Degradation of mutant Ndc10 is mediated by the E3 ligase Doa10 at the endoplasmic reticulum/nuclear envelope membrane. An autonomous degradation motif was localized to the C-terminal region of Ndc10. The motif is composed of two indispensable elements: a hydrophobic surface of an amphipathic helix and a loosely structured, hydrophobic C-terminal tail.
Background: Initiation of protein degradation by the proteasome requires the presence of an unstructured motif, usually located at the termini of the substrate. Results: Disruption of a hydrophobic sequence within such an unstructured motif abolishes proteasomal degradation.
Conclusion:The hydrophobicity requirement reveals a novel feature of degradation initiation sites. Significance: Diverse proteasome substrates can be selectively stabilized by fusing a disrupted degradation initiation site to their termini.
NEDD4-2 (NEDD4L), a ubiquitin protein ligase of the Nedd4 family, is a key regulator of cell surface expression and activity of the amiloride-sensitive epithelial Na+ channel (ENaC). While hypomorphic alleles of Nedd4-2 in mice show salt-sensitive hypertension, complete knockout results in pulmonary distress and perinatal lethality due to increased cell surface levels of ENaC. We now show that Nedd4-2 deficiency in mice also results in an unexpected progressive kidney injury phenotype associated with elevated ENaC and Na+Cl− cotransporter expression, increased Na+ reabsorption, hypertension and markedly reduced levels of aldosterone. The observed nephropathy is characterized by fibrosis, tubule epithelial cell apoptosis, dilated/cystic tubules, elevated expression of kidney injury markers and immune cell infiltration, characteristics reminiscent of human chronic kidney disease. Importantly, we demonstrate that the extent of kidney injury can be partially therapeutically ameliorated in mice with nephron-specific deletions of Nedd4-2 by blocking ENaC with amiloride. These results suggest that increased Na+ reabsorption via ENaC causes kidney injury and establish a novel role of NEDD4-2 in preventing Na+-induced nephropathy. Contrary to some recent reports, our data also indicate that ENaC is the primary in vivo target of NEDD4-2 and that Nedd4-2 deletion is associated with hypertension on a normal Na+ diet. These findings provide further insight into the critical function of NEDD4-2 in renal pathophysiology.
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