RUNDC1

RUNDC1 is a negative regulator of autophagy that acts at the terminal autophagosome-lysosome fusion step (PMID:37684417, PMID:37876308). It localizes to mature autophagosomes, colocalizing with LC3 in both human cell lines and zebrafish, positioning it at the fusion-competent compartment prior to lysosome engagement (PMID:37876308). Mechanistically, RUNDC1 interacts with ATG14 and stimulates ATG14 homo-oligomerization, clasping the ATG14-STX17-SNAP29 complex; this prevents ATG14 dissociation and blocks VAMP8 from binding the STX17-SNAP29 complex, thereby preventing assembly of the fusogenic STX17-SNAP29-VAMP8 SNARE complex (PMID:37684417, PMID:37876308). Phosphorylation of RUNDC1 at Ser379 is required for this inhibitory activity (PMID:37684417). An earlier RNAi screen additionally identified RUNDC1 as a negative regulator of p53 transcriptional activity and p53-mediated apoptosis, but no molecular mechanism for this link has been characterized in the available corpus (PMID:16929179).

1. 2006 Low

   Before any mechanistic role was known, a functional screen asked whether RUNDC1 influences stress-response transcription, establishing it as a negative regulator of p53 transcriptional output.

   Evidence High-throughput RNAi screen with p53 reporter and overexpression apoptosis readout in human cells

   PMID:16929179

   Open questions at the time

   • Single lab RNAi screen with no molecular mechanism identified for the RUNDC1-p53 link
   • No direct physical interaction or pathway intermediate defined
   • Not connected to the later-defined autophagy function
2. 2023 High

   The core question of RUNDC1's molecular function was answered by identifying it as an ATG14 interactor that blocks autophagosome-lysosome fusion, defining the SNARE-clasping mechanism.

   Evidence Reciprocal co-IP, gain/loss-of-function in human cells and zebrafish, in vitro liposome and autophagosome-lysosome fusion reconstitution assays

   PMID:37684417 PMID:37876308

   Open questions at the time

   • Upstream signals controlling RUNDC1 engagement of ATG14 not defined
   • Structural basis of the ATG14 homo-oligomerization clasp unresolved
3. 2023 Medium

   How RUNDC1 inhibitory activity is regulated was partially addressed by showing Ser379 phosphorylation is required to drive ATG14 homo-oligomerization and SNARE blockade.

   Evidence Site-directed mutagenesis of Ser379 with SNARE-assembly and fusion functional readouts

   PMID:37684417

   Open questions at the time

   • The kinase (writer) and phosphatase (eraser) acting on Ser379 are not identified
   • Conditions triggering Ser379 phosphorylation are unknown
4. 2023 Medium

   Where RUNDC1 acts within the autophagy pathway was established by localizing it to mature LC3-positive autophagosomes prior to lysosome fusion.

   Evidence LC3 colocalization imaging and subcellular localization in cell lines and zebrafish

   PMID:37876308

   Open questions at the time

   • Mechanism of recruitment to the autophagosome membrane not defined
   • Whether localization depends on Ser379 phosphorylation or ATG14 binding untested

• Whether the autophagy regulatory function and the p53 regulatory function reflect a single unified mechanism or two independent activities remains unresolved.
• No molecular bridge between RUNDC1 autophagy function and p53 regulation
• Identity of the Ser379 kinase remains unknown
• No structural model of the RUNDC1-ATG14 interaction