KLHDC1

KLHDC1 is a cytoplasmic substrate receptor for a Cul5-type ubiquitin ligase complex that targets truncated SELENOS—a selenoprotein that fails UGA/Sec decoding and therefore lacks its catalytic selenocysteine—for proteasomal degradation, coupling protein quality control of defective selenoproteins to ROS homeostasis (PMID:32200094). Loss of KLHDC1 decreases ER stress-induced cell death and alters cellular ROS levels, placing its degradative activity within the maintenance of oxidoreductase capacity (PMID:32200094). KLHDC1 localizes predominantly to the cytoplasm, distinguishing it from its nuclear paralog KLHDC2; it neither binds actin nor inhibits LZIP/CREB3-mediated transcription (PMID:16964437). Its substrate-binding pocket is functionally distinct from the U-shaped C-degron binding site of KLHDC2, as KLHDC1 does not engage the C-degron ligands recognized by KLHDC2 (PMID:39396041).

1. 2006 Medium

   Established that KLHDC1 is functionally distinct from its paralog KLHDC2, resolving whether the two paralogs share localization and activity.

   Evidence Subcellular fractionation, immunofluorescence, actin-binding and transcriptional reporter assays

   PMID:16964437

   Open questions at the time

   • Did not identify a positive molecular function or substrate for KLHDC1
   • No interacting partners or complex membership defined
   • Cellular role left unassigned
2. 2020 Medium

   Assigned KLHDC1 a concrete molecular role as the substrate-receptor subunit of a Cul5-type ubiquitin ligase that degrades truncated, Sec-lacking SELENOS, linking selenoprotein quality control to ROS homeostasis.

   Evidence siRNA knockdown in U2OS cells with cell death and ROS readouts; identification as a Cul5-type ligase component targeting truncated SELENOS

   PMID:32200094

   Open questions at the time

   • Single-lab study; degradation mechanism not reconstituted in vitro
   • Scope of substrate repertoire beyond SELENOS unknown
   • Structural basis of truncated-SELENOS recognition undefined
3. 2024 Medium

   Demonstrated that KLHDC1's substrate-binding pocket does not recognize the C-degron ligands engaged by KLHDC2, defining its specificity as mechanistically separate from the KLHDC2 C-degron pathway.

   Evidence Selectivity profiling of KLHDC2-targeting ligands against KLHDC1/KLHDC3/KLHDC10 in binding and degradation assays with structure-based ligand design

   PMID:39396041

   Open questions at the time

   • A negative selectivity result; positive degron specificity of KLHDC1 not mapped
   • No KLHDC1 structure provided
   • Does not define how truncated SELENOS is recognized

• The structural basis of KLHDC1 substrate recognition and the full repertoire of its physiological substrates remain undefined.
• No structure of the KLHDC1 substrate-binding pocket
• Substrate degron and recognition determinants for truncated SELENOS unmapped
• Composition of the full Cul5-type ligase complex not characterized