Sulfhydryl Involvement in Fusion Mechanisms - Fusion of Biological Membranes and Related Problems - page 491

erythrocyte ghosts could restore cell fusion in the presence of A 2 C (Glaser
and Kosower, 1986).
The same laboratory has probed the role of calpain in myoblast fusion
into multinucleated myotubes. It was demonstrated that whereas the
level of calpain polypeptide does not undergo major changes during rat L8
myoblast differentiation and fusion, the level of calpastatin, the endogenous
inhibitor of calpain, decreased dramatically during the stages before and
during myoblast fusion (Barnoy et al., 1996). Calpain activity was stimulated
during this period of time. Again, when calpain activity was inhibited either
directly, by inhibitors such as calpeptin or E-64d, or indirectly, through incu-
bation of the myoblasts with TGF-

(which appears to prevent the diminu-
tion of calpastatin levels that accompanies myoblast differentiation),
protein degradation within the myoblasts and myoblast fusion are inhibited
(Barnoy et al., 1997; Barnoy et al., 1998).

β

2.2. Experimental Systems with Thiol-Reagent- or
Disulfide-Reagent-Modified Proteins of Unknown Identity

2.2.1. Frog Neuromuscular Junction

There are numerous systems where protein thiols have been implicated
in cellular membrane-fusion events, but the targets of modification have
not been identified. In the amphibian neuromuscular junction the thiol-
oxidizing agent diamide stimulates the frequency and amplitude size of
miniature endplate potentials (transmitter release) in a temperature-depen-
dent fashion (Publicover and Duncan, 1981). The stimulation is reversed by
incubation of the frog cutaneous pectoris nerve-muscle preparation with
dithioerythritol and does not appear to be acting through effects on mito-
chondria. The authors conclude that oxidation of protein thiols promotes
vesicle-plasma membrane fusion (Publicover and Duncan, 1981).

2.2.2. Mammalian Sperm-Egg Fusion

Mammalian fertilization culminates with the fusion of a spermatazoon
that has penetrated the zona pellucida with the plasma membrane of an
egg. The effects of the reactive oxygen species, superoxide and hydrogen
peroxide, on mouse sperm motility and sperm-egg fusion have been inves-
tigated. The reactive oxygen species were generated by addition of xanthine
oxidase at various concentrations to hypoxanthine; the higher the concen-
tration of xanthine oxidase the higher the level of the reactive oxygen
species. At high levels of these species sperm motility and sperm-egg fusion