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A more recent version of this article appeared on April 1, 2003
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Submitted on August 10, 2002
Revised on October 23, 2002
Accepted on December 9, 2002
1 Institute of Parasitology, University of Zürich, CH-8057 Zürich, Switzerland
2 Institute of Parasitology, University of Zürich, CH-8057 Zürich, Switzerland (present address: Department of Parasitology, Harbin Medical University, 150086, Harbin, P.R.C.)
3 Electron Microscopy Unit, Institutes of Veterinary Anatomy and Virology, University of Zürich, CH-8057 Zürich
* Corresponding author. E-mail address: ahehl{at}vetparas.unizh.ch.
Transmission of the protozoan parasite Giardia intestinalis to vertebrate hosts presupposes the encapsulation of trophozoites into an environmentally resistant and infectious cyst form. We have previously shown that cyst wall proteins (CWPs) were faithfully sorted to large encystation-specific vesicles (ESVs), despite the absence of a recognizable Golgi apparatus. Here we demonstrated that sorting to a second constitutively active pathway transporting variant-specific surface proteins (VSPs) to the surface depended on the cytoplasmic VSP tail. Moreover, pulsed ER export of chimeric reporters containing functional signals for both pathways showed that protein sorting was accomplished at, or very early after, export from the ER. Correspondingly, we found that a limited number of novel transitional ER-like structures together with small transport intermediates were generated during encystation. Co-localization of transitional ER regions and early ESVs with COPII, and of maturing ESVs with COPI and clathrin, strongly suggested that ESVs form by fusion of ER-derived vesicles and subsequently undergo maturation by retrograde transport. Taken together, the data supported the hypothesis that in Giardia a primordial secretory apparatus is in operation where proteins are sorted in the early secretory pathway, and the developmentally induced ESVs carry out at least some Golgi functions.
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