Calnuc binds to LRP9 and affects its endosomal sorting.
Julie Brodeur1, Heidi Larkin, Rémi Boucher
1Department of Pharmacology, Faculty of Medicine, Université de Sherbrooke, QC, Canada.
Traffic (Copenhagen, Denmark)
|June 6, 2009
Summary
Calnuc, a calcium-binding protein, interacts with LRP9, a transmembrane protein. Calnuc
Area of Science:
- Cell Biology
- Molecular Biology
- Protein Interactions
Background:
- Calnuc (Calreticulin) is a ubiquitous Ca(++)-binding protein involved in various cellular processes.
- LDLR-related protein 9 (LRP9) is a transmembrane protein that cycles between the TGN and endosomes.
- The interaction and functional relationship between Calnuc and LRP9 were previously unknown.
Purpose of the Study:
- To characterize the molecular interaction between Calnuc and LRP9.
- To investigate the role of Calnuc in the trafficking and sorting of LRP9.
Main Methods:
- Biochemical assays to determine the binding domains of Calnuc and LRP9.
- Confocal microscopy to visualize colocalization of Calnuc and LRP9.
- Small interfering RNA (siRNA) to deplete Calnuc and observe effects on LRP9 trafficking.
- Rescue experiments using siRNA-resistant Calnuc.
Main Results:
- The N-terminus of Calnuc directly interacts with an arginine-rich region in the cytosolic tail of LRP9.
- Calnuc and LRP9 colocalize at the trans-Golgi network (TGN) and early endosomes.
- Calnuc depletion leads to missorting of LRP9 to late endosomes/lysosomes and enhanced degradation.
- The cytoplasmic pool of Calnuc is crucial for proper LRP9 endosomal sorting.
Conclusions:
- Calnuc directly binds to LRP9 and plays a critical role in its endosomal trafficking.
- Cytoplasmic Calnuc prevents the lysosomal degradation of LRP9 by ensuring correct sorting.
- This study reveals a novel function of Calnuc in receptor trafficking.
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