DPH3

DPH3 (Kti11) is a small CSL-type zinc finger protein that serves as an electron and iron donor essential for both diphthamide biosynthesis on eEF2 and Elongator-dependent tRNA wobble uridine modification. In its reduced state, DPH3 donates electrons to the [4Fe-4S] cluster of the radical-SAM Dph1–Dph2 heterodimer, enabling the first biosynthetic step of diphthamide, and can additionally donate an iron atom to repair oxygen-damaged [3Fe-4S] clusters back to functional [4Fe-4S] form (PMID:24422557, PMID:34154323). DPH3 is reduced by the NADH-dependent cytochrome b5 reductase Cbr1, coupling cellular metabolic state to translational fidelity, and forms a structurally characterized heterodimer with the RCC1-like β-propeller protein Kti13, which constrains access to the DPH3 iron center and modulates electron transfer (PMID:27694803, PMID:25543256, PMID:25604895). A separation-of-function mutation demonstrates that DPH3 operates in two genetically distinct complexes—one with Dph1/Dph2 for diphthamide synthesis and one with Elongator subunits for tRNA modification—and Dph3 knockout in mice causes embryonic lethality by E11.5 with loss of eEF2 diphthamide modification (PMID:18627462, PMID:16648478).

1. 2002 Medium

   Establishing the genetic link between KTI11/DPH3 and Elongator: deletion of KTI11 phenocopied Elongator-minus cells and showed synthetic growth defects with ELP3 deletion, placing DPH3 in the Elongator-dependent pathway before its biochemical role was known.

   Evidence Yeast gene disruption and synthetic genetic interaction analysis in S. cerevisiae

   PMID:11994165

   Open questions at the time

   • No biochemical mechanism identified
   • Physical interaction with Elongator not demonstrated
   • Role in tRNA modification vs. diphthamide biosynthesis not distinguished
2. 2006 High

   Demonstrating physiological essentiality: Dph3 knockout mice die by E11.5 with verified loss of diphthamide on eEF2, establishing that DPH3 is essential for mammalian development and confirming its in vivo requirement for diphthamide biosynthesis.

   Evidence Knockout mouse generation with embryonic phenotype analysis and biochemical verification of diphthamide loss

   PMID:16648478

   Open questions at the time

   • Whether lethality is due to diphthamide loss, tRNA modification defects, or both is unresolved
   • Tissue-specific requirements not defined
3. 2008 Medium

   Resolving that DPH3 operates in two separable complexes: co-immunoprecipitation showed DPH3 interacts with both Dph1/Dph2 and Elongator subunits Elp2/Elp5, and a separation-of-function C-terminal truncation mutation dissociated these interactions, demonstrating dual complex membership.

   Evidence Co-immunoprecipitation with separation-of-function mutagenesis in S. cerevisiae; co-purification of Kti13 with Kti11

   PMID:18466297 PMID:18627462

   Open questions at the time

   • Stoichiometry and architecture of the Dph1-Dph2-Dph3 complex not defined
   • Whether Kti13 participates in both complexes unclear
4. 2014 High

   Defining the biochemical mechanism: in vitro reconstitution with purified components and EPR spectroscopy demonstrated that reduced DPH3 donates electrons to the [4Fe-4S] cluster of Dph1-Dph2, enabling the radical-SAM-dependent first step of diphthamide biosynthesis, establishing DPH3 as a physiological electron donor rather than a structural cofactor.

   Evidence In vitro reconstitution with purified yeast Dph1, Dph2, Dph3; EPR spectroscopy; mutagenesis of iron-binding residues

   PMID:24422557

   Open questions at the time

   • In vivo electron transfer rate not measured
   • How DPH3 is itself re-reduced in vivo not yet identified
5. 2014 High

   Structural basis of DPH3 regulation by Kti13: crystal structures of the Kti11–Kti13 heterodimer revealed that the RCC1-like β-propeller of Kti13 restricts access to the DPH3 iron center, modulating electron transfer, and confirmed that both metal coordination and heterodimerization are required for diphthamide and tRNA modification functions.

   Evidence X-ray crystallography (2.9 Å and 1.45 Å resolution) with interface mutagenesis validated in vitro and in vivo

   PMID:25543256 PMID:25604895

   Open questions at the time

   • How Kti13-mediated restriction of iron access is dynamically regulated is unknown
   • No structure of the full Dph1-Dph2-Dph3 ternary complex
6. 2016 High

   Identifying the upstream reductase: Cbr1 was identified as the NADH-dependent cytochrome b5 reductase that reduces DPH3, linking cellular NADH/NAD+ ratio to both diphthamide biosynthesis and tRNA modification, completing the electron transfer chain from metabolism to translational control.

   Evidence Proteomic identification of Cbr1 as DPH3 interactor; in vitro NADH-dependent reduction assay; genetic validation in yeast

   PMID:27694803

   Open questions at the time

   • Whether Cbr1 is the sole reductase for DPH3 in vivo is not established
   • Regulation of Cbr1-DPH3 interaction not characterized
7. 2021 High

   Revealing a dual function as iron donor: beyond electron transfer, DPH3 donates an iron atom to repair oxygen-damaged [3Fe-4S] clusters in Dph1-Dph2 back to functional [4Fe-4S], explaining how diphthamide biosynthesis operates under aerobic conditions.

   Evidence In vitro reconstitution with EPR, Mössbauer, and X-ray absorption spectroscopy under aerobic vs. anaerobic conditions

   PMID:34154323

   Open questions at the time

   • Whether iron donation occurs in vivo under physiological O2 tensions not confirmed
   • Source of iron reloading onto DPH3 after donation is unknown

• Key unresolved questions include: the structural basis of DPH3 engagement with the Dph1-Dph2 complex versus Elongator, how DPH3 is reloaded with iron after donation, and whether the developmental lethality in mice reflects loss of diphthamide, tRNA modification, or both pathways.
• No ternary Dph1-Dph2-Dph3 structure available
• Relative contribution of diphthamide vs. tRNA modification to embryonic lethality not dissected
• Iron reloading mechanism onto DPH3 unknown