REXO1

REXO1 (EloA-BP1) is a nuclear DEDDh-family exoribonuclease homolog that physically associates with the transcription elongation machinery. It was identified through its direct binding to the N-terminal ~120-amino-acid region of Elongin A and to the N-terminus of transcription factor SII, and it colocalizes with Elongin A in the nucleus (PMID:12943681). Despite this interaction, purified REXO1 has no detectable effect on the rate of transcription elongation in vitro (PMID:12943681). Structurally, REXO1 belongs to a Rex3-related clade of DEDDh exoribonucleases distinguished by a KIX domain and a cysteine- and histidine-rich (CHORD) domain flanking a C-terminal DEDD catalytic domain; its closest functional reference is yeast Rex3, whose KIX domain region is required for RNase MRP processing in vivo (PMID:40455837). The specific RNA substrates of human REXO1 have not been characterized in the available corpus.

1. 2003 Medium

   Establishing that an uncharacterized exonuclease-domain protein could be linked to transcription elongation, REXO1 was found to bind the N-terminus of Elongin A and SII and to share nuclear localization with Elongin A.

   Evidence Yeast two-hybrid screening, in vitro binding assay, and double immunofluorescence in COS7 cells

   PMID:12943681

   Open questions at the time

   • Interaction shown by Y2H and in vitro binding without reciprocal endogenous co-IP validation
   • Functional consequence of the Elongin A/SII interaction not established
   • RNA substrate or enzymatic activity not tested
2. 2003 Medium

   To test whether REXO1 modulates the elongation complex it binds, its effect on transcription was assayed directly, showing it does not alter elongation rate in vitro and decoupling the binding interaction from any elongation-rate function.

   Evidence In vitro transcription elongation assay with purified REXO1

   PMID:12943681

   Open questions at the time

   • Negative result does not exclude effects on elongation processivity, pausing, or termination
   • Does not address a role in vivo or under stress conditions
   • Catalytic exonuclease activity of REXO1 itself not assayed
3. 2025 Medium

   Placing REXO1 in a structural and evolutionary framework, modeling assigned it to a Rex3-related DEDDh clade defined by KIX and CHORD domains adjacent to a DEDD catalytic domain, linking it via the yeast Rex3 ortholog to RNase MRP-type RNA processing.

   Evidence Protein structure modeling, sequence comparison, and in vivo RNA-processing analysis of yeast Rex3 deletion mutants

   PMID:40455837

   Open questions at the time

   • Domain assignment for human REXO1 rests on structural homology, not direct biochemistry
   • RNase MRP processing role demonstrated only for yeast Rex3, not human REXO1
   • No human RNA substrate identified

• The endogenous RNA substrates and catalytic role of human REXO1, and how its Elongin A/SII binding integrates with its predicted exoribonuclease activity, remain undefined.
• No human RNA substrate identified
• Exoribonuclease activity of human REXO1 not directly demonstrated
• Functional link between transcription-factor binding and RNA processing unresolved