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A more recent version of this article appeared on June 1, 2002
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Submitted on October 2, 2001
Revised on February 20, 2002
Accepted on March 20, 2002
1 Centre d'Immunologie de Marseille-Luminy, CNRS-INSERM-Univ-Med, Campus de Luminy, Case 906, 13288 Marseille Cedex 09, France, and Division of Molecular Cell Biology, Department of Biology, University of Oslo, Norway
2 Division of Molecular Cell Biology, Department of Biology, University of Oslo, Norway
3 Centre d'Immunologie de Marseille-Luminy, CNRS-INSERM-Univ-Med, Campus de Luminy, Case 906, 13288 Marseille Cedex 09, France
4 Istituto di Chimica Biomolecolare del CNR, Comprensorio Olivetti, Edificio 70, I-80078 Pozzuoli (Napoli), Italy
* Corresponding author. E-mail address: oddmund.bakke{at}bio.uio.no.
The MHC class II associated Invariant chain (Ii) has been shown to inhibit endocytic transport and to increase the size of endosomes. We have recently found that this property has a significant impact on antigen processing and presentation. Here, we show in a cell-free endosome fusion assay that expression of Ii can increase fusion after PI3-kinase activity is blocked by wortmannin. In live cells wortmannin was also not able to block formation of the Ii induced enlarged endosomes. The effects of Ii on endosomal transport and morphology depend upon elements within the cytoplasmic tail. Data from mutagenesis analysis and NMR-based structure calculations of the Ii cytoplasmic tail demonstrate that free negative charges that are not involved in internal salt bridges were essential for both interactions between the tails and for the formation of enlarged endosomes. This correlation indicate that it is interactions between the Ii cytoplasmic tails that are involved in endosome fusion. The combined data from live cells, cell-free assays and molecular dynamic simulations suggest that Ii molecules on different vesicles can promote endosome docking and fusion and thereby control endosomal traffic of membrane proteins and endosomal content.
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