TRNAU1AP

TRNAU1AP (SECp43) is an RNA-binding protein that orchestrates selenocysteine biosynthesis and incorporation by assembling supramolecular complexes of selenocysteine pathway factors and regulating their subcellular localization. It specifically binds tRNA[Ser]Sec and forms ternary and higher-order complexes with SLA/LP, EFsec, SBP2, SEPSECS, SEPHS1, and SEPHS2, promoting the 2'-O-methylation (Um34) of tRNA[Ser]Sec required for stress-related selenoprotein expression and facilitating nuclear-cytoplasmic shuttling of pathway components (PMID:10606267, PMID:16230358, PMID:16508009, PMID:28414460). Constitutive deletion in mice is embryonic lethal and neuron-specific loss impairs motor function, establishing essential roles in development and neuronal physiology (PMID:26043259). Beyond selenoprotein synthesis, TRNAU1AP directly modulates hundreds of endogenous alternative splicing events, as demonstrated by eCLIP, tethered-function assays, and RNA-seq (PMID:38168984).

1. 1999 High

   The identification of TRNAU1AP as a specific tRNA[Ser]Sec-binding protein established the existence of a dedicated accessory factor for selenocysteine biology beyond the core translation machinery.

   Evidence Affinity purification with anti-SECp43 antibody from HeLa extracts followed by RNA sequencing and Northern blot

   PMID:10606267

   Open questions at the time

   • Identity of the co-purified 48-kDa partner was not determined
   • Direct vs. indirect RNA binding not resolved
   • No functional assay for selenoprotein synthesis included
2. 2005 High

   Demonstrating that TRNAU1AP is required for Um34 methylation of tRNA[Ser]Sec and forms a ternary complex with SLA/LP revealed its mechanistic role in tRNA maturation and factor recruitment, explaining how it controls stress-related selenoprotein expression.

   Evidence RNAi knockdown in NIH3T3 and TCMK-1 cells with tRNA methylation assay, co-IP, and subcellular fractionation

   PMID:16230358

   Open questions at the time

   • The methyltransferase catalyzing Um34 was not identified in this context
   • Whether TRNAU1AP directly contacts the methyltransferase is unknown
   • Mechanism of nuclear import promotion for SLA/LP not defined
3. 2006 High

   Showing that TRNAU1AP promotes EFsec–SBP2 interaction and increases selenocysteine incorporation established it as a scaffold that organizes the selenoprotein translation machinery and controls subcellular distribution of pathway components.

   Evidence In vitro binding assay, co-IP in vivo, co-transfection with immunofluorescence, selenoprotein expression assays

   PMID:16508009

   Open questions at the time

   • Stoichiometry of the supramolecular complex not determined
   • Whether scaffold function requires RNA binding is unclear
   • No reconstitution of selenocysteine incorporation with purified components
4. 2015 High

   Conditional knockout studies revealed that constitutive loss of TRNAU1AP is embryonic lethal and neuron-specific deletion impairs motor function, yet hepatocyte-specific deletion does not alter selenoprotein levels, exposing unexplained tissue-specific dependencies.

   Evidence Cre-lox conditional knockout mice with 75Se metabolic labeling, Western blot, tRNA methylation analysis, behavioral testing

   PMID:26043259

   Open questions at the time

   • Truncated Δ7,8 protein product may retain partial function, confounding interpretation of liver results
   • Molecular basis of neuronal vulnerability not identified
   • Cause of embryonic lethality (selenoprotein-dependent or independent) not resolved
5. 2016 Medium

   Loss- and gain-of-function experiments in cardiomyocyte-like cells linked TRNAU1AP to cell survival via selenoprotein-dependent anti-apoptotic signaling, extending its functional relevance beyond hepatocytes and neurons.

   Evidence siRNA knockdown and overexpression in H9c2 cells with annexin V apoptosis assay, caspase-3 activity, and Western blot for PI3K/Akt and Bcl-2 family

   PMID:28101579

   Open questions at the time

   • PI3K/Akt pathway activation is correlative; direct mechanism linking TRNAU1AP to Akt not shown
   • Effects not distinguished from general selenoprotein deficiency
   • Single cell line and single lab
6. 2017 High

   SAXS and mutagenesis defined TRNAU1AP as a two-RRM globular protein that dimerizes via K166/P167 and interacts with SEPSECS, SEPHS1, and SEPHS2, providing the first structural framework for its scaffold function.

   Evidence SAXS, phage display for dimerization interface, BRET in mammalian cells, co-IP

   PMID:28414460

   Open questions at the time

   • No high-resolution structure of the full-length protein or its RNA-bound state
   • Functional consequence of dimerization disruption on selenoprotein synthesis not tested in cells
   • Whether dimerization is required for splicing function is unknown
7. 2024 High

   The discovery that TRNAU1AP modulates hundreds of alternative splicing events via direct RNA binding fundamentally expanded its functional repertoire beyond selenocysteine biology.

   Evidence Tethered-function splicing reporters, eCLIP, RNA-seq, and AP-MS in human cells

   PMID:38168984

   Open questions at the time

   • RNA sequence/structure determinants of splicing targets not defined
   • Whether splicing and selenocysteine functions are coordinated or independent is unknown
   • Physiological significance of splicing regulation in vivo not tested

• It remains unknown how TRNAU1AP's dual functions in selenocysteine biosynthesis and alternative splicing are coordinated, what determines its tissue-specific essentiality, and what its high-resolution RNA-bound structure looks like.
• No high-resolution co-crystal or cryo-EM structure with tRNA[Ser]Sec or mRNA substrates
• Mechanistic basis of embryonic lethality (selenoprotein vs. splicing vs. both) unresolved
• Whether splicing regulation contributes to neuronal motor phenotype is untested