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Epithelial septate junction assembly relies on melanotransferrin iron binding and endocytosis in Drosophila

Abstract

Iron is an essential element in many biological processes. In vertebrates, serum transferrin is the major supplier of iron to tissues, but the function of additional transferrin-like proteins remains poorly understood. Melanotransferrin (MTf) is a phylogenetically conserved, iron-binding epithelial protein. Elevated MTf levels have been implicated in melanoma pathogenesis. Here, we present a functional analysis of MTf in Drosophila melanogaster. Similarly to its human homologue, Drosophila MTf is a lipid-modified, iron-binding protein attached to epithelial cell membranes, and is a component of the septate junctions that form the paracellular permeability barrier in epithelial tissues. We demonstrate that septate junction assembly during epithelial maturation relies on endocytosis and apicolateral recycling of iron-bound MTf. Mouse MTf complements the defects of Drosophila MTf mutants. Drosophila provides the first genetic model for the functional dissection of MTf in epithelial junction assembly and morphogenesis.

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Figure 1: MTf is a septate junction component.
Figure 2: MTf co-localizes with early and recycling endosomes.
Figure 3: Endocytosis is required for MTf localization.
Figure 4: MTf is a GPI-anchored iron-binding protein.
Figure 5: Uptake of MTf induces septate junction assembly in MTf mutant cells.

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Acknowledgements

We are indebted to S. Baumgartner, M. González-Gaitán, R. Fehon, G. Beitel, M. Bhat, F. Missirlis, H. Bellen, M. Hortsch, D. Ready, V. Auld, the Bloomington Drosophila Stock Center, the Drosophila Genomics Resource Center (DGRC; Indiana), and the Developmental Studies Hybridoma Bank (DSHB; Iowa) for Drosophila strains, clones and antibodies. We thank M. Sahlin for initial help with iron detection, T. Astlind for the EPR measurements and M. Björk and I. Granell for expert technical assistance. We are grateful to members of the Samakovlis lab for suggestions. K.S. was supported by an EMBO postdoctoral fellowship. The work was supported by grants from the G. Gustafsson Foundation, Vetenskapsrådet and Cancerfonden to C.S.

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C.S., K.S. and K.T. designed the experiments. K.T. performed the experiments. S.W. identified the tracheal overelongation phenotype of the MTf P-element mutant. A.G. analysed the EPR data. C.S., K.T. and K.S. wrote the paper.

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Correspondence to Christos Samakovlis.

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The authors declare no competing financial interests.

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Tiklová, K., Senti, KA., Wang, S. et al. Epithelial septate junction assembly relies on melanotransferrin iron binding and endocytosis in Drosophila. Nat Cell Biol 12, 1071–1077 (2010). https://doi.org/10.1038/ncb2111

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