Possible Roles of Long-chain Fatty Acyl-CoA Esters in the Fusion of Biomembranes - Fusion of Biological Membranes and Related Problems - page 196

palmitoylated proteins in Sindbis Virus infected chicken embryo cells, a
growing number of viral proteins, including the influenza virus hemagglu-
tinin, have been shown to acquire covalently bound fatty acids (Lambrecht
and Schmidt, 1986; Schmidt and lambrecht, 1985). Hemagglutinin and neu-
roamidase are the two major envelope spike glycoproteins of the influenza
A virus. Despite the fact that hemagglutinin is very well characterized with
regards to its structure, biosynthesis, and posttranslational modifications
(Bullough et al., 1994), the role of palmitoylation of hemagglutinin is only
poorly understood. Hemagglutinin is synthesized as a 550 amino acid pre-
cursor but is subsequently cleaved into two subunits HA1 and HA2 which
associates to form trimers (Klenk and Garten, 1994). HA1 forms the glob-
ular domain of the hemagglutinin spike and is responsible for binding to
sialic acid-containing receptors on host cells, while HA2 is responsible for
fusion activity. Under fusion-inducing conditions the pH decreases slightly
which results in a conformational change of HA2 which exposes a fusion
peptide that binds hydrophobically to the target membrane and the viral
membrane (Chernomordik et al., 1997). These interactions are believed to
induce membrane fusion, a process that is sensitive to changes in the lipid
environment and specific mutations in the fusion protein (Chernomordik
et al., 1997; Kemble et al., 1994). HA2 contains three conserved cysteine
residues in the cytoplasmic domain of the protein in close proximity to its
transmembrane region. Each cysteine residue is palmitoylated through
a thioester bond (Ponimaskin and Schmidt, 1998; Philipp et al., 1995;
Steinhauer et al., 1991; Veit et al., 1991; Naeve and Williams, 1990). Acyla-
tion of hemagglutinin does not affect biosynthesis, transport or recep-
tor binding, and the reported effects on fusion are inconsistent. Using
site-directed mutagenesis Naeve and Williams (1990) showed that modifi-
cation of any of the three cysteines abolished its membrane fusion activity.
This supports the observation that deacylation of hemagglutinin inhibited
viral fusion activity (Lambrecht and Schmidt, 1986). Recently it was shown
that fatty acylation is important in pore flickering, an early event in the
fusion process (Melikyan et al., 1997). Additionally, elimination of acylation
sites in hemagglutinin prevents formation of some infectious virus subtypes
(Zurcher et al., 1994) whereas no effect was observed on other subtypes
(Jin et al., 1997, 1996). In some acylation mutants of hemagglutinin
syncytia formation was suppressed but no effect was reported on mem-
brane fusion and pore formation (Fisher et al., 1998). However, in some
studies acylation did not alter the properties of hemagglutinin to any extent
(Naim et al., 1992;Veit et al., 1991). Despite the inconsistency in the reported
effects of hemagglutinin acylation, it is interesting that of the known
HA serotypes, the five that have been examined with respect to acylation