SARNP

SARNP (CIP29/THO1) is a conserved RNA-binding component of the TREX mRNA export complex that couples cotranscriptional mRNP assembly to nuclear export (PMID:20844015, PMID:16738307). Within TREX, an ATP-dependent interaction mediated by UAP56/DDX39B bridges SARNP and Aly, with UAP56 forming a trimeric complex that scaffolds SARNP onto the THO complex (PMID:20844015, PMID:31313837). SARNP engages DDX39B through tandem DDX39B-interacting motifs in its C-terminal RNA-binding domain, enabling a single SARNP molecule to simultaneously bind up to five DDX39B molecules and stimulate DDX39B/Sub2 helicase activity by acting as a rigid scaffold that promotes its oligomerization on RNA (PMID:37578863, PMID:41543169). This activity is required for efficient nuclear export of mRNAs, with GC-rich transcripts most dependent on SARNP, and its recruitment to mRNA is splicing- and cap-dependent, consistent with cotranscriptional loading (PMID:20844015, PMID:37578863). The protein has a bipartite architecture: an N-terminal SAP DNA-binding domain and a distinct C-terminal RNA-binding domain that carries the helicase-stimulating motifs (PMID:27303905, PMID:16738307). Beyond export, SARNP is phosphorylated on a conserved serine in an ATM-dependent manner upon DNA double-strand breaks, though it is dispensable for DNA end-joining (PMID:28715428), and through interaction with pinin it downregulates E-cadherin to promote epithelial-to-mesenchymal transition (PMID:31313837).

1. 2002 Medium

   Before its RNA-export role was known, the question was what cellular process CIP29 participates in; identifying an N-terminal SAP DNA-binding motif and a cell-cycle-promoting effect gave the first functional foothold.

   Evidence cDNA cloning and GFP overexpression with flow-cytometry cell cycle analysis in HEK293 and UT7/Epo cells

   PMID:11922608

   Open questions at the time

   • Overexpression readout does not establish endogenous function
   • SAP domain DNA-binding activity not biochemically demonstrated
   • No mechanistic link to mRNA biology yet
2. 2006 High

   The yeast ortholog Tho1 was placed in the mRNA biogenesis pathway by showing it binds transcribed chromatin in a THO- and RNA-dependent manner and genetically suppresses THO-mutant export and recombination defects, establishing a conserved RNA-export-linked function.

   Evidence Multicopy suppressor genetics in hpr1Δ yeast, ChIP, and in vitro RNA binding localizing activity to the C-terminal half

   PMID:16738307

   Open questions at the time

   • Direct partners within THO/TREX not biochemically defined
   • Conservation to human protein assumed, not shown here
3. 2010 High

   The architecture of human TREX was resolved by showing UAP56 bridges CIP29 and Aly into an ATP-dependent trimeric complex, defining how SARNP is incorporated and that assembly requires ATP.

   Evidence Proteomics of immunopurified TREX plus recombinant protein reconstitution and ATP-dependence assays; localization to nuclear speckles with splicing/cap-dependent RNA recruitment

   PMID:20844015

   Open questions at the time

   • Structural basis of the bridging interaction unknown
   • Functional consequence of SARNP within TREX for export not directly tested
4. 2016 High

   Two structural and functional questions were addressed: the domain architecture of the protein was defined atomically, and a viral context for SARNP recruitment was identified.

   Evidence NMR structures of the yeast Tho1 SAP and C-terminal RNA-binding domains; Co-IP and siRNA knockdown showing CIP29 in the KSHV ORF57 vRNP

   PMID:27189710 PMID:27303905

   Open questions at the time

   • NMR structures lack RNA or partner-bound complexes
   • How ORF57 redirects SARNP to viral mRNAs not mechanistically defined
5. 2017 Medium

   Whether SARNP participates in the DNA damage response was tested, revealing ATM-dependent phosphorylation on a conserved serine but no requirement for end-joining, narrowing its DDR role to a regulated modification of uncertain output.

   Evidence Xenopus egg extract DSB system with ATM inhibition, phospho-site mapping, and DNA end-joining assays

   PMID:28715428

   Open questions at the time

   • Functional consequence of the phosphorylation unknown
   • Whether phosphorylation modulates mRNA export untested
6. 2019 Medium

   The functional output of human SARNP in mRNA metabolism and disease was tied together: it binds UAP56 and Aly, promotes splicing and export, and drives EMT by repressing E-cadherin via pinin.

   Evidence Reciprocal Co-IP, overexpression and shRNA knockdown with splicing/export assays, and a tumor xenograft model

   PMID:31313837

   Open questions at the time

   • Mechanism linking export function to E-cadherin repression unclear
   • Single-lab functional data without structural detail
7. 2023 High

   The molecular logic of SARNP–DDX39B engagement was solved structurally, showing tandem DDX39B-interacting motifs let one SARNP bind up to five DDX39B molecules, with GC-rich mRNAs most export-dependent on SARNP.

   Evidence X-ray crystallography of a Tho1/DDX39B/RNA complex, biochemical binding assays, and RNA-seq from SARNP knockdown cells

   PMID:37578863

   Open questions at the time

   • Why GC-rich mRNAs are preferentially affected not resolved
   • In vivo stoichiometry of the high-order complex unknown
8. 2026 High

   The enzymatic purpose of the scaffolding was established: the Tho1/SARNP C-terminal domain stimulates DDX39B/Sub2 helicase activity by promoting its oligomerization on RNA through two essential conserved α-helical motifs.

   Evidence In vitro helicase and oligomerization assays, α-helical motif mutagenesis, and cross-species functional complementation

   PMID:41543169

   Open questions at the time

   • How helicase stimulation translates to directional mRNP packaging in cells untested
   • Regulation of this activity (e.g. by phosphorylation) unknown

• It remains unknown how SARNP's DNA-damage phosphorylation and its EMT/E-cadherin functions mechanistically connect to its core TREX helicase-scaffolding role.
• No unifying model linking DDR phosphorylation to export activity
• SAP domain function in vivo undefined
• Selectivity for GC-rich transcripts unexplained