Sulfhydryl Involvement in Fusion Mechanisms - Fusion of Biological Membranes and Related Problems - page 509

homotrimer (Figure 2B) (Wahlberg et al., 1992). It is noteworthy that cleav-
age of PE2 appears to be necessary for the dissociation of the heterodimer
under normal conditions (Salminen et al., 1992). Perhaps El and PE2 form
a difficult-to-detect cystine bridge, similar to the MuLV SU-TM disulfide
bond, that cannot rearrange under normal conditions, but does rearrange
during solubilization of the proteins.

4. CONCLUSION

Although much of the evidence for the roles of protein thiols and thiol-
disulfide exchange in membrane fusion is indirect, it appears probable that
additional cellular membrane fusion-promoting proteins containing cat-
alytically important or thiol-reagent-accessible cysteine residues, besides
the N -ethylmaleimide-sensitive factors and cysteine proteases, will be iden-
tified. Of equal importance and probability will be the ongoing investiga-
tion of the occurrence and prevalence of thiol-disulfide exchange reactions
in viral glycoproteins during the process of membrane fusion. Such studies
may not only illuminate a critical and common biochemical step in mem-
brane-fusion but may also lead to the identification of reagents that can
specifically inhibit the thiol-disulfide exchange and thereby prevent viral
entry.

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