PACRG

PACRG is a conserved axonemal inner-junction protein that, together with FAP20, bridges the A- and B-tubules of ciliary doublet microtubules to stabilize B-tubule dynamics, support inner dynein arm assembly, and enable ciliary motility (PMID:16278296, PMID:17654607, PMID:31116684). Structurally, PACRG adopts a helical repeat fold, binds both assembled doublet microtubules and free tubulin heterodimers, and catalyzes inner junction formation; its crystal structure with MEIG1 reveals a loop-mediated interaction interface essential for PACRG stability (PMID:33529594, PMID:18387367). In elongating spermatids, PACRG forms a manchette-associated complex with MEIG1 and DNALI1 that transports cargo proteins such as SPAG16L for sperm flagellum assembly, with MEIG1 stabilizing PACRG against proteasomal degradation and UCHL3/DNAH10 further regulating PACRG levels through deubiquitination (PMID:25715396, PMID:37083624, PMID:41058558). Outside its ciliary roles, PACRG promotes TNF-induced canonical NF-κB signaling by stabilizing the LUBAC complex, interacting with HOIP, HOIL-1L, and SHARPIN, and functionally substituting for SHARPIN to restore linear ubiquitylation and protect cells from TNF-induced apoptosis (PMID:32019898).

1. 2005 High

   The fundamental question of whether PACRG has a structural role in cilia was answered when knockdown in Trypanosoma brucei demonstrated that PACRG is required for outer doublet microtubule integrity and flagellar motility, establishing it as a core axonemal component.

   Evidence RNAi knockdown of both PACRG homologues in T. brucei with GFP localization and TEM

   PMID:16278296

   Open questions at the time

   • Precise position within the doublet was unknown
   • Mechanism of structural contribution unresolved
   • Mammalian relevance not yet established
2. 2007 High

   The question of where exactly PACRG sits within the doublet was resolved by immuno-electron microscopy placing it at the inner junction between A- and B-tubules, defining its structural niche.

   Evidence Immuno-EM and Sarkosyl fractionation of Chlamydomonas axonemes

   PMID:17654607

   Open questions at the time

   • Binding partners at the inner junction not identified
   • Whether PACRG directly contacts tubulin was untested
3. 2008 Medium

   Whether PACRG directly engages tubulin was answered by in vitro reconstitution showing high-affinity binding to α/β-tubulin heterodimers and microtubule bundling activity, establishing PACRG as a bona fide tubulin-binding protein.

   Evidence Co-sedimentation and fluorescence microscopy with purified tubulin

   PMID:18387367

   Open questions at the time

   • In vivo relevance of bundling activity unclear
   • Binding domain not mapped
   • No structural data on the PACRG-tubulin interface
4. 2012 Medium

   Whether PACRG functions in vertebrate cilia was established when morpholino knockdown in Xenopus produced left-right asymmetry defects and neural tube closure defects, linking PACRG to developmental ciliopathy phenotypes.

   Evidence Morpholino knockdown in Xenopus with phenotypic scoring and localization

   PMID:23351225

   Open questions at the time

   • Morpholino approach lacks genetic knockout confirmation
   • Downstream signaling pathways not defined
   • No mammalian loss-of-function model at this point
5. 2015 High

   The question of how PACRG participates in spermatogenesis was answered by demonstrating that PACRG and MEIG1 form a manchette complex that transports SPAG16L cargo for flagellum assembly, with reciprocal stabilization between the partners.

   Evidence Yeast two-hybrid, reciprocal co-IP, and knockout mouse analysis with proteasome inhibition

   PMID:25715396

   Open questions at the time

   • Motor driving manchette transport not identified
   • Full cargo repertoire unknown
   • Whether other axonemal proteins require this complex was untested
6. 2016 Medium

   Three studies collectively expanded PACRG's functional scope: it was placed within a radial spoke–inner dynein arm regulatory cascade in Chlamydomonas, mapped to a MEIG1 hydrophobic binding interface by mutagenesis, and shown to influence G-protein signaling and longevity in C. elegans nonmotile cilia.

