiogalactopyranoside (0.5 mM) and lactose (2.5 g/L) together in a defined medium at 32 °C, which appeared to be a promising cultivation strategy, was reproduced in 5 L bioreactor culture, leading to a yield of 146.0 mg PspA4Pro/g dry cell weight. After purification, the cell extract generated from ClearColi led to 98% purity PspA4Pro, which maintained secondary structure and biological function. ClearColi is a potential host for industrial recombinant protein production.

Key points

• ClearColi can produce as much PspA4Pro as conventional E. coli BL21(DE3) cells.

• 10.5 g PspA4Pro produced in ClearColi bioreactor culture using a defined medium.

• Functional PspA4Pro (98% of purity) was obtained in ClearColi bioreactor culture.

Virucidal nano-perforator of viral membrane trapping viral RNAs in the endosome

Membrane-disrupting agents that selectively target virus versus host membranes could potentially inhibit a broad-spectrum of enveloped viruses, but currently such antivirals are lacking. Here, we develop a nanodisc incorporated with a decoy virus receptor that inhibits virus infection. Mechanistically, nanodiscs carrying the viral receptor sialic acid bind to influenza virions and are co-endocytosed into host cells. At low pH in the endosome, the nanodiscs rupture the viral envelope, trapping viral RNAs inside the endolysosome for enzymatic decomposition. In contrast, liposomes containing a decoy receptor show weak antiviral activity due to the lack of membrane disruption. The nanodiscs inhibit influenza virus infection and reduce morbidity and mortality in a mouse model. Our results suggest a new class of antivirals applicable to other enveloped viruses that cause irreversible physical damage specifically to virus envelope by viruses' own fusion machine. In conclusion, the lipid nanostructure provides another dimension for antiviral activity of decoy molecules.