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Membrane Biology Laboratory, Institute of Molecular and Cell Biology, Singapore 119076, Singapore
Yeast Bet1p participates in vesicular transport from the endoplasmic reticulum to the Golgi apparatus and functions as a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)
associated with ER-derived vesicles. A mammalian
protein (rbet1) homologous to Bet1p was recently identified, and it was concluded that rbet1 is associated with
the Golgi apparatus based on the subcellular localization of transiently expressed epitope-tagged rbet1. In
the present study using rabbit antibodies raised against the cytoplasmic domain of rbet1, we found that the majority of rbet1 is not associated with the Golgi apparatus as marked by the Golgi mannosidase II in normal
rat kidney cells. Rather, rbet1 is predominantly associated with vesicular spotty structures that concentrate in
the peri-Golgi region but are also present throughout the cytoplasm. These structures colocalize with the
KDEL receptor and ERGIC-53, which are known to be
enriched in the intermediate compartment. When the
Golgi apparatus is fragmented by nocodazole treatment, a significant portion of rbet1 is not colocalized
with structures marked by Golgi mannosidase II or the
KDEL receptor. Association of rbet1 in cytoplasmic
spotty structures is apparently not altered by preincubation of cells at 15°C. However, upon warming up
from 15 to 37°C, rbet1 concentrates into the peri-Golgi
region. Furthermore, rbet1 colocalizes with vesicular
stomatitis virus G-protein en route from the ER to the
Golgi. Antibodies against rbet1 inhibit in vitro transport of G-protein from the ER to the Golgi apparatus
in a dose-dependent manner. This inhibition can be
neutralized by preincubation of antibodies with recombinant rbet1. EGTA is known to inhibit ER-Golgi
transport at a stage after vesicle docking but before the
actual fusion event. Antibodies against rbet1 inhibit
ER-Golgi transport only when they are added before
the EGTA-sensitive stage. These results suggest that
rbet1 may be involved in the docking process of ER-
derived vesicles with the cis-Golgi membrane.
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