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Eukaryotic Cell, February 2005, p. 432-442, Vol. 4, No. 2
1535-9778/05/$08.00+0     doi:10.1128/EC.4.2.432-442.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Receptor for Retrograde Transport in the Apicomplexan Parasite Toxoplasma gondii

Stacy L. Pfluger,1,2 Holly V. Goodson,2,3 Jennifer M. Moran,1,2 Christine J. Ruggiero,1 Xin Ye,1 Krista M. Emmons,1 and Kristin M. Hager1,2*

Department of Biological Sciences,1 Center for Tropical Disease and Research Training,2 Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana3

Received 23 August 2004/ Accepted 16 November 2004

Toxoplasma gondii and its apicomplexan relatives (such as Plasmodium falciparum, which causes malaria) are obligate intracellular parasites that rely on sequential protein release from specialized secretory organelles for invasion and multiplication within host cells. Because of the importance of these unusual membrane trafficking pathways for drug development and comparative cell biology, characterizing them is essential. In particular, it is unclear what role retrieval mechanisms play in parasite membrane trafficking or where they operate. Previously, we showed that T. gondii’s beta-COP (TgBCOP; a subunit of coatomer protein complex I, COPI) and retrieval reporters localize exclusively to the zone between the parasite endoplasmic reticulum (ER) and Golgi apparatus. This suggested the existence of an HDEL receptor in T. gondii. We have now identified, cloned, and sequenced this receptor, TgERD2. TgERD2 localizes in a Golgi or ER pattern suggestive of the HDEL retrieval reporter (K. M. Hager, B. Striepen, L. G. Tilney, and D. S. Roos, J. Cell Sci. 112:2631-2638, 1999). A functional assay reveals that TgERD2 is able to complement the Saccharomyces cerevisiae ERD2 null mutant. Retrieval studies reveal that stable expression of a fluorescent exogenous retrieval ligand results in a dispersal of ßCOP signal throughout the cytoplasm and, surprisingly, results in ßCOP staining of the vacuolar space of the parasite. In contrast, stable expression of TgERD2GFP does not appear to disturb ßCOP staining. In addition to TgERD2, Toxoplasma contains two more divergent ERD2 relatives. Phylogenetic analysis reveals that these proteins belong to a previously unrecognized ERD2 subfamily common to plants and alveolate organisms and as such could represent mediators of parasite-specific retrieval functions. No evidence of class 2 ERD2 proteins was found in metazoan organisms or fungi.

* Corresponding author. Mailing address: Center for Tropical Disease Research and Training, Department of Biological Sciences, 216 Galvin Life Sciences Building, University of Notre Dame, Notre Dame, IN 46556-0369. Phone: (574) 631-8135. Fax: (574) 631-7413. E-mail: khager1{at}nd.edu.

Eukaryotic Cell, February 2005, p. 432-442, Vol. 4, No. 2
1535-9778/05/$08.00+0     doi:10.1128/EC.4.2.432-442.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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