RIBC2

RIBC2 is a microtubule inner protein (MIP) of the doublet microtubules of motile cilia and sperm flagella, where it supports ciliary and flagellar motility (PMID:40265983). In mouse sperm flagella, RIBC2 and its paralog RIBC1 reside in the inner lumen of doublet microtubules, and concurrent loss of both proteins reduces sperm motility and fertility without producing gross axonemal ultrastructural defects, indicating that RIBC2 contributes to motility through a mechanism not resolvable at the level of overall axonemal architecture; its function in this context depends on RIBC1 (PMID:40265983). In multi-ciliated epithelial cells, RIBC2 physically interacts with creatine kinase B (Ckb) and Ybx2 and is required for their localization to the ciliary axoneme, and loss of either partner disrupts ciliary beating and cilia-driven fluid flow, placing RIBC2 upstream of an axonemal beating module (PMID:36508087). Beyond these motile-cilia roles, no further biochemical mechanism for RIBC2 has been characterized in the available corpus.

1. 2020 Low

   Established that RIBC2 is expressed and localized specifically in motile ciliated cells, first placing it in the motile cilia context.

   Evidence Transcriptomic network deconvolution plus in situ mRNA and protein localization in motile ciliated cells

   PMID:32616903

   Open questions at the time

   • Localization without functional consequence established
   • No interaction partners or molecular activity defined
   • Subcompartment within the cilium not resolved
2. 2022 Medium

   Identified physical partners of RIBC2 and showed it is required to recruit them to the axoneme, linking RIBC2 to ciliary beating.

   Evidence Reciprocal IP/MS, morpholino knockdown in Xenopus, immunostaining, and fluorescent-bead flow assays

   PMID:36508087

   Open questions at the time

   • Mechanism by which RIBC2 recruits Ckb/Ybx2 to the axoneme unknown
   • How partner localization translates into beating not defined
   • Single-lab Xenopus model only
3. 2025 Medium

   Defined RIBC2 as a doublet-microtubule inner protein required, together with RIBC1, for sperm flagellar motility and fertility.

   Evidence Single and double CRISPR/Cas9 knockout mice, sperm motility and fertility analysis, TEM, and cryo-EM/AI-based localization

   PMID:40265983

   Open questions at the time

   • No gross ultrastructural defect, so the molecular basis of motility loss is unresolved
   • Functional interdependence with RIBC1 not mechanistically explained
   • Whether the Ckb/Ybx2 module operates in flagella as in cilia untested
4. 2026 Low

   Extended RIBC2 function beyond cilia by showing it promotes esophageal cancer cell proliferation, migration, and invasion.

   Evidence siRNA silencing in esophageal cancer cell lines, proliferation/migration/invasion assays, in vivo experiments, transcriptome sequencing

   PMID:41666178

   Open questions at the time

   • No molecular mechanism or pathway placement for the tumor phenotype
   • Relationship to its ciliary MIP role unknown
   • Single-lab observation

• How RIBC2 as a microtubule inner protein mechanistically supports axonemal beating, and how its ciliary role relates to its proliferative role in cancer cells, remains unresolved.
• No structural model of how RIBC2 stabilizes or tunes doublet microtubules
• Recruitment mechanism for Ckb/Ybx2 undefined
• No link established between MIP function and tumor cell phenotype