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Vol. 14, Issue 4, 1433-1447, April 2003

The Secretory Apparatus of an Ancient Eukaryote: Protein Sorting to Separate Export Pathways Occurs Before Formation of Transient Golgi-like Compartments

Matthias Marti,^* Yajie Li,^* Elisabeth M. Schraner, Peter Wild, Peter Köhler,^* and Adrian B. Hehl^*§

 ^*Institute of Parasitology, University of Zürich, CH-8057 Zürich, Switzerland, and  Electron Microscopy Unit, Institutes of Veterinary Anatomy and Virology, University of Zürich, CH-8057 Zürich, Switzerland.

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 were faithfully sorted to large encystation-specific vesicles (ESVs), despite the absence of a recognizable Golgi apparatus. Here, we demonstrate 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 endoplasmic reticulum (ER) export of chimeric reporters containing functional signals for both pathways showed that protein sorting was done at or very soon 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. Colocalization of transitional ER regions and early ESVs with coat protein (COP) II 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. Together, the data supported the hypothesis that in Giardia, a primordial secretory apparatus is in operation by which proteins are sorted in the early secretory pathway, and the developmentally induced ESVs carry out at least some Golgi functions.

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Present address: Department of Parasitology, Harbin Medical University, 150086, Harbin, P.R.C.

^§ Corresponding author. E-mail address: ahehl{at}vetparas.unizh.ch.

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Molecular Biology of the Cell
Vol. 14, 1433-1447, April 2003
Copyright © 2003 by The American Society for Cell Biology

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