GSBS Dissertations and Theses

Approval Date

November 1999

Document Type

Doctoral Dissertation

Department

Graduate School of Biomedical Sciences, Molecular Genetics & Microbiology

Subjects

Newcastle disease virus; Membrane Fusion; Academic Dissertations; Dissertations, UMMS

Abstract

For many paramyxoviruses, including Newcastle disease virus (NDV), syncytia formation requires the expression of both surface glycoproteins (HN and F) in the same cell, and evidence suggests that fusion involves a specific interaction between the HN and F proteins (23, 73). Because a potential interaction in paramyxovirus infected cells has never been clearly demonstrated, such an interaction was explored in Chapter 2 using coimmunoprecipitation and crosslinking. Both HN and F proteins could be precipitated with heterologous antisera after a five minute radioactive pulse as well as after a two hour chase in non-radioactive media, but at low levels. Chemical crosslinking increased detection of complexes containing HN and F proteins at the cell surface. After crosslinking, intermediate as well as high molecular weight species containing both proteins were precipitated with monospecific antisera. Precipitation of proteins with anti-HN after crosslinking resulted in the detection of complexes which electrophoresed in the stacker region of the gel, from 160-300 kD, at 150 kD and at 74 kD. Precipitates obtained with anti-F after crosslinking contained species which migrated in the stacker region of the gel, between 160-300 kD, at 120 kD and at 66 kD. The 3-4 discrete complexes ranging in size from 160-300 kD contained both HN and F proteins when precipitated with either HN or F antisera. That crosslinking of complexes containing both HN and F proteins was not simply a function of overexpression of viral glycoproteins at the cell surface was addressed by demonstrating crosslinking at early time points post infection, when levels of viral surface glycoproteins are low. Use of cells infected with an avirulent strain of NDV showed that chemically crosslinked HN and F proteins were precipitated independent of cleavage of F0. Furthermore, under conditions that maximized HN protein binding to its receptor, there was no change in the percentages of HN and F0 proteins precipitated with heterologous antisera, but a decrease in F1 protein precipitated was observed upon attachment. These data argue that the HN and F proteins interact in the RER. Upon attachment of the HN protein to its receptor, the HN protein undergoes a conformational change which causes a subsequent change in the associated F protein, releasing the hydrophobic fusion peptide into the target membrane and initiating fusion.

Chapter 3 explores the stalk region of the NDV HN protein, which has been implicated in both fusion promotion and virus specificity of that activity. The NDV F protein contains two heptad repeat motifs which have been shown by site-directed mutagenesis to be critical for fusion (7, 51, 57). Heptad repeat motifs mediate protein-protein interactions by enabling the formation of coiled-coils. Upon analysis of the stalk region of the NDV HN protein, we identified two heptad repeats. Secondary structure analysis of these repeats suggested the potential for these regions to form alpha-helices. To investigate the importance of this sequence motif for fusion promotion, we mutated the hydrophobic "a" position amino acids of each heptad repeat to alanine or methionine. In addition, hydrophobic amino acids in other positions were also changed to alanine. Every mutant protein retained levels of attachment activity that was greater than or equal to the wild-type protein and bound to conformation-specific monoclonal as well as polyclonal antisera. Neuraminidase activity was variably affected. Every mutation, however, showed a dramatic decrease in fusion promotion activity. The phenotypes of these mutant proteins indicate that individual amino acids within the heptad repeat region of the stalk domain of the HN protein are important for the fusion promotion activity of the protein. These data are consistent with the idea that the HN protein associates with the F protein via specific interactions between the heptad repeat regions of both proteins.

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