Mitochondrial membrane proteins that are imported from the cytosol contain insoluble transmembrane domains (TMDs) and thus rely on factors to prevent their aggregation in the cytosol and to route them for degradation upon import failure. Hegde and colleagues report that ubiquilins (UBQLNs) perform this function. UBQLNs interacted with the TMDs of mitochondrial membrane proteins in a dynamic manner, providing a mechanism by which client proteins can be targeted to mitochondria (by unknown factors) while remaining soluble. However, this mechanism was time-limited as UBQLNs also recruited an E3 ligase through their carboxy-terminal ubiquitin-associating domain. Ubiquitylation served as a 'fate switch' as it blocked mitochondrial import and enabled the amino-terminal ubiquitin-like domain of UBQLNs to engage with the proteasome more efficiently, thus favouring degradation.
References
Itakura, E. et al. Ubiquilins chaperone and triage mitochondrial membrane proteins for degradation. Mol. Cell 63, 21–33 (2016)
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Baumann, K. Triaging mitochondrial membrane proteins. Nat Rev Mol Cell Biol 17, 463 (2016). https://doi.org/10.1038/nrm.2016.103
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DOI: https://doi.org/10.1038/nrm.2016.103