The Paramyxovirus SV5 Small Hydrophobic (SH) Protein Is Not Essential for Virus Growth in Tissue Culture Cells

The SH gene of the paramyxovirus SV5 is located between the genes for the glycoproteins, fusion protein (F) and hemagglutinin-neuraminidase (HN), and the SH gene encodes a small 44-residue hydrophobic integral membrane protein (SH). The SH protein is expressed in SV5-infected cells and is oriented in membranes with its N terminus in the cytoplasm. To study the function of the SH protein in the SV5 virus life cycle, the SH gene was deleted from the infectious cDNA clone of the SV5 genome. By using the recently developed reverse genetics system for SV5, it was found that an SH-deleted SV5 (rSV5ΔSH) could be recovered, indicating the SH protein was not essential for virus viability in tissue culture. Analysis of properties of rSV5ΔSH indicated that lack of expression of SH protein did not alter the expression level of the other virus proteins, the subcellular localization of F and HN, or fusion competency as measured by lipid mixing assays and a new content mixing assay that did not require the use of vaccinia virus. The growth rate, infectivity, and plaque size of rSV5 and rSV5ΔSH were found to be very similar. Although SH is shown to be a component of purified virions by immunoblotting, examination of purified rSV5ΔSH by electron microscopy did not show an altered morphology from SV5. Thus in tissue culture cells the lack of the SV5 SH protein does not confer a recognizable phenotype.