DSK2

Toxoplasma gondii DSK2 (TGGT1_223680) is a UBL-UBA shuttle protein of the ubiquitin-proteasome system that localizes to the cytoplasm and nucleus and functions to escort ubiquitinylated substrates to the proteasome for degradation (PMID:32808330). It acts redundantly with the shuttle proteins DDI1 and RAD23: combined loss of the three proteins causes accumulation of ubiquitinylated proteins (PMID:32808330). Because all reported mechanistic phenotypes derive from a triple knockout, the individual contribution of DSK2 has not been isolated in the available corpus.

1. 2020 Medium

   Establishing whether T. gondii encodes functional ubiquitin-shuttle machinery, this work identified DSK2 as a cytoplasmic/nuclear UBL-UBA shuttle protein whose loss (with DDI1 and RAD23) triggers accumulation of ubiquitinylated proteins, placing it in the proteasomal degradation route.

   Evidence Triple-gene knockout (DDI1, RAD23, DSK2), subcellular localization, and ubiquitinated-protein accumulation assay in TGGT1

   PMID:32808330

   Open questions at the time

   • DSK2-specific contribution cannot be separated from DDI1/RAD23 in a triple-KO
   • no direct substrate of DSK2 identified
   • no reconstitution of DSK2-proteasome escort in vitro
2. 2020 Low

   Testing whether shuttle-protein function extends beyond proteostasis, deletion of the shuttle proteins including DSK2 produced asynchronous cell division, linking the ubiquitin-shuttle machinery to coordination of synchronous parasite replication.

   Evidence Triple-gene knockout (DDI1, RAD23, DSK2) with cell division phenotype analysis in TGGT1

   PMID:32808330

   Open questions at the time

   • phenotype observed only in triple-KO; DSK2-specific role not attributed
   • mechanism linking shuttle function to division timing unknown
3. 2021 Low

   Probing the downstream consequences of shuttle-protein loss, mutation of the shuttle proteins including DSK2 caused nuclear accumulation of ubiquitinated proteins, aberrant DNA replication, and mistargeting of the virulence effector ROP18, which in turn led to aberrant host Irgb6 recruitment to the parasitophorous vacuole membrane.

   Evidence Shuttle-protein mutant analysis, subcellular localization of ubiquitinated proteins, ROP18 localization, and host Irgb6 recruitment assay in TGGT1

   PMID:34727385

   Open questions at the time

   • findings derive from triple-KO; DSK2-specific role not separable from DDI1/RAD23
   • mechanistic link between shuttle-protein loss and ROP18 mistargeting not defined
   • whether DSK2 directly handles ROP18 as a substrate is untested

• The specific, non-redundant contribution of DSK2 — its direct substrates and how its loss individually affects proteostasis, cell division, and virulence-effector targeting — remains unresolved because all phenotypes to date derive from combined deletion with DDI1 and RAD23.
• no single DSK2 knockout phenotype reported
• no DSK2-specific substrate identified
• no structural or biochemical characterization of DSK2 UBL/UBA domains