Sulfhydryl Involvement in Fusion Mechanisms - Fusion of Biological Membranes and Related Problems - page 503

analogous to that proposed for the influenza virus hemagglutinin protein,
and membrane fusion ensues (Figure 1B).
There are three lines of evidence suggesting that such a scenario may
be correct. The first is that the SU-TM disulfide is labile in the presence of
detergents, but it is stabilized by N -ethylmabhide treatment prior to solu-
bilization (Gliniak et al., 1991; Pinter and Fleissner, 1977; Pinter et al., 1997;
Pinter et al., 1978) or by reducing the pH during solubilization (Opstelten et
al., 1998).This suggests that the blocking of a free sulfhydryl,presumably the
second cysteine of the CXXC motif, stabilizes the preexistent intermolecu-
lar cystine bridge (Figure 1C). The second is that the products of protease
digestion of the disulfide-linked Friend MuLV SU and TM indicates that the
bond is between a half-cystine in the disulfide exchange motif and one in the
CX 6 CC motif (Pinter et al., 1997).The third is that investigations of the disul-
fide bonds in the isolated Friend MuLV and Friend Mink-Cell Focus-induc-
ing virus SU proteins indicate that the two cysteines in the CXXC motif are
linked in a cystine bridge (Linder et al., 1992;Linder et al., 1994). Such cystine
bridges are known to be highly strained and are found heretofore only in
proteins with thiol-disulfide exchange activity (Darby and Creighton, 1995;
Zapun et al., 1993). The idea that thiol-disulfide exchange accompanies
receptor binding is as yet unsupported by direct experimental data.
However, it is plausible considering the conservation and placement of the
motifs and the analogies with ricin and diphtheria toxin: Interchain disulfide
bond reduction appears to be a rate-limiting step in membrane penetration
by both toxins (Lewis and Youle, 1986; Papini et al., 1993). In contrast to the
HIV glycoproteins, which appear to be dependent on exogenous enzymatic
activities for the catalysis of disulfide-bond rearrangements (Ryser et al.,
1994), the MuLV and other homologous Envs would possess a thiol-disul-
fide-exchange enzyme active site as part of their structures. Other than the
regions of the two cysteine-containing motifs and a GXDP motif (Gallaher
et al., 1995), the MuLV envelope proteins share essentially no sequence iden-
tity with those of the more distant HTLV-1, BLV, and M-PMV, suggesting
that these conserved motifs have coevolved and are likely to play similar
roles in all cases. Although a covalent interaction between SU and TM has
not been found for some of the more distantly related viruses (Brody et al.,
1994), these studies have not utilized conditions capable of stabilizing a labile
disulfide bond.
It has been suggested that the SU-TM disulfide bond is not “labile” to
reduction but merely appears to rearrange as an artifact of solubilization
(Opstelten et al., 1998). It is therefore, in fact, a “stable” disulfide when
present in intact glycoprotein complexes on the surface of cells or virions.
Nevertheless, the SU-TM linkage is “labile” in the sense that extra care is