Cloning, expression, purification and characterization of the recombinant nucleocapsid protein from SARS-CoV-2 and its combination with a CpG ODN-39M

Several studies have reported the ability of the nucleocapsid (N) protein from the SARS-CoV-2 virus to interact with RNA in vitro. Nevertheless, its capacity to interact with deoxyribonucleic acid has yet to be widely described. In the present work, the nucleocapsid protein from the SARS-CoV-2 Delta variant was obtained in E. coli. The recombinant protein was purified, immune identified and incubated with different quantities of the ODN-39M, a CpG ODN with adjuvant properties. As a result, a curve of precipitation was obtained. The analysis by agarose gel electrophoresis of the mixtures revealed that ODN-39M retarded its mobility, whereas the protein increased its migration. When a sample of N+ODN-39M was crosslinked and analyzed by SDS-PAGE, high molecular weight aggregates were detected. Accordingly, upon analysis by Transmission Electron Microscopy, particles of approximately 20 nm were visualized in both samples of N protein, with and without ODN-39M. However, aggregates of particles were abundant in the mixture. Notably, such structures kept the ability of N protein to be recognized by human sera of COVID-19 convalescent donors. We can assert that nucleocapsid protein can bind to deoxyribonucleic acid. Such interaction changed the protein conformation, contributing to the previously reported immunogenicity in mice of N+ODN-39M complexes. Keywords: Nucleocapsid, SARS-CoV-2, ODN-39M, Particles