The biologically active form of the fusion glycoprotein F from Newcastle disease virus (NDV) comprises two polypeptides, F and F (derived from a precursor polypeptide F by a post translational cleavage event), which are covalently linked together (F) by disulphide bonds. This feature was exploited in a two-dimensional SDS-polyacrylamide gel electrophoretic analysis to orientate the position of the cleavage event within F. Separation of proteins from NDV-infected CEF in the first dimension in the absence of reducing agent resolved F protein from all NDV-induced proteins other than F. Reduction of the first dimension gel with 2-mercaptoethanol, followed by electrophoresis in the second dimension, resolved F (55K), F (12.5K) and F (64K) proteins. The only polypeptides other than F and F which fell below the diagonal, indicating the positions of the polypeptides from infected cells, were two minor glycoproteins designated HN (51.5K) and HN (27.5K) which took up positions vertically beneath the major haemagglutinin-neuraminidase glycoprotein HN (75K). Dual isotope labelling experiments with NDV-infected chick embryo fibroblasts, which had previously received a salt shock to effect synchronization of polypeptide initiation upon release of salt shock, revealed the following orientations within the parent molecules: NH―F―F―COOH and NH―HN―HN―COOH.

The existence of intermolecular disulphide bonds, orientation and relative lengths of the two NDV HN fragments is analogous to the HA and HA proteins of influenza virus haemagglutinin.


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