ABSTRACT
Membrane fusion (Fig. 11.1) is an essential event in many biological
processes of eukaryotic cells (Jahn et al., 2003), such as for
instance, in the vesicular release of hormones. Membrane fusion
is thought to begin with the formation of a hemifusion stalk,
an intermediate structure connecting the outer leaflets of fusing
membranes (for a review see Chernomordik and Kozlov, 2008).
The hemifusion stalk then develops into a fusion neck (pore), the
membrane bilayer channel connecting a spherical vesicle and a
plasma membrane, through which cargo molecules may diffuse
from the vesicle lumen into the cell exterior (Jorgacˇevski et al.,
2010). After formation, the fusion neck either closes and allows
the vesicle to be reused in another round of (transient) exocytosis
(Ceccarelli et al., 1973), or it fully opens, leading to full fusion
exocytosis, that is, complete merging of the vesicle membrane
with the target plasma membrane (see Heuser and Reese, 1973,
and Fig. 11.1). The mechanisms by which the initial fusion neck
between the vesicle and the plasma attains stability are still
poorly understood (Jorgacˇevski et al., 2010). Formation of the
fusion neck at first glance may be considered to be energetically
unfavourable by the argument that the repulsive electrostatic forces
between two closely opposed phospholipid bilayers need to be
overcome in order to reach metastable transition states leading
to fusion neck formation (Kozlov and Markin, 1983). However,
in cellular systems the repulsive electrostatic force between like-
charged membrane surfaces can be reduced or even changed into
an attractive force, for example, by protein-mediated interactions
between like-charged membrane surfaces, where the proteins (or
some other biological nanoparticles, such as lipoproteins) should
have a distinctive internal charge distribution (see Chapter 14 and
Kim et al., 2008; May et al., 2008; Urbanija et al., 2007, 2008a,b).
Another example of possible mediators that may reduce the energy
barrier between the vesicles and the target plasma membrane in
regulated exocytosis is soluble N-ethylmaleimide-sensitive factor attachment protein (SNARE) receptor (Duman and Forte, 2003; Jahn
and Scheller, 2006; Martens et al., 2007).
