A conserved late endosome–targeting signal required for Doa4 deubiquitylating enzyme function

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Abstract
Enzyme specificity in vivo is often controlled by subcellular localization. Yeast Doa4, a deubiquitylating enzyme (DUB), removes ubiquitin from membrane proteins destined for vacuolar degradation. Doa4 is recruited to the late endosome after ESCRT-III (endosomal sorting complex required for transport III) has assembled there. We show that an N-terminal segment of Doa4 is sufficient for endosome association. This domain bears four conserved elements (boxes A-D). Deletion of the most conserved of these, A or B, prevents Doa4 endosomal localization. These mutants cannot sustain ubiquitin-dependent proteolysis even though neither motif is essential for deubiquitylating activity. Ubiquitin-specific processing protease 5 (Ubp5), the closest paralogue of Doa4, has no functional overlap. Ubp5 concentrates at the bud neck; its N-terminal domain is critical for this. Importantly, substitution of the Ubp5 N-terminal domain with that of Doa4 relocalizes the Ubp5 enzyme to endosomes and provides Doa4 function. This is the first demonstration of a physiologically important DUB subcellular localization signal and provides a striking example of the functional diversification of DUB paralogues by the evolution of alternative spatial signals.