Influence of intron and exon splicing enhancers on mammalian cell expression of a truncated spike protein of SARS-CoV and its implication for subunit vaccine development

Abstract

The spike (S) protein of severe acute respiratory syndrome coronavirus (SARS-CoV) is important for vaccine development. A truncated S protein of the TW1 strain, S TR2 (88 kDa), carrying three S fragments (S74–253, S294–739, and S1129–1255) was investigated to study the influences of intron and exon splicing enhancers to improve S TR2 protein expression in mammalian cells. Our results showed that S TR2 protein expression with the use of an 138 base-pair intron addition increased by 1.9-, 2.5-, and 4.1-fold in Vero E6, QBI-293A cells, and CHO/dhFr− cells (dihydrofolate reductase [ dhfr] gene deficient CHO cells), respectively. Using the exon splicing enhancers, including a bidirectional splicing enhancer (BSE) or an exon splicing enhancer derived from the EDA alternative exon of the fibronectin gene (EDA ESE), were also found to increase S TR2 protein expression in CHO/dhFr− cells by 1.7- and 2.6-fold. Nevertheless, combination of the intron and the exon splicing enhancers resulted in suppressing the intron-enhancing e S TR2 protein expression in in CHO/dhFr− cells. Our studies also demonstrated the S TR2 protein was mainly as the Endo H-sensitive glycoprotein (115 kDa) expressed in Vero E6, QBI-293A, and CHO/dhFr− cells. However, only a minor form of the Endo H-resistant glycoproteins (∼130 kDa) was detected in CHO/dhFr− cells. Taken together, our results indicated that intron had a better enhancing effect on S TR2 protein expression than exon splicing enhancers, and the expression of ∼130 kDa S TR2 glycoprotein was enhanced by the intron addition into the expression vector construct. Results of the present study can provide an optimal strategy to enhance SARS-CoV S protein expression in mammalian cells and may contribute to the development of SARS-CoV subunit vaccine.