   Evidence Chlamydomonas mutant sliding assays; site-directed mutagenesis with pull-downs; C. elegans epistasis and behavioral assays

   PMID:26726850 PMID:27193298 PMID:27770595

   Open questions at the time

   • C. elegans signaling role not confirmed in mammals
   • Structural basis of radial spoke interaction unknown
   • How PACRG couples to G-protein signaling unresolved
7. 2019 High

   A central structural question—how the inner junction is built—was resolved by cryo-electron tomography showing that PACRG and FAP20 together constitute the inner junction bridge along all nine doublets, and that their absence specifically impairs inner dynein arm b assembly and ciliary motility.

   Evidence Cryo-ET of Chlamydomonas pacrg mutants, in vitro sliding assay with rescue by exogenous protein

   PMID:31116684

   Open questions at the time

   • Atomic-resolution structure of PACRG in situ not yet available
   • Mechanism of inner dynein arm recruitment by PACRG/FAP20 unknown
8. 2020 High

   An unexpected non-ciliary function was uncovered: PACRG stabilizes the LUBAC ubiquitin ligase complex and promotes TNF-induced NF-κB signaling, functionally substituting for SHARPIN in SHARPIN-deficient cells to restore linear ubiquitylation and suppress apoptosis.

   Evidence Reciprocal co-IP, NF-κB reporter assays, SHARPIN-deficient cell rescue, TNF receptor complex purification

   PMID:32019898

   Open questions at the time

   • Whether PACRG and SHARPIN act redundantly in vivo unknown
   • How ciliary versus NF-κB functions are partitioned in different tissues unresolved
   • Structural basis of PACRG–LUBAC interaction not determined
9. 2021 High

   The atomic structure of human PACRG–MEIG1 revealed a helical repeat fold and showed that PACRG simultaneously binds doublet microtubules and recruits free tubulin to catalyze inner junction formation, providing the first high-resolution mechanistic model for inner junction biogenesis.

   Evidence X-ray crystallography and single-molecule fluorescence microscopy

   PMID:33529594

   Open questions at the time

   • Full in situ atomic model of PACRG within the doublet not achieved
   • Whether tubulin recruitment is rate-limiting for inner junction assembly unknown
10. 2023 High

    The manchette transport machinery was further defined when DNALI1 was shown to interact with and stabilize PACRG, and its loss caused mislocalization of the entire MEIG1/PACRG/SPAG16L complex, establishing DNALI1 as an upstream organizer.

    Evidence Reciprocal co-IP, conditional knockout mouse, immunofluorescence

    PMID:37083624

    Open questions at the time

    • Motor protein driving the DNALI1–PACRG complex along manchette microtubules not identified
    • Whether DNALI1 functions as an inner arm dynein light chain in this context is unclear
11. 2025 Medium

    Regulation of PACRG protein levels during spermiogenesis was further elaborated by showing that UCHL3 deubiquitinates and stabilizes PACRG, with DNAH10 bridging the UCHL3–PACRG interaction to facilitate intra-manchette transport.

    Evidence Co-IP, pull-down, localization in DNAH10-deficient mice

    PMID:41058558

    Open questions at the time

    • Specific ubiquitin linkage type cleaved by UCHL3 on PACRG not defined
    • Whether DNAH10 motor activity is required for transport unknown
    • Single lab finding

• Key unresolved questions include how PACRG's ciliary and NF-κB signaling functions are differentially regulated across tissues, whether PACRG loss causes ciliopathy in mammals, and the complete cargo repertoire transported by the MEIG1–PACRG–DNALI1 manchette complex.
• No mammalian PACRG knockout phenotype fully characterized
• Tissue-specific regulation of ciliary versus NF-κB roles unknown
• Complete manchette transport cargo inventory not established