Membrane fusion, the merging of two initially distinct membranes to form one
common lipid bilayer, is a fundamental mechanism of life. It occurs many times each
day within every eukaryotic cell as part of essential daily homeostatic processes, as
well as between individual cells, such as sperm and egg during fertilisation. The fusion
mechanism is, however, also crucial to the development of many diseases. All
enveloped viruses, and indeed many other obligate intracellular parasites, must fuse
their own surrounding lipid bilayer with the membrane of their host's target cell in
order to gain cell entry and thus the ability to replicate. These infections produce
disease states, and possibly even death, in the host species
Despite the clear importance of fusion, the precise molecular events that
occur during this process are still not known. Fusion proteins of viruses have
recently become popular tools for use in fusion studies. More specifically, several
viruses have known fusion peptides, the sections of these proteins which confer their
fusogenic activity. This thesis examines the structure and function of the putative
fusion peptide of the retrovirus Feline leukaemia virus, (FeLV), using a variety of
mainly biophysical techniques.
The structural effects of the FeLV fusion peptide on lipid polymorphism
were studied. Using differential scanning calorimetry, ³¹P nuclear magnetic
resonance and time-resolved X-ray diffraction this peptide was found to induce
changes in lipid conformation and motion similar to those of known fusogens: it
favoured the formation of non-bilayer lipid conformations which have a relatively
large negative curvature, namely the inverted hexagonal phase and isotropic lipid
states. Moreover, using X-ray diffraction, a new lipid phase was observed in the
presence of the FeLV peptide
Neutron diffraction studies revealed a change in the packing of lipid
molecules within a bilayer and also possible thinning ofthe bilayer, both ofwhich
were induced by interaction with the FeLV fusion peptide.
Fusogenic activity for this putative viral fusion peptide was demonstrated,
using fusion assays, which measured the merging of lipid membranes in the presence
ofthe FeLV fusion peptide.
These findings are discussed in the light ofthe current concepts ofthe fusion
mechanism. They add support to two currently favoured theories of fusion:
precession by a fusion peptide as a means of inducing the initial destabilisation of a
bilayer, and the formation ofhighly bent, high energy lipid intermediates, such as the
'modified stalk', in the multistep fusion pathway.
Circular dichroism was employed to determine the secondary structure ofthe
FeLV fusion peptide under a variety of experimental conditions. This peptide was
observed to flip readily between a-helical and p sheet conformations. This suggests
that structural plasticity may be an important dynamic property offusion peptides.
Possible relationships between peptide structure and function are discussed