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Traffic. 2015 Jun;16(6):604-16. doi: 10.1111/tra.12273. Epub 2015 Apr 14.

A Conserved Di-Basic Motif of Drosophila Crumbs Contributes to Efficient ER Export.

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Max Planck Institute of Molecular Cell Biology and Genetics, Pfotenhauerstr.108, 01307, Dresden, Germany.
Present address: Membrane Traffic and Cell Division, Institut Pasteur, 28 rue du Dr Roux, 75724 Paris, France.


The Drosophila type I transmembrane protein Crumbs is an apical determinant required for the maintenance of apico-basal epithelial cell polarity. The level of Crumbs at the plasma membrane is crucial, but how it is regulated is poorly understood. In a genetic screen for regulators of Crumbs protein trafficking we identified Sar1, the core component of the coat protein complex II transport vesicles. sar1 mutant embryos show a reduced plasma membrane localization of Crumbs, a defect similar to that observed in haunted and ghost mutant embryos, which lack Sec23 and Sec24CD, respectively. By pulse-chase assays in Drosophila Schneider cells and analysis of protein transport kinetics based on Endoglycosidase H resistance we identified an RNKR motif in Crumbs, which contributes to efficient ER export. The motif identified fits the highly conserved di-basic RxKR motif and mediates interaction with Sar1. The RNKR motif is also required for plasma membrane delivery of transgene-encoded Crumbs in epithelial cells of Drosophila embryos. Our data are the first to show that a di-basic motif acts as a signal for ER exit of a type I plasma membrane protein in a metazoan organism.


COPII; Crumbs; Drosophila; ER export; Sar1; di-basic motif

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