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in these cells. Over the years, the term “fusion pore” has been loosely referred
to plasma membrane dimples that originate following a secretory stimulus or
to the continuity or channel established between opposing lipid membrane
during membrane fusion. Therefore, for clarity, the term “porosome” was
assigned to depressions.
(a)
(b)
(c)
(d)
Figure 5.3. Porosomes dilate to allow expulsion of vesicular contents. (a and b) AFM
micrographs and section analysis of a pit and two out of the four depressions or
porosomes, showing enlargement of porosomes following stimulation of secretion.
(c) Exposure of live cells to gold-conjugated amylase antibody (Ab) results in speciic
localization of gold to these secretory sites. Note the localization of amylase-speciic
immunogold at the edge of porosomes. (d) AFM micrograph of pits and porosomes
with immunogold localization is also demonstrated in cells immunolabeled and then
ixed. Blue arrowheads point to immunogold clusters and the yellow arrowhead
points to a depression or porosome opening. 7
The porosome structure, at the cytosolic compartment of the plasma
membrane in the exocrine pancreas 10 and in neurons, 9 has also been
determined at near-nanometre resolution in live tissue. To determine the
morphology of porosomes at the cytosolic compartment of pancreatic
acinar cells, isolated plasma membrane preparations in near-physiological
buffered solution have been imaged at high resolution using AFM. 10 These
studies reveal scattered circular disks measuring 0.5-1 μm in diameter, with
inverted cup-shaped structures within. 10 The inverted cups at the cytosolic
compartment of isolated pancreatic plasma membrane preparations range
in height from 10 to 15 nm. On several occasions, ZGs ranging in size from
0.4 to 1 μm in diameter were observed in association with one or more of
the inverted cups, suggesting the circular disks to be pits and the inverted
cups to be porosomes. To further conirm that the cup-shaped structures are
porosomes, where secretory vesicles dock and fuse, immuno-AFM studies
were performed. Target membrane proteins SNAP-23 15 and syntaxin 16
(t-SNARE) and secretory vesicle-associated membrane protein v-SNARE
or VAMP 17 are part of the conserved protein complex involved in fusion of
 
 
 
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