Codon optimization of the human papillomavirus type 58 L1 gene enhances the expression of soluble L1 protein in Saccharomyces cerevisiae
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- Kim, H.J., Kwag, H. & Kim, H. Biotechnol Lett (2013) 35: 413. doi:10.1007/s10529-012-1097-y
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The effect of codon optimization of L1 gene on the production of the L1 protein of human papillomavirus (HPV) was investigated in a yeast expression system. Saccharomyces cerevisiae was transformed with a plasmid containing either the wild type (WS)-HPV type 58 L1 (HPV58 L1) gene or a codon-optimized (MO)-HPV58 L1 gene. The proportion of soluble L1 protein expressed from MO-HPV58 L1 was significantly higher than that expressed from WS-HPV58 L1. Moreover, the amount of purified MO-HPV58 L1 protein recovered was 2.5-fold higher than the amount of WS-HPV58 L1 protein. Codon optimization of HPV58 L1 gene thus increases the proportion of soluble L1 protein and the amount of purified product that can be used as antigen to generate vaccines.
KeywordsCodon optimizationHuman papillomavirusL1 proteinSaccharomyces cerevisiae
L1 protein is the major capsid protein of human papillomavirus (HPV), which can self-assemble into a virus-like particle (VLP) with a structure similar to that of naturally occurring HPV virions (Conway and Meyers 2009). Therefore, recombinant L1 proteins expressed in insect cells (Sf9) and Saccharomyces cerevisiae (S. cerevisiae) have been used as antigens in prophylactic vaccines (Garland and Smith 2010). The S. cerevisiae expression system that generates VLPs has a low production cost and short culture period (Park et al. 2008). However, productivity is sub-maximal. Improved fermentation technology and promoter systems are considered important for enhancing productivity (Kim et al. 2011; Mattanovich et al. 2004), but there has been little investigation into the possibility of codon optimization, which has been studied as a means of enhancing transgene expression for two decades, mostly in E. coli expression systems.
There are two strategies for codon optimization: one is to alter codons in a way that reduces mRNA secondary structure, and the other is to use the preferred codons of the relevant species (Angov 2011). Reducing mRNA secondary structure increases protein expression by increasing translation initiation (Kozak 2005), while changing to high frequency usage codons increases the binding of isoacceptor tRNA molecules to mRNA (Bulmer 1987; Ikemura 1985). There is disagreement over which strategy is more useful in enhancing transgene expression. The results of a recent study strongly suggested that reducing mRNA secondary structure was more effective in enhancing transgene expression than codon changes per se (Kudla et al. 2009).
The L1 ORFs of high-risk HPVs contain two putative start codons (ATGs) (Webb et al. 2005). The second in-frame ATG is located 78 nucleotides downstream of the first. The short L1 gene, which uses the second in-frame ATG as start codon, has been used to produce HPV L1 VLP (Kirnbauer et al. 1993; Zhou et al. 1991). The long L1 gene, which contains both ATGs, does not produce L1 VLPs efficiently (Webb et al. 2005), perhaps because of some effect of the nucleotide sequence between the first and second ATG on L1 production.
Materials and methods
Construction of plasmids containing the WL-, WS-, and MO-HPV58 L1 genes
To construct the plasmid containing the WL-HPV58 L1 gene, the DNA sequence of the gene (containing both the first and second ATGs) was amplified from HPV58 DNA. (Matsukura and Sugase 1990). The 1,575 bp PCR product was ligated into pGEM-T-easy vector (Promega, USA) and subsequently into vector YEGα-MCS, using the EcoRI and SalI sites, to create YEGα-MCS-WL-HPV58 L1 (Supplementary Fig. 1). To construct the plasmid containing the WS-HPV58 L1 gene, the DNA sequence of the gene (containing only the second ATG) was amplified from the HPV58 DNA above. The 1,497 bp PCR product was ligated into pGEM-T-easy vector and subsequently into YEGα-MCS, using HindIII and SalI sites, to create YEGα-MCS-WS-HPV58 L1 (Supplementary material I). We confirmed that the WL- and WS-HPV58 L1 sequences were identical to those of the relevant sections of HQ357755.1 in GenBank. The MO-HPV58 L1 gene (1,497 bp) was synthesized by Blue Heron Biotechnology Inc. (USA) and ligated into YEGα-MCS, using HindIII and SalI sites, to create YEGα-MCS-MO-HPV58 L1 (Supplementary material I). HPV58 DNA was kindly provided by Dr. Jong Sup Park (College of Medicine, Catholic University of Korea, Seoul, South Korea), and the YEGα-MCS vector was kindly provided by Prof. Hyun Ah Kang (Chung-Ang University, Seoul, South Korea).
Calculation of folding free energies and codon adaptation indices
The folding free energies and codon adaptation indices (CAIs) of L1 mRNAs transcribed from WL-HPV58 L1, WS-HPV58 L1 and MO-HPV58 L1 were calculated using the software provided by the Vienna RNA WebServer (http://rna.tbi.univie.ac.at/) and the E-CAI server program (http://genomes.urv.es/CAIcal/E-CAI/), respectively.
