Disappearance of neuraminidase and hemagglutinin activities in NDV-infected chick embryo cell monolayer culture treated with inhibitors of protein synthesis. Brief report

Dynamics of functional maturation and inactivation of HN glycoprotein in NDV-infected chick embryo fibroblasts

In avirulent NDV strain-infected chick embryo cells treated with cycloheximide at different intervals post infection a decrease of the level of hemagglutinating (HA) and neuraminidase (Nase) activities was observed. Studies on this system led to conclusion that the HA-Nase (HN) glycoprotein molecules are unstable and the actual amount of the functionally active (mature) HN entities is determined by a dynamic equilibrium between the antidromic processes of the HN functional maturation and inactivation. Kinetic studies on the actual intracellular levels of the HA and Nase activities using 5 min intervals of their detection after the cycloheximide treatment permitted to uncouple the processes of the HN maturation and inactivation. Analytical part of the studies made it possible to compute quantitative parameters of the involved processes: (a) pool size of the functionally nonactive HN precursors, (b) time needed for their functional maturation, and (c) rate of their inactivation.

Effect of inhibitors of proteases and glycosidases on functional lability of newly synthesized hemagglutinin-neuraminidase glycoprotein in avirulent NDV strain-infected cells

Inhibition of either glycosylation or the protein synthesis leads to sharp drop of hemagglutinating (HA) and neuraminidase (Nase) activities in Newcastle disease virus (NDV)-infected cells. The drop can be prevented by tosyl-lysyl-chloromethyl-ketone, an inhibitor of trypsin-like proteases, or D-galactonolactone, an inhibitor of glycosidases, if added together with cycloheximide. Tosyl-lysyl-chloromethyl-ketone when added alone increases the level of both the HA and Nase activities as compared with non-treated virus-infected cells. It has been suggested that the actual level of the HA and Nase activities in the virus-infected cells is a result of dynamic equilibrium between the synthesis and post-translational functional maturation of the hemagglutinin-neuraminidase (HN) molecules, on the one hand, and the protease and glycosidase-mediated functional inactivation, on the other.

Strain-specific degradation of a viral glycoprotein in Newcastle disease virus-infected cells

In cells infected with mesogenic or lentogenic strain of Newcastle disease virus the level of neuraminidase and hemagglutinin activities sharply decreased after the addition of cycloheximide. With two velogenic strains such decreases did not occur. The infected cells were labelled with 14C-amino acids (leucine or valine) and further incubated with an excess of unlabelled precursor. Polyacrylamide gel analysis revealed a decrease of the peak correspondig to the "large" glycoprotein after the chase in cells infected with meso- or lentogenic strain (Beaudette, B1). In the cells infected with velogenic strains (Italia, Herts) no such decrease was observed. The degradation of the "large" glycoprotein as the cause of the decrease of hemagglutinin and neuraminidase activities in cycloheximide-treated cells and its possible relation to virulence is discussed.