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The bacterium Clostridium botulinum type C produces a progenitor toxin (C16S
toxin) that binds to O-linked sugar chains terminating with sialic acid on the
surface of HT-29 cells prior to internalization [A. Nishikawa, N. Uotsu, H.
Arimitsu, J.C. Lee, Y. Miura, Y. Fujinaga, H. Nakada, T. Watanabe, T. Ohyama, Y.
Sakano, K. Oguma, Biochem. Biophys. Res. Commun. 319 (2004) 327-333] [21]. Based
on this, it was hypothesized that the C16S toxin is internalized via
clathrin-coated pits. To examine this possibility, the internalized toxin was
observed with a fluorescent antibody using confocal laser-scanning microscopy.
The confocal images clearly indicated that the C16S toxin was internalized
mainly via clathrin-coated pits and localized in early endosomes. The toxin was
colocalized with caveolin-1 which is one of the components of caveolae, however,
implying the toxin was also internalized via caveolae. The confocal images also
showed that the neurotoxin transported to the endosome was transferred to the
Golgi apparatus. However, the non-toxic components were not merged with the
Golgi marker protein, TGN38, implying the neurotoxin was dissociated from
progenitor toxin in endosomes. These results suggested that the C16S toxin was
separated to the neurotoxin and other proteins in endosome and the neurotoxin
was further transferred to the Golgi apparatus which is the center for protein
sorting.
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