Spontaneous fusions to prv43 can suppress the export defect of pseudorabies virus gIII signal peptide mutants.

We have devised an enrichment scheme for the isolation of export-competent derivatives of pseudorabies virus glycoprotein gIII signal peptide mutants. Enrichment is based upon a growth advantage imparted upon gIII-containing virions compared with virions lacking the glycoprotein. Each of identified derivatives suppressed the gIII signal peptide defect by fusing the gIII gene in frame to the prv43 gene that lay immediately upstream; the result was the synthesis of a Prv43-gIII hybrid protein. The deduced Prv43 protein is predicted to span a membrane multiple times, and it appeared that the gIII portion of each hybrid used a hydrophobic domain of Prv43 protein to initiate its export. For at least two of the isolates, the hybrid protein was efficiently translocated across the endoplasmic reticulum membrane but appeared to be poorly exported out of the endoplasmic reticulum. Nonetheless, the prv43-gIII fusions encoded a gIII species that was localized to the virus envelope. Because the gIII portion of each hybrid protein must be exposed on the virion surface to provide a growth advantage, our results also suggest a preliminary membrane topology for wild-type Prv43 protein.