PPP1R35

PPP1R35 is a resident centrosomal protein localized to the proximal centriole lumen, where it serves dual roles in centriole elongation and centriole-to-centrosome conversion (CCC). It acts downstream of the microcephaly protein RTTN to promote distal centriole elongation and upstream of CEP295 to drive CCC, with its putative PP1-interacting motif being dispensable for both localization and CCC function (PMID:30168418, PMID:30230954). Loss of PPP1R35 in mouse embryos abolishes primary cilia formation, disrupts notochord development, and causes cell-cycle stalling, confirming an essential in vivo role in centriole homeostasis and ciliogenesis (PMID:32628936). PPP1R35 is a direct phosphorylation substrate of CDKL5 and interacts with the centrosomal protein CEP131, connecting centriole biogenesis to CDKL5 kinase signaling (PMID:39136782).

1. 2018 High

   Defining where PPP1R35 resides within the centriole and what happens when it is lost established it as a core structural regulator of centriole elongation acting in complex with the microcephaly protein RTTN.

   Evidence Quantitative super-resolution microscopy, siRNA knockdown, BioID proximity proteomics, and Co-IP in human cells

   PMID:30168418

   Open questions at the time

   • Whether PPP1R35 has catalytic activity or acts solely as a scaffold remains unknown
   • Structural basis of the PPP1R35–RTTN interaction is unresolved
   • How PPP1R35 recruits or stabilizes distal centriole components is not mechanistically defined
2. 2018 High

   Demonstrating that PPP1R35-null cells form nascent centrioles that fail to recruit CEP295 and disintegrate after mitosis placed PPP1R35 as the earliest known factor required for centriole-to-centrosome conversion, independent of its putative PP1-binding motif.

   Evidence CRISPR/Cas9 knockout, epistasis analysis of CEP295 recruitment, and domain mutagenesis in human cells

   PMID:30230954

   Open questions at the time

   • The molecular mechanism by which PPP1R35 promotes CEP295 recruitment is unknown
   • Whether PPP1R35 engages PP1 phosphatase in any other context remains untested
   • The relationship between centriole elongation and CCC functions of PPP1R35 is not disentangled
3. 2020 High

   In vivo validation in a mouse knockout model confirmed that PPP1R35 is essential for primary cilia formation, neural tube patterning, and cell-cycle progression, extending the cell-culture findings to organismal physiology.

   Evidence Homozygous loss-of-function mouse model with histology, immunofluorescence, and molecular marker analysis

   PMID:32628936

   Open questions at the time

   • Tissue-specific and conditional requirements for PPP1R35 have not been examined
   • Whether the cell-cycle delay is a direct consequence of centriole loss or a checkpoint response is unresolved
   • No human Mendelian disease link has been established
4. 2024 Medium

   Identification of PPP1R35 as a direct CDKL5 kinase substrate and CEP131 interaction partner connected the centriole elongation/cilia pathway to CDKL5 signaling, suggesting a regulatory input controlling PPP1R35 function.

   Evidence Unbiased phosphoproteomics, in vitro kinase assay, and Co-IP in iPSC-derived neurons

   PMID:39136782

   Open questions at the time

   • CEP131 interaction relies on a single Co-IP without reciprocal validation
   • Functional consequence of CDKL5-mediated phosphorylation on PPP1R35 activity or localization is unknown
   • Whether CDKL5 phosphorylation regulates centriole elongation or CCC has not been tested

• The molecular mechanism by which PPP1R35 coordinates centriole elongation and centriole-to-centrosome conversion — including its direct binding partners on the centriole wall and how CDKL5 phosphorylation modulates its function — remains to be defined.
• No structural model of PPP1R35 or its complexes exists
• Whether PPP1R35 dysfunction underlies a human ciliopathy or microcephaly syndrome is untested
• The relationship between CDKL5-dependent phosphorylation and the PPP1R35–RTTN or PPP1R35–CEP295 pathways has not been examined