Revertant analysis of a temperature-sensitive mutant of Newcastle disease virus with defective glycoproteins: implication of the fusion glycoprotein in cell killing and isolation of a neuraminidase-deficient hemagglutinating virus.

Biological and molecular properties of a temperature-sensitive mutant (C1) of Newcastle disease virus and its revertants were analyzed. C1 exhibited three temperature-sensitive alterations (plaque formation, virion assembly, and cytopathogenicity) and several defects which were also present at the permissive temperature. C1 virions contained low amounts of hemagglutinin-neuraminidase glycopeptides and consequently were deficient in hemagglutinating and neuraminidase activities. These virions also contained defective fusion glycoproteins which rendered them poorly hemolytic and slow to penetrate cultured chicken embryo cells. The biological activities of the membrane glycoproteins were recovered sequentially in a series of plaque-forming revertants. The coreversion of hemolysis, membrane-penetrating activities, and cytopathogenicity in the first-step revertant (S1) suggested that fusion glycoproteins were major contributors to cellular destruction. This revertant also provided evidence of a role for fusion glycoproteins in virion assembly. From S1 we isolated a large-plaque-forming revertant (L1) that assembled wild-type amounts of biologically active hemagglutinin-neuraminidase glycoproteins into virions. Although it was normal for hemagglutination, L1 had less than 3% of the neuraminidase activity of the wild type, demonstrating that these two activities can be uncoupled genetically. The neuraminidase deficiency of L1 did not impair its virulence in ovo or its reproduction in cultured cells.