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Published online 5 February 2001. doi:10.1083/jcb.152.3.563
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© The Rockefeller University Press, 0021-9525/2001//563 $5.00
The Journal of Cell Biology, Volume 152, Number 3, , 2001 563-578

Original Article

Identification and Characterization of an Escorter for Two Secretory Adhesins in Toxoplasma gondii



Matthias Reissa, Nicola Viebiga, Susan Brechta, Marie-Noelle Fourmauxb, Martine Soeteb, Manlio Di Cristinac, Jean François Dubremetzb, and Dominique Soldatia

a Center for Molecular Biology, University of Heidelberg, Heidelberg D-63120, Germany
b Institute of Biology CNRS, Institut Pasteur de Lille, Lille, 59019 France
c Department of Cell Biology, Imperial College of Science, Technology, and Medicine, London, SW7 2AZ United Kingdom
ZMBH, Im Neunheimer Feld 282, P.O. Box 106249, Heidelberg D-69120, Germany.49-6221-54-58-9249-6221-54-68-70

soldati{at}zmbh.uni-heidelberg.de

The intracellular protozoan parasite Toxoplasma gondii shares with other members of the Apicomplexa a common set of apical structures involved in host cell invasion. Micronemes are apical secretory organelles releasing their contents upon contact with host cells. We have identified a transmembrane micronemal protein MIC6, which functions as an escorter for the accurate targeting of two soluble proteins MIC1 and MIC4 to the micronemes. Disruption of MIC1, MIC4, and MIC6 genes allowed us to precisely dissect their contribution in sorting processes. We have mapped domains on these proteins that determine complex formation and targeting to the organelle. MIC6 carries a sorting signal(s) in its cytoplasmic tail whereas its association with MIC1 involves a lumenal EGF-like domain. MIC4 binds directly to MIC1 and behaves as a passive cargo molecule. In contrast, MIC1 is linked to a quality control system and is absolutely required for the complex to leave the early compartments of the secretory pathway. MIC1 and MIC4 bind to host cells, and the existence of such a complex provides a plausible mechanism explaining how soluble adhesins act. We hypothesize that during invasion, MIC6 along with adhesins establishes a bridge between the host cell and the parasite.

Key Words: parasite • Toxoplasma gondii • protein targeting • regulated secretion • EGF-like domain

© 2001 The Rockefeller University Press

M. Reiss and N. Viebig contributed equally to this study.

Abbreviations used in this paper: CD, cytoplasmic domain; GPI, glycosyl-phosphatidyl-inositol; HFF, human foreskin fibroblast; IFA, indirect immunofluorescence assay; PBSFCS, 10% PBS FCS; RH, wild-type parasite of RH strain; TM-CDMIC6, transmembrane and CD of the MIC6; TRAP, thrombospondin-related adhesive protein.


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