4. Discussion
The recombinant proteins produced in E.coli often aggregate or
degrade rapidly because of their inability to form correct tertiary
structures due to anomalies in protein folding (Kim, Kweon, Lee, Park,
& Seo, 2005; Nishihara, Kanemori, Yanagi, & Yura, 2000). In this
study, we used the chaperone protein pTf16 to increase the expression of
VP3 in supernatant and the results showed that the solubility of the
protein was significantly improved. In fact, it has been demonstrated
that chaperone over-production can slow the rate of protein expression
in E.coli by sequestering nascent polypeptides for prolonged
periods (Hu et al., 2007). The effect of the combination of the
chaperone protein pTf16 with the recombinant vector is similar to the
effect of the low temperature and reduced inducer concentration which
slows down the production but increases the proportion of functional
heterologous protein in the process of recombinant protein production
(Hu et al., 2007; Shibui & Nagahari, 1992).
The most important chaperones in E.coli include pTf16, GrpE,
GroEL, DnaK, DnaJ, and GroES. The molecular chaperone TF expressed by
pTf16 was initially identified as a protein that binds to certain
precursor proteins and facilitates their transport to membrane vesicles
(Crooke & Wickner, 1987). Some studies have shown that TF may play a
role in protein folding because it is related to nascent peptides and
50S ribosomes (Hartl & Hayer-Hartl, 2002; Hesterkamp, Hauser, Lütcke,
& Bukau, 1996). In addition, TF is related to GroEL, which can enhance
the binding of GroEL substrate and promote protein folding or
degradation (Kandror, Sherman, Moerschell, & Goldberg, 1997; Kandror,
Sherman, Rhode, & Goldberg, 1995). The effectiveness of these
chaperones on protein folding, stability, and aggregation has also been
demonstrated (Maeng, Nam, & Kim, 2011; Nishihara, Kanemori, Kitagawa,
Yanagi, & Yura, 1998; Veisi et al., 2015). In addition to chaperone
proteins, fusion tagging technology also facilitates the expression of
soluble proteins in E.coli . Davis et al (Davis, Elisee,
Newham, & Harrison, 1999). used the NusA solubilizing label to increase
the solubility of bovine growth hormone by up to 90%, De (De Marco,
Stier, Blandin, & de Marco, 2004) and his colleagues showed that the
fusion protein purified by NusA fusion tag was more stable and higher in
content than GST fusion tag.
When selecting the optimal expression vector, we found that the NusA
fusion protein also affect the solubilization . Previous studies have
reported that the NusA solubilizing tag can significantly improve the
solubility of recombinant proteins in E.coli (De Marco et al.,
2004; Zacharchenko, Barsukov, Rigden, Bennett, & Mayans, 2016).
Considering the size of the fusion protein and the difficulty of
subsequent purification, the chaperone pTf16 was finally selected for
later experiments.
In addition, during protein
purification, we found that the mounting effect of VP3 is not ideal.
This may be due to the fact that VP3 is too large, which prevents the
enrichment of VP3 on Ni-NTA, resulting in VP3 not fully binding with
Ni-NTA. We then incubated the supernatant with the Ni-NTA for 2 h and
reduced the flow rate during elution, which improved the VP3 bounding to
Ni-NTA to a certain extent. The purity of the VP3 could reach 70% after
purification, and it can produce certain immunogenicity. This study lays
the foundation for further research on the structure and function of VP3
and other proteins of bluetongue virus. Apart from this, it provided
materials for the preparation of BTV-1 VP3 monoclonal antibody.
In conclusion, we expressed four recombinant proteins in E. coli,including His-VP3, His-VP3-Tf, His-VP3-NusA and TRX-His-VP3. Compared
with His-VP3 without molecular chaperone, His-VP3 with molecular
chaperone pTf16 increased its expression in the supernatant by about
30%. The VP3 purified by Ni-NTA has certain immunogenicity by Dot-ELISA
and IFA test.
ACKNOWLEDGEMENTS
This work was supported by the National Key Research and Development
Program (2017YFD0502304-4); ‘1125 talent gathering plan’ project of
Zhengzhou. We thank Dr. Yankai Liu for revising the grammatical errors
and reorganizing this manuscript. Dr. Liu held a doctorate from Queen
Mary University of London and now he is an associate professor of
bioinformatics at Zhengzhou University.
CONFLICT OF INTEREST
The authors declare that they have no competing interests.
ETHICAL APPROVAL
Ethical statement is not applicable because sample collection has been
gathered.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the
corresponding author upon reasonable request.