Specific Roles for Lipids in Virus Fusion and Exit Examples from the Alphaviruses - Fusion of Biological Membranes and Related Problems - page 417

Table 1
Summary of events during SFV low pH-induced fusion

Order of events Assay

1. Acid treatment
2. E2-E1 dimer dissociation

sucrose gradient sedimentation,
co-immunoprecipitation

3. E2 alterations trypsin sensitivity

El alterations Ab epitope exposure (enhanced by cholesterol and

sphingolipid)
trypsin resistance, gradient sedimentation, SDS-
PAGE (enhanced by cholesterol and sphingolipid)
liposome coflotation (cholesterol-dependent)
observed as lag time after virus-lipid binding
lipid mixing, content mixing (requires cholesterol
and sphingolipid)

El homotrimerization

4. Lipid bilayer interaction
5. Other unknown steps
6. Fusion

Events are numbered in the probable sequence of their occurrence during the fusion reaction. The multi-
ple events listed under some numbers have not yet been separated kinetically. Modified from (Kielian, 1995).

environment or a fusion-inactive neutral pH environment. The fusion reac-
tion can be slowed by using sub-optimal conditions of pH and temperature,
thus enabling careful dissection of the spike protein conformational
changes involved. While the precise order of the conformational changes is
not completely clear, a great deal of useful information has been gained by
the analysis of these types of virus fusion reactions. Our current under-
standing of the spike protein conformational changes induced by low pH
exposure and their relationship to fusion is summarized as follows and in
Table 1. The lipid dependence of these conformational changes will be dis-
cussed in section 5.1.
The first spike protein alteration observed after low pH exposure is a
change in the normally very stable but non-covalent E2-E1 dimer interac-
tion. The acid-treated dimer dissociates upon solubilization in non-ionic
detergent, and a loss of E2-E1 co-immunoprecipitation and co-flotation on
sucrose gradients is observed. This dimer dissociation appears critical for
virus fusion, since virus mutants that have a more acidic pH threshold for
dimer dissociation also have a more acidic pH threshold for the subsequent
conformational changes in El and for membrane fusion (Glomb-Reinmund
and Kielian, 1998a). The p62-E1 dimer requires a considerably lower pH to
trigger dissociation compared to the mature E2-E1 dimer (Wahlberg et al.,
1989), and this increased dimer stability is responsible for the more acidic
pH required to trigger the fusion of virus or spike protein mutants con-