Conformational change in a viral glycoprotein during maturation due to disulfide bond disruption

The fusion glycoprotein of Newcastle disease virus is synthesized as an inactive precursor, F0. During intracellular transport and maturation, F0 undergoes a conformational change resulting from the loss of intramolecular disulfide bonds. F0 is also cleaved to yield F1, F2, the active, membrane-fusing form of the protein. Two monoclonal antibodies were used to explore this conformational change and its relationship to cleavage. These antibodies failed to precipitate the pulse-labeled fusion protein but did precipitate the F0 and the F1, F2 forms of the "chase" fusion protein. Use of the inhibitors carbonylcyanide m-chlorophenylhydrazone and monensin showed that the fusion protein acquired the ability to react with the monoclonal antibodies after it left the rough endoplasmic reticulum but before it left the medial Golgi membranes and before it was cleaved. The acquisition of antigenicity correlates with the disruption of intramolecular disulfide bonds during transit through the cell. This correlation was directly confirmed. The pulse-labeled fusion protein could be recognized by both monoclonal antibodies if the protein was first reduced. The formation and disruption of intramolecular disulfide bonds as a posttranslational modification of glycoproteins is discussed.