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A more recent version of this article appeared on October 1, 2007
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Submitted on July 18, 2007
Revised on July 25, 2007
Accepted on July 31, 2007
Hospital for Sick Children Research Institute, Department of Biochemistry and Laboratory Medicine and Pathobiology, University of Toronto, Toronto, ON, Canada, M5G 1X8
Monitoring Editor: Sandra Schmid
Lysosomal targeting is fundamental for the regulated disposal of ubiquitinated membrane proteins from the cell surface. To elucidate ubiquitin (Ub) configurations that are necessary and sufficient as multi-vesicular body (MVB)/lysosomal sorting motifs, the intraendosomal destination and transport kinetics of model transmembrane cargo molecules bearing mono-, multi-mono- or poly-ubiquitinated cytoplasmic tails were determined. Monomeric CD4-chimeras with K63-linked poly-Ub chains and tetrameric CD4-mono-Ub chimeras were rapidly targeted to the lysosome. In contrast, lysosomal delivery of CD4 chimeras exposing K48-linked Ub chains was delayed, while delivery of mono-ubiquitinated CD4 chimeras was undetectable. Similar difference was observed in the lysosomal targeting of mono- versus poly-ubiquitinated invariant chain and CD4 ubiquitinated by the MARCH (Membrane-Associated RING-CH) IV Ub ligase. Consistent with this, Hrs (hepatocyte growth factor regulated tyrosine kinase phosphorylated substrate), an endosomal sorting adaptor, binds preferentially to K63-Ub chain and negligibly to mono-Ub. These results highlight the plasticity of Ub as a sorting signal and its recognition by the endosomal sorting machinery, and together with previous data, suggest a dynamic role for assembly and disassembly of Ub chains of specific topology in lysosomal cargo sorting.
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