MPLKIP

MPLKIP (TTDN1/C7orf11) is a nuclear protein with dual roles in mitotic progression and pre-mRNA splicing, and its loss causes non-photosensitive trichothiodystrophy through a mechanism distinct from the TFIIH-dependent photosensitive form (PMID:15645389, PMID:16977596, PMID:37507019). In mitosis, MPLKIP physically interacts with the polo-like kinase Plk1 via a C-terminal consensus Plk1-binding motif; this interaction depends on prior Cdk1-mediated phosphorylation, including at Thr120 and at Ser93/Ser104 (PMID:17310276). MPLKIP co-localizes with Plk1 at the centrosome during mitosis and at the midbody during cytokinesis, and its dysregulation by knockdown or overexpression produces multipolar spindles and multinucleation, establishing a role in mitotic and cytokinetic fidelity (PMID:17310276). In a second role, MPLKIP directly binds and stabilizes the RNA lariat debranching enzyme DBR1, raising steady-state DBR1 levels in primary cells and stimulating its catalytic efficiency ~19-fold in vitro through engagement of DBR1's intrinsically disordered C-terminal domain (PMID:37800682, PMID:37507019). Through its interaction with DBR1 and core splicing factors, MPLKIP supports proper pre-mRNA splicing, and its loss impairs splicing and keratinocyte differentiation in human skin equivalents, linking the molecular defect to the trichothiodystrophy phenotype (PMID:37800682, PMID:37507019).

1. 2005 Medium

   Established that MPLKIP is a nuclear protein and, contrary to the canonical photosensitive trichothiodystrophy paradigm, does not act directly on TFIIH, signaling a distinct disease mechanism.

   Evidence Comparative immunofluorescence localization in patient-relevant cells

   PMID:15645389

   Open questions at the time

   • Did not define molecular function
   • Subnuclear distribution and binding partners unresolved
2. 2007 Medium

   Defined a phosphorylation-dependent mitotic role by showing MPLKIP binds Plk1 through a C-terminal Plk1-binding motif requiring Cdk1 priming phosphorylation.

   Evidence Yeast two-hybrid screen, site-directed mutagenesis of Thr120/Ser93/Ser104, in-cell phosphorylation assays

   PMID:17310276

   Open questions at the time

   • Whether MPLKIP is a Plk1 substrate or scaffold not resolved
   • Downstream mitotic targets of the complex unknown
   • Single lab, two-hybrid based interaction
3. 2007 Medium

   Connected the Plk1 interaction to function by localizing MPLKIP to the centrosome and midbody and showing perturbation causes spindle and cytokinesis defects.

   Evidence Immunofluorescence co-localization, siRNA knockdown and overexpression

   PMID:17310276

   Open questions at the time

   • Mechanism by which MPLKIP loss causes multipolarity unclear
   • Relationship between mitotic defects and trichothiodystrophy not established
4. 2007 Medium

   Confirmed in patient cells that MPLKIP disease is independent of UV response and TFIIH stability, separating it mechanistically from photosensitive TTD.

   Evidence UV survival assay and TFIIH western blot in patient-derived cells

   PMID:16977596

   Open questions at the time

   • Did not identify the causal molecular pathway
   • Negative result; positive function still undefined
5. 2016 Low

   Provided clinical-genetic support that a phosphorylation-site-disrupting splice variant is pathogenic, reinforcing the functional importance of MPLKIP phosphorylation sites.

   Evidence Splicing assay, Sanger and exome sequencing of patient variant

   PMID:26880286

   Open questions at the time

   • Single study with limited mechanistic depth
   • Does not directly demonstrate functional loss of the phosphorylation-dependent activity
6. 2023 High

   Identified a splicing-related function by showing MPLKIP binds and stabilizes the debranching enzyme DBR1 and core splicing factors, with loss impairing splicing and keratinocyte differentiation.

   Evidence AP-MS interaction proteomics, western blot in MPLKIP-deficient fibroblasts, human skin equivalent differentiation model

   PMID:37800682

   Open questions at the time

   • Mechanism of DBR1 stabilization not defined
   • Spectrum of mis-spliced transcripts driving the phenotype not fully mapped
7. 2023 High

   Defined the biochemical mechanism of MPLKIP action on splicing by showing it directly activates DBR1 catalysis ~19-fold via DBR1's intrinsically disordered C-terminal domain.

   Evidence In vitro debranching assay with purified proteins, NMR of the disordered domain, cross-species domain dissection (Entamoeba histolytica Dbr1)

   PMID:37507019

   Open questions at the time

   • Structure of the MPLKIP-DBR1 complex unknown
   • How activation integrates with the Plk1/mitotic role not addressed

• Whether the mitotic Plk1-dependent role and the splicing/DBR1 activation role represent independent functions or a unified mechanism remains unresolved.
• No experiment links the centrosomal/cytokinesis function to DBR1 activation
• Relative contribution of each role to trichothiodystrophy not quantified