Syncollin is a 13 kDa
protein that is highly expressed in the exocrine pancreas.
Syncollin normally exists as a doughnut-shaped homo-oligomer (quite probably a hexamer) in close association with the
luminal surface of the zymogen granule membrane. In the present study, we examine the effect of expression of
syncollin in AtT-20 neuroendocrine cells, which do not normally express this
protein. Efficient expression was achieved by
infection of the cells with adenoviral constructs encoding either untagged or GFP (
green fluorescent protein)-tagged
syncollin. Both forms of the
protein were sorted into
corticotropin (
ACTH)-positive secretory vesicles present mainly at the
tips of cell processes. Neither form affected basal
corticotropin secretion or the constitutive secretion of exogenously expressed secreted
alkaline phosphatase. In contrast, regulated secretion of
corticotropin was inhibited (by 49%) by untagged but not by GFP-tagged
syncollin. In parallel, untagged
syncollin caused a 46% reduction in the number of secretory vesicles present at the
tips of the cell processes.
Syncollin-GFP was without effect. We could also show that native
syncollin purified from rat pancreas was capable of permeabilizing erythrocytes. We suggest that
syncollin may induce uncontrolled permeabilization of
corticotropin-containing vesicles and subsequently destabilize them. Both forms of
syncollin were tightly membrane-associated and appeared to exist as homooligomers. Hence, the lack of effect of
syncollin-GFP on regulated exocytosis suggests that the GFP tag interferes in a subtler manner with the properties of the assembled
protein.