ROPN1L

ROPN1L is an A-kinase anchoring scaffold of the motile axoneme that organizes PKA-dependent signaling required for sperm flagellar motility and male fertility (PMID:23303679). In the sperm fibrous sheath it binds AKAP3 and functions redundantly with its paralog ROPN1, such that single loss of either protein causes only modest motility defects with compensatory upregulation of the other, whereas combined loss abolishes motility, thins and shreds the principal piece, and depletes AKAP3, establishing that ROPN1L and ROPN1 together are required to incorporate AKAP3 into the fibrous sheath and to support PKA-dependent phosphorylation events during capacitation (PMID:23303679). ROPN1L acts downstream of capacitation signaling: phosphorylated FSCB binds ROPN1L/ROPN1 with high affinity and suppresses their SUMOylation, and ROPN1L cysteines undergo reversible oxidative modification within the cAMP/PKA redox pathway during capacitation (PMID:27398160, PMID:34360666). Beyond the fibrous sheath, ROPN1L is a constituent of the radial spoke 1 stalk of the flagellar axoneme (PMID:39849482), and its role in ciliary structure and motility extends to motile cilia in a metazoan model, where its knockdown shortens cilia and reduces swimming (PMID:42079374).

1. 2013 High

   Established that ROPN1L is an AKAP3-binding fibrous sheath scaffold acting redundantly with ROPN1 to enable PKA signaling and sperm motility, answering whether these paralogs are functionally required and how they relate.

   Evidence Single and double knockout mice with sperm motility assays, Western blot, immunofluorescence and morphology

   PMID:23303679

   Open questions at the time

   • Does not resolve the direct molecular consequence of AKAP3 loss on specific PKA substrates
   • Compensation mechanism between ROPN1L and ROPN1 at the regulatory level is undefined
2. 2016 Medium

   Placed ROPN1L downstream of PKA-mediated FSCB phosphorylation by showing phospho-FSCB binds ROPN1L and suppresses its SUMOylation to regulate motility.

   Evidence Co-immunoprecipitation and SUMOylation-inhibition rescue with sperm motility assays

   PMID:27398160

   Open questions at the time

   • SUMOylation sites on ROPN1L not mapped
   • Single-lab finding without reciprocal validation of the FSCB-ROPN1L interaction
3. 2021 Medium

   Linked ROPN1L to redox signaling by demonstrating reversible cysteine oxidation during capacitation.

   Evidence Fluorescent gel-based redox proteomics, Western blot, flow cytometry in bull sperm

   PMID:34360666

   Open questions at the time

   • Functional consequence of ROPN1L oxidation not tested
   • Specific oxidized cysteines and their effect on AKAP3/PKA scaffolding unknown
4. 2025 Medium

   Identified ROPN1L as a structural component of the radial spoke 1 stalk, extending its role beyond the fibrous sheath to the axonemal radial spoke.

   Evidence Protein mass spectrometry, Western blotting and electron microscopy in IQUB-deficient patients and Iqub-/- mice

   PMID:39849482

   Open questions at the time

   • Whether ROPN1L is structurally essential for RS1 assembly versus a passenger is not resolved
   • Relationship between its RS1 role and its fibrous sheath/AKAP3 role is unclear
5. 2025 Low

   Connected ROPN1L abundance to spermiogenic flagellar assembly programs via NSUN7-dependent regulation.

   Evidence Proteomic profiling of sorted spermatids from Nsun7 knockout mice

   PMID:41516134

   Open questions at the time

   • Single proteomics dataset with no direct functional follow-up on ROPN1L
   • Whether NSUN7 regulates ROPN1L directly or indirectly is unknown
6. 2025 Low

   Identified ROPN1L as a marker of multi-ciliated cell identity in a neoplastic context, suggesting expression beyond sperm.

   Evidence Single-cell RNA sequencing, immunohistochemistry and electron microscopy in breast cancer

   PMID:39936988

   Open questions at the time

   • No functional mechanistic experiment on ROPN1L in these cells
   • Marker association does not establish a functional role
7. 2026 Medium

   Demonstrated a conserved requirement for ROPN1L in motile cilia structure and motility in a non-mammalian metazoan.

   Evidence RNAi knockdown in Schmidtea mediterranea with swim rate and cilia length measurement

   PMID:42079374

   Open questions at the time

   • Molecular partners of ROPN1L in planarian cilia not defined
   • Whether the AKAP3/PKA mechanism operates in non-sperm cilia is untested

• How ROPN1L coordinates its dual localization to the fibrous sheath and the radial spoke 1 stalk, and whether its PKA-scaffolding mechanism generalizes to somatic motile cilia, remains unresolved.
• No structural model of ROPN1L within RS1 or its AKAP3 complex
• Functional importance of capacitation-induced SUMOylation and redox modifications in vivo not established