GPKOW

GPKOW is a nuclear RNA-binding protein and an essential component of the human spliceosome required for pre-mRNA splicing (PMID:25296192). It directly binds both pre-mRNA and the spliceosomal DEAH-box helicase DHX16/hPRP2, and immuno-depletion of GPKOW inactivates the spliceosome while DHX16 remains bound, with splicing activity restored by recombinant GPKOW add-back (PMID:25296192). The physical GPKOW–DHX16 contact is mediated by the G-patch domain, yet G-patch mutants that lose this interaction retain splicing activity and still suppress a DHX16 dominant-negative defect, indicating GPKOW functionally modulates DHX16 in vivo independently of stable physical contact (PMID:25296192). RNA engagement through the KOW1 domain contributes directly to splicing function, as KOW1 mutations reduce both RNA binding and activity in vitro and in vivo (PMID:25296192). GPKOW's RNA-binding activity is regulated by phosphorylation: it is a direct partner and substrate of the PKA catalytic subunit Cβ2, which phosphorylates it at two sites, and its RNA binding in cells is PKA-sensitive, linking PKA signaling to GPKOW function (PMID:21880142).

1. 2011 Medium

   Whether GPKOW is connected to a signaling pathway was unknown; identifying it as a PKA substrate established a regulatory link between PKA signaling and its RNA-binding activity.

   Evidence Yeast two-hybrid with PKA Cβ2 bait, in vitro PKA kinase assay, phosphosite mutagenesis, and RNA immunoprecipitation

   PMID:21880142

   Open questions at the time

   • PKA Cβ2 interaction rests on yeast two-hybrid without described reciprocal validation
   • Functional consequence of phosphorylation on splicing not tested
   • Physiological signals driving this phosphorylation not defined
2. 2014 High

   Whether GPKOW has a role in splicing was unknown; depletion-rescue established it as an essential spliceosomal factor that directly binds pre-mRNA and the helicase DHX16.

   Evidence Co-IP, RNA-binding assay, immuno-depletion from HeLa nuclear extracts with recombinant add-back, in vitro splicing assay

   PMID:25296192

   Open questions at the time

   • Spliceosomal assembly step at which GPKOW acts not pinpointed
   • Structural basis of pre-mRNA recognition not resolved
3. 2014 High

   Whether the G-patch–DHX16 physical contact is required for function was unknown; G-patch mutagenesis showed the interaction is dispensable for splicing activity, dissociating physical binding from functional modulation.

   Evidence G-patch site-directed mutagenesis, Co-IP, in vitro splicing assay, in vivo dominant-negative DHX16 suppression assay

   PMID:25296192

   Open questions at the time

   • Mechanism by which GPKOW modulates DHX16 without stable contact unexplained
   • Whether transient or alternative contacts substitute for the G-patch not addressed
4. 2014 High

   The contribution of RNA binding to GPKOW function was unknown; KOW1 mutagenesis tied reduced RNA binding to reduced splicing activity, establishing KOW1-mediated RNA interaction as functionally relevant.

   Evidence KOW1 site-directed mutagenesis, RNA-binding assay, in vitro splicing assay, in vivo rescue assay

   PMID:25296192

   Open questions at the time

   • RNA sequence/structure specificity of KOW1 binding undefined
   • Effect was modest, leaving other RNA-contacting regions unidentified

• How PKA phosphorylation, KOW1 RNA binding, and DHX16 modulation are integrated to control splicing in cells remains unresolved.
• No structural model of GPKOW within an assembled spliceosome
• Functional impact of PKA phosphorylation on splicing output untested
• Genome-wide RNA targets and physiological splicing program of GPKOW uncharacterized