Cells Persistently Infected with Newcastle Disease Virus III. Thermal Stability of Hemagglutinin and Neuraminidase of a Mutant Isolated from Persistently Infected L Cells

Abstract

Data were obtained which indicated the possible cause of the defective elution from erythrocytes of the mutant virus (NDV pi ) isolated from L cells persistently infected with the Herts strain of Newcastle disease virus (NDV o ). The chicken erythrocyte receptors for the mutant and wild-type viruses were equally sensitive to the action of Vibrio cholera filtrate neuraminidase; this suggests that the failure of NDV pi to elute from chicken erythrocytes is not due to a specific neuraminidase-resistant receptor for this virus on the erythrocyte membrane. There was no difference in the enzyme content of the intact virions of NDV o and NDV pi when tested with a soluble substrate, indicating that the inefficient elution of NDV pi was not due to a reduced enzyme content. The neuraminidase activity of intact NDV pi virions was significantly more stable at 55 C than the enzyme of NDV o virions, whereas the dissociated enzymes of the two viruses were inactivated at the same rate. On the basis of these findings, it seems likely there is a structural difference between the two viruses. The neuraminidase protein of the mutant NDV pi may be incorporated into the viral envelope in such a manner that it is prevented from reacting with the substrate in the erythrocyte membrane, although it can react with a soluble substrate. The hemagglutinin activity of both intact and disrupted NDV pi was significantly more resistant to thermal inactivation than that of the wild-type NDV o . This finding suggests a genetic difference in the hemagglutinin protein of the two viruses.