Yeast transformation and cell culture
Saccharomyces cerevisiae Y2805 was transformed with YEGα-MCS-WL-HPV58L1, YEGα-MCS-WS-HPV58 L1 and YEGα-MCS-MO-HPV58 L1 as described previously (Soni et al. 1993), and transformants were selected on SD-ura medium (a synthetic complete medium without uracil). Fifteen colonies on SD-ura plate were selected for each gene and culture in 5 ml SD-ura broth for 2 days. Thereafter nine clones with high growth rates among 15 clones were selected for each gene. The nine clones were further cultured in 5 ml SD-ura broth for 1 day and mixed with the other clones transformed with the same gene. The mixtures were then inoculated into 150 ml YPDG medium containing 1 % (w/v) yeast extract, 2 % (w/v) peptone, 8 % (w/v) glucose and 1 % (w/v) galactose (Kim et al. 2011) and grown in flasks at 30 °C with shaking at 230 rpm for 48, 96, or 144 h. Cell densities were calculated from the OD600 values.
Preparation of cell lysates
For measurement of total L1 protein level, cell lysates were prepared without removing cell debris, while for measurement of soluble L1 protein level cell lysates were prepared with cell debris removed. For both preparations cells were harvested from 1 ml of cultures by centrifugation at 10,000×g for 10 min, mixed with disruption buffer (10 mM NaH2PO4, 150 mM NaCl, 1.7 mM EDTA, 0.01 % Tween 80, pH 7.2) and disrupted by vortexing with glass beads (BioSpec Products, Inc., USA). Cell lysates for measurement of total L1 protein level were then diluted 50-fold with disruption buffer. Cell lysates for measurement of soluble L1 protein level were centrifuged at 12,000×g for 10 min to remove cell debris, then diluted 20-fold with disruption buffer.
SDS-PAGE and western blotting
Diluted cell lysates were mixed with Laemmli sample buffer (Laemmli 1970) and heated for 12 min at 77 °C to perform SDS-PAGE (Cook et al. 1999). (Heating at 77 °C increases the resolution of L1 protein band.) Proteins were fractionated on 12 % acrylamide gel. The fractionated proteins were visualized with silver staining. Western blotting was performed as described previously (Kim et al. 2011). The band corresponding to L1 protein was detected using rabbit anti-HPV16 L1 polyclonal antibody (Pab) followed by HRP-conjugated anti-rabbit IgG (Bethyl laboratories, USA). L1 band intensities were determined with NIH open source software Image J (http://rsbweb.nih.gov/ij/), and calculations were performed as described previously (Kim et al. 2011). To determine intracellular BiP levels, 50 μg protein per well was loaded onto gels for western blotting. Tubulin was used as an internal control. Anti-KAR2 antibody (Santa Cruz Biotechnology, USA) and anti-tubulin antibody (Abcam, UK) were used for detection of BiP and tubulin, respectively.
Purification of L1 proteins
L1 proteins were purified from cultures after 48, 96, and 144 h as described previously (Kim et al. 2010). Briefly, protein in the cell lysates was precipitated with 40 % saturated (NH4)2SO4. The precipitated samples were clarified by removing precipitated contaminants and the L1 proteins were isolated by cation-exchange chromatography using P-11 resin (Whatman, UK). The purified proteins were analyzed by SDS-PAGE (Laemmli 1970), and western blotting. For SDS-PAGE and western blot, proteins were fractionated on 12 % acrylamide gel.
Determination of protein concentration
Protein concentrations were determined with a Bradford protein assay kit (Bio-Rad Laboratories, USA) with bovine serum albumin as standard.
Transmission electron microscopy (TEM)
Purified HPV58 L1 was dialyzed against PBS containing 0.33 M NaCl and 0.01 % Tween 80, and negatively stained with 2 % phosphotungstic acid (Kim et al. 2011). Electron microscopy was performed on a LIBRA 120 energy-filtering transmission electron microscope (Carl Zeiss, Germany) at a final magnification of 50,000×.
Evaluation of immunogenicity of purified HPV58 L1 proteins
Six-week-old female BALB/c mice were divided into three groups (PBS, WS-HPV58 L1 and MO-HPV58 L1), each consisting of 7–14 mice and immunized subcutaneously with PBS or purified HPV58 L1 proteins four times at 2-week intervals. Those in the PBS group received 100 μl PBS containing 200 μg Al(OH)3 per dose while those in the WS-HPV58 L1 and MO-HPV58 L1 groups received 10 ng purified WS-HPV58 L1 or MO-HPV58 L1 and 200 μg Al(OH)3 per dose.
