TMUB2

TMUB2 is a ubiquitin-like domain-containing transmembrane protein that functions in endoplasmic reticulum-associated degradation (ERAD) of misfolded ER membrane proteins (PMID:32738194). It is a component of an ER membrane quality-control complex that includes the ubiquitin ligase RNF185, TMUB1, and TMEM259/Membralin, and that cooperates with the cytosolic ubiquitin ligase UBE3C and the p97 ATPase to ubiquitinate and extract a subset of misfolded ER membrane substrates for degradation (PMID:32738194). Beyond its membership in this RNF185-Membralin ERAD complex, no further mechanistic detail for TMUB2 has been characterized in the available corpus.

1. 2016 Low

   An initial interaction screen placed TMUB2 in the candidate interactome of the PRL-1/PRL-3 oncogenic phosphatases, raising the possibility of a role in phosphatase-associated signaling.

   Evidence Yeast two-hybrid screen with BLAST-based identification, in which TMUB2 was recovered as a candidate partner but not validated biochemically

   PMID:27882103

   Open questions at the time

   • TMUB2-PRL interaction was not confirmed by immunoprecipitation or any independent biochemical method
   • no functional consequence of a TMUB2-PRL association was demonstrated
   • yeast two-hybrid hits frequently reflect indirect or spurious binding
2. 2020 High

   Defining TMUB2's molecular function, it was established as a subunit of an ER membrane ERAD complex that drives degradation of a subset of misfolded ER membrane proteins, anchoring the gene to protein quality control.

   Evidence Genome-wide CRISPR-Cas9 screen, biochemical fractionation and mass spectrometry for complex membership, plus functional ERAD assays with model substrates

   PMID:32738194

   Open questions at the time

   • the specific contribution of the TMUB2 ubiquitin-like domain to complex assembly or substrate handoff is not resolved
   • the full substrate repertoire selected by this complex is not defined
   • no structural model of TMUB2 within the RNF185-Membralin complex is available

• How TMUB2 specifically recognizes or coordinates substrate ubiquitination within the RNF185-Membralin-UBE3C-p97 axis, and whether its earlier reported phosphatase association is biologically relevant, remain unresolved.
• mechanistic role of the ubiquitin-like domain is uncharacterized
• no reconciliation of ERAD function with the 2016 PRL phosphatase screen
• no quantitative substrate spectrum or kinetics reported