In vitro and in vivo expression of rubella virus glycoprotein E2: the signal peptide is contained in the C-terminal region of capsid protein.
The 24 subgenomic mRNA of rubella virus (RV) specifies a polyprotein which is post-translationally processed to three structural protein species E1, E2, and capsid. E1 and E2 are membrane glycoproteins forming the virion spikes. In the polyprotein, E2 and E1 are both preceded by stretches of uncharged, mainly nonpolar amino acids which probably function as signal peptides mediating translocation into the endoplasmic reticulum. We have previously shown that translocation of E1 is reinitiated by a signal peptide located in the carboxy-terminus of E2 (Hobman et al., 1988, J. Virol. 62, 4259-4264). A cDNA from RV encoding the entire E2 gene fused to the capsid N-terminus has been constructed, allowing expression of RV E2 in vitro and in vivo. The resulting protein is efficiently translocated into canine pancreatic microsomes and is glycosylated when expressed in vitro. In vivo some of the N-linked sugars are processed to complex types. Cell surface immunofluorescence indicates that RV E2 is transported to the plasma membrane in COS cells. Oligonucleotide-directed mutagenesis was used to create a cDNA lacking 163 nucleotides immediately 5' to the E2 coding region. This deletion mutant failed undergo translocation into microsomes in vitro and was unstable when expressed in COS cells. The results imply that a signal peptide domain for RV E2 is contained in the carboxyl terminus of the capsid.