Determination of anti-HPV58 L1 IgG titer
After four immunizations sera were obtained, anti-HPV58 L1 IgG titers were determined as described previously (Kim et al. 2012). 96-well ELISA plate was coated with purified WS-HPV58 L1 VLP (100 ng/well) and blocked with 3 % BSA in PBS-T (PBS containing 0.5 % Tween 20). Thereafter, serial dilution of the mice sera were incubated with the coated L1 VLPs for 1 h at 37 °C. The anti-HPV58 L1 antibody bound with coated HPV58 L1 VLPs was detected using HRP-conjugated anti-mouse IgG polyclonal antibody (Bethyl, USA), and the reactions were developed using o-phenylenediamine (Sigma. USA). The anti-HPV58 L1 IgG titers were determined by end-point titrations. To determine anti-HPV58 neutralizing antibody titers of mice sera HPV pseudovirus (PsV)-based assays were performed as described previously (Pastrana et al. 2004).
Comparison of the WL-, WS- and MO-HPV58 L1 genes
Comparison of L1 protein expression and amounts of L1 protein recovered after purification
Cells transformed with WS-HPV58 L1 yielded the highest level of total L1 protein and those transformed with WL-HPV58 L1 produced some L1 protein but most of the L1 protein produced in either case was lost after removing cell debris (Fig. 1). In contrast, the L1 protein produced in cells transformed with MO-HPV58 L1 remained after removal of cell debris (Fig. 1). The absolute level of soluble L1 protein was thus highest in cells transformed with MO-HPV58 L1 (Fig. 2a). No L1 protein was purified from cells transformed with WL-HPV58 L1 (Fig. 2b), and the amount of L1 protein purified from cells transformed with MO-HPV58 L1 was 2.5-fold higher than the amount purified from cells transformed with WS-HPV58 L1 (Fig. 2b). The amount of L1 protein purified from 150 ml culture of cells transformed with MO-HPV58 L1 gene for 144 h was 0.48 mg (Fig. 2b). This amount of L1 protein corresponds to 24 human doses for vaccination.
Comparison of soluble L1 protein levels in different transformed clones
Conformation and immunogenicity of purified WS- and MO-HPV58 L1 proteins
Codon optimization of HPV58 L1 gene reduces the intracellular BiP level
We have compared levels of L1 proteins derived from the WL-HPV58 L1, WS-HPV58 L1 and MO-HPV58 L1 genes in S. cerevisiae. It has previously been unclear whether L1 protein is produced from the long L1 gene in yeast. We found that L1 protein was produced from WL-HPV58 L1 at a detectable level but none of that protein was soluble (Fig. 1). Overall expression of L1 protein was highest from the WS-HPV58 L1 gene but the proportion of soluble L1 protein was significantly lower than for the MO-HPV58 L1 gene (Fig. 1). Expression of soluble L1 protein was highest from the MO-HPV58 L1 gene, despite the low level of total L1 protein produced. These results suggest that the nucleotide sequence between the first and second ATGs hinders the expression of L1 protein, and that the codon composition of the wild type L1 gene of HPV58 also reduces soluble L1 protein production.
High levels of non-functional protein are associated with increased cellular stress responses (Mattanovich et al. 2004). In the endoplasmic reticulum (ER), the quality of newly synthesized protein is controlled by post-translational modifications such as glycosylation and disulfide bond formation (Feige and Hendershot 2011; Hulsmeier et al. 2011). Failure of quality control for newly synthesized protein results in accumulation of misfolded proteins, which in excess causes endoplasmic reticulum (ER) stress (Ron and Walter 2007). The BiP protein derived from the KAR2 gene is a well-known marker of ER stress in yeast (Lajoie et al. 2011; Lindl et al. 2007; Mattanovich et al. 2004). We found that the BiP level in cells transformed with MO-HPV58 L1 genes was significantly lower than in cells transformed with WL- or WS-HPV58 L1 genes (Fig. 6). Based on these findings, it is likely that the codon optimization of the MO-HPV58 L1 gene reduces cell stress by decreasing the amount of misfolded protein produced, and the reduced cell stress enhances the performance of the cellular machinery.
Codon usage frequency and secondary structure of mRNA have been thought as critical factors to affect the expression levels of heterogeneous genes. CAI value of WS-HPV58 L1 gene for yeast is higher than those for human and mammals, indicating that the codon composition of L1 gene is relatively favorable for expression in yeast: CAI of WS-HPV58 L1 gene for yeast is 0.83, while those for human and rat are 0.73 and 0.67, respectively. Therefore, it was thought that there is little margin for increasing L1 protein expression in codon usage optimization for yeast. In this study, we used modified L1 DNA designed to reduce the mRNA secondary structure and confirmed that the strategy is useful to obtain the soluble L1 protein.
There have been numerous studies aimed at enhancing protein expression using codon optimized genes. However, there has been little research on codon optimization in S. cerevisiae. Our results suggest that the proportion of soluble protein and amount of final product should be taken into account in codon optimization for S. cerevisiae. We found that the codon optimized MO-HPV58 L1 gene enhanced the expression of soluble L1 protein in S. cerevisiae, and that this protein could be purified and has potential for use as a prophylactic vaccine.