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Studies on the processing of rubella virus structural proteins by analysis of the endoproteolytic cleavage sites McDonald, Helen L


Rubella virus is a small enveloped positive strand RNA virus. Two species of viral RNA are found in infected cells: a full-length genomic RNA and a subgenomic species corresponding to the 3' one third of the genomic RNA molecule. The 24S subgenomic RNA specifies a polyprotein which is cotranslationally processed by endoproteolytic cleavage by host signal peptidase to yield three structural proteins, El, E2 and capsid. El and E2 are membrane glycoproteins forming the virion spikes, and C protein binds to 40S genomic RNA to form a nucleocapsid. El and E2 proteins contain N-linked oligosaccharide as a consequence of their passage through the endoplasmic reticulum (ER) and Golgi apparatus. According to the signal hypothesis, translocation of secretory and membrane proteins into the ER is mediated by a hydrophobic signal peptide. The signal peptides for E2 and El have been localized by in vivo expression of El and E2 cDNAs. Oligonucleotide-directed mutagenesis was used to define the cleavage sites between C, E2, and El, as well as the effect of the cleavages on the transport and processing of E2 and El. The expression of the cleavage site mutants was studied in vitro and in vivo. It was found that uncleaved precursor polypeptides were retained in the ER and very little E2 or El polypeptide was observed at either the Golgi apparatus or the plasma membrane. The E2 and El polypeptides can cross the ER membrane without the cleavage of the signal peptide while the transport of E2 and El beyond the ER requires the cleavage of E2 from C and El from E2. The C-termini of the C and E2 proteins, which were not previously defined, have been partially characterized. Capsid protein does not appear to undergo further proteolytic processing after it is cleaved from E2 by signal peptidase, but E2 may be processed at a second cleavage site at its C-terminus by a trypsin-like enzyme.

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