The effect of cations on the alkaline dissociation of tobacco mosaic virus

Improving the large scale purification of the HIV microbicide, griffithsin

BACKGROUND

Griffithsin is a broad spectrum antiviral lectin that inhibits viral entry and maturation processes through binding clusters of oligomannose glycans on viral envelope glycoproteins. An efficient, scaleable manufacturing process for griffithsin active pharmaceutical ingredient (API) is essential for particularly cost-sensitive products such as griffithsin -based topical microbicides for HIV-1 prevention in resource poor settings. Our previously published purification method used ceramic filtration followed by two chromatography steps, resulting in a protein recovery of 30%. Our objective was to develop a scalable purification method for griffithsin expressed in Nicotiana benthamiana plants that would increase yield, reduce production costs, and simplify manufacturing techniques. Considering the future need to transfer griffithsin manufacturing technology to resource poor areas, we chose to focus modifying the purification process, paying particular attention to introducing simple, low-cost, and scalable procedures such as use of temperature, pH, ion concentration, and filtration to enhance product recovery.

RESULTS

We achieved >99% pure griffithsin API by generating the initial green juice extract in pH 4 buffer, heating the extract to 55°C, incubating overnight with a bentonite MgCl2 mixture, and final purification with Capto™ multimodal chromatography. Griffithsin extracted with this protocol maintains activity comparable to griffithsin purified by the previously published method and we are able to recover a substantially higher yield: 88 ± 5% of griffithsin from the initial extract. The method was scaled to produce gram quantities of griffithsin with high yields, low endotoxin levels, and low purification costs maintained.

CONCLUSIONS

The methodology developed to purify griffithsin introduces and develops multiple tools for purification of recombinant proteins from plants at an industrial scale. These tools allow for robust cost-effective production and purification of griffithsin. The methodology can be readily scaled to the bench top or industry and process components can be used for purification of additional proteins based on biophysical characteristics.

Proton dependence of tobacco mosaic virus dissociation by pressure

Tobacco mosaic virus (TMV) is an intensely studied model of viruses. This paper reports an investigation into the dissociation of TMV by pH and pressure up to 220 MPa. The viral solution (0.25 mg/ml) incubated at 277 K showed a significant decrease in light scattering with increasing pH, suggesting dissociation. This observation was confirmed by HPLC gel filtration and electron microscopy. The calculated volume change of dissociation (DeltaV) decreased (absolute value) from -49.7 ml/mol of subunit at pH 3.8 to -21.7 ml/mol of subunit at pH 9.0. The decrease from pH 9.0 to 3.8 caused a stabilization of 14.1 kJ/mol of TMV subunit. The estimated proton release calculated from pressure-induced dissociation curves was 0.584 mol H(+)/mol of TMV subunit. These results suggest that the degree of virus inactivation by pressure and the immunogenicity of the inactivated structures can be optimized by modulating the surrounding pH.

Calcium and potassium ion binding by tobacco mosaic virus ribonucleic acid

Calcium and potassium ion titration experiments were performed on solutions of tobacco mosaic virus RNA using ion-specific electrodes. The data obtained were analyzed using Scatchard and Klotz plots for the number of binding sites per nucleotide (n), and the apparent stability constant for complex formation, beta Me. The experimental design also allowed for the determination of the number of protons released per metal ion bound, chi. The calcium ion titration in water yielded values of 0.45 for n, 6.03 for log beta Ca and 0.24 for chi. When this titration was repeated in 0.01 M-KCl, the values were found to be 0.11 for n, 5.08 for log beta Ca and zero for chi. An aqueous potassium titration was also performed, with values for n, log beta K and chi of 0.25, 2.96 and less than 0.10, respectively.

Calcium ion binding by isolated tobacco mosaic virus coat protein

Calcium ion titrations were performed on solutions of tobacco mosaic virus coat protein using a calcium-specific ion-exchange electrode. Isolated coat protein was found incapable of binding calcium ions under equilibrium conditions at pH values above its iso-ionic point (pH 4.3 to 4.6). However, calcium ions were found to bind to coat protein under non-equilibrium conditions, which suggests that the isolated coat protein has the proper conformation to bind calcium ions at the iso-ionic point.

Calcium ion binding by tobacco mosaic virus

Calcium ion titrations were performed on solutions of tobacco mosaic virus using a calcium-specific ion-exchange electrode. Scatchard analyses were used to obtain the number of calcium ion binding sites per protein subunit (n) and the apparent stability constant for complex formation (beta' Ca). These experiments were performed on unbuffered solutions, in either water or 0.01 M-KCl, to allow a determination of the number of hydrogen ions released per calcium ion bound (chi). The results indicate that near neutrality, the virus particle possesses two calcium ion binding sites per subunit having apparent stability constants greater than 10(4) M-1. The results are interpreted as if these two sites are non-identical and titrate independently. The higher affinity site for the virus in water has a value of log beta' Ca, which varies from about 8.5 at pH 8.5 to about 3.9 at pH 5.0, and for the virus in 0.01 M-KCl has a value that varies from about 6.2 at pH 8.0 to about 3.7 at pH 5.5. The higher affinity site for the virus in water binds up to two competing hydrogen ions, one with an apparent pKH value greater than 8.5 and the other with a value that varies from 6.0 at pH 5.5 to 7.3 at pH 8.0. For the virus in 0.01 M-KCl, only the competing hydrogen ion binding with an apparent pKH value greater than 8.5 remains. The results could be interpreted as indicating that the electrical charge on the virus particle has a constant value in the pH range 5.5 to 8.0 despite the fact that hydrogen ion titration curves for the intact virus particle indicate that the charge should vary from about -1 per subunit at pH 5.5 to about -4 at pH 8.0.

The origin of multiple polypeptides of molecular weight below 110 000 encoded by tobacco mosaic virus RNA in the messenger-dependent rabbit reticulocyte lysate

Multiple polypeptides encoded by tobacco mosaic virus (TMV) RNA in the messenger-dependent rabbit reticulocyte lysate are not attributable to contaminating 3'-coterminal RNA fragments, multiple leaky termination codons or endonuclease activity opening-up legitimate or spurious internal initiation sites. Quantitative analysis of polypeptides encoded over a range of added RNA concentrations from 0.09 microgram X ml-1 to 180 micrograms X ml-1 compared with those synthesized in response to size-fractionated RNAs from a crude virus preparation, or with RNA extracted from the alkali-stable fraction of TMV suggest that apart from four legitimate virus-coded products of apparent Mr approx. 165 000, 110 000, 30 000 and 17 500 all other polypeptides arise from the overlapping 5'-proximal cistrons either by (i) site-selective endonucleolytic cleavage, (ii) sense codon misreading, or (iii) specific regions of secondary structure on TMV RNA which impede ribosome translocation.