Dimerisation of glycoprotein Erns of classical swine fever virus is not essential for viral replication and infection

The pestivirus glycoprotein Erns, a ribonuclease, is expressed on the surface of virions and in infected cells as a disulfide-linked homodimer. Erns is involved in the infection process and its RNase activity is probably involved in viral replication and pathogenesis. The most C-terminal cysteine residue forms an intermolecular disulfide bond with another Erns monomer, resulting in an Erns dimer. To study the function of dimerisation of Erns for viral replication, the cysteine residue at amino acid position 438 was mutated into a serine residue. The mutated C438S gene was cloned into a vector containing an infectious cDNA copy of the CSFV C-strain genome. Using reverse genetics, a mutant virus was generated that only expressed monomeric Erns, confirming that Cys 438 is essential for homodimerisation. Characterization of this mutant virus and of a baculovirus-expressed C438S mutant protein indicated that the loss of the dimeric state of Erns reduced the affinity of binding of virions and Erns to heparan sulphate (HS), the receptor for Erns on the cell surface of SK6 cells. This suggests that interaction of virus-bound Erns homodimers with membrane associated HS may be a joined action of the two HS-binding domains (one in each monomer) present in the homodimer.