TULP2

TULP2 is a testis-enriched RNA-binding protein that functions in spermatid differentiation and sperm motility, with its loss causing male sterility marked by elongated spermatid apoptosis, restricted spermatid release, and defective sperm tail structures (PMID:33763418). It is expressed specifically in round and elongating spermatids of the adult testis (PMID:35603654). Mechanistically, TULP2 acts as a coordinator of intraflagellar transport during spermiogenesis: it interacts with IFT components including IFT20, IFT80, IFT70A, BBS7, DYNLT2B, and HDAC6, and its loss mislocalizes IFT20 in testicular spermatids and spermatozoa and IFT70A and IFT80 in spermatozoa (PMID:40613306). Correct folding of TULP2 depends on the chaperonin CCT, whose subunit CCT8 binds TULP2 through its apical domain; CCT8 knockdown causes TULP2 to form cytoplasmic aggregates and impairs its ciliogenic function (PMID:33763418, PMID:40613306). At the ultrastructural level, TULP2 deletion produces abnormal outer dense fiber structure in sperm tails without affecting mitochondrial sheath formation (PMID:35619658), and at the functional level TULP2 is required for sperm capacitation, the acrosome reaction, and fertilization (PMID:40613306). TULP2 protein abundance is governed by a conserved acetylation/ubiquitination switch shared across the TULP family, in which acetylation stabilizes the protein and ubiquitination at the unacetylated site promotes its turnover (PMID:33187986).

1. 2000 Low

   An initial question was where TULP2 protein resides in cells; ectopic expression placed it in the nucleus, an early and isolated localization clue.

   Evidence GFP-tagged TULP2 expression and fluorescence imaging in transfected cells

   PMID:11000483

   Open questions at the time

   • Single overexpression experiment with GFP tag, no endogenous validation
   • No functional consequence demonstrated
   • Conflicts with later spermatid cytoplasmic/ciliary localization data
2. 2020 Medium

   How TULP2 protein levels are controlled was unknown; assays showed its stability is set by a conserved acetylation/ubiquitination switch shared across the TULP family.

   Evidence Protein-interaction network analysis with acetylation/ubiquitination assays and pharmacological modulation, primarily for TULP3 and extended to TULP2

   PMID:33187986

   Open questions at the time

   • TULP2 data secondary to TULP3 findings
   • Specific regulated lysines for TULP2 not mapped
   • Physiological context of the switch in spermatids untested
3. 2021 High

   The cellular role of TULP2 was undefined; a knockout established it as a testis-specific, spermatid-localized RNA-binding protein essential for spermatid differentiation and sperm motility.

   Evidence Tulp2 knockout mice with transcriptome sequencing and IP-MS

   PMID:33763418

   Open questions at the time

   • Direct RNA targets and binding mode not defined
   • Causal link between transcript regulation and tail defects unresolved
4. 2021 Medium

   It was unclear how TULP2 attains a functional state; IP-MS identified CCT8 as a partner, implicating the CCT chaperonin in folding TULP2.

   Evidence Immunoprecipitation and mass spectrometry in testis

   PMID:33763418

   Open questions at the time

   • Single Co-IP/MS in original report
   • Folding dependence not yet demonstrated functionally at this stage
5. 2021 Medium

   Whether TULP2 is functionally interchangeable with its ciliary paralog TULP3 was tested; rescue assays showed TULP2 cannot restore ciliogenesis and only partially restores ciliary protein trafficking, defining limited functional equivalence.

   Evidence Rescue experiments in TULP3-knockout RPE-1 cells with ciliary localization assays for ARL13B, INPP5E, GPR161, IFT140

   PMID:34462398

   Open questions at the time

   • Heterologous somatic cell context, not spermatids
   • Basis for partial rescue not mechanistically explained
6. 2022 High

   The structural target of TULP2 loss in the sperm tail was unknown; two independent KO lines identified abnormal outer dense fiber structure while sparing the mitochondrial sheath.

   Evidence Two independent Tulp2 KO lines with scanning and transmission electron microscopy

   PMID:35619658

   Open questions at the time

   • Molecular link between TULP2 and ODF assembly unresolved
   • Whether the defect is formation or maintenance not distinguished
7. 2022 Medium

   The precise cell types expressing TULP2 were refined; validated antibody staining localized it to round and elongating spermatids.

   Evidence Immunofluorescence with validated polyclonal antibody and Western blot

   PMID:35603654

   Open questions at the time

   • Subcellular compartment within spermatids not resolved at this stage
   • Temporal dynamics across spermiogenesis not mapped
8. 2025 High

   The mechanistic role of TULP2 was clarified as an IFT coordinator: it binds multiple IFT components and its loss mislocalizes them, while CCT8 apical-domain binding is required to keep TULP2 soluble and functional.

   Evidence Co-IP, CCT8 knockdown with localization analysis, and quantitative proteomics in Tulp2 KO mice

   PMID:40613306

   Open questions at the time

   • Direct versus indirect nature of each IFT interaction not fully resolved
   • How TULP2 organizes IFT cargo movement mechanistically unknown
9. 2025 Medium

   Downstream physiological consequences were defined; TULP2 KO mice fail in sperm capacitation, acrosome reaction, and fertilization.

   Evidence Tulp2 knockout mouse analysis with sperm function assays

   PMID:40613306

   Open questions at the time

   • Single lab
   • Mechanistic chain from IFT/ODF defects to capacitation failure not delineated

• How TULP2's RNA-binding activity, IFT coordination, and outer dense fiber assembly are mechanistically integrated during spermiogenesis remains unresolved.
• No direct RNA target set defined
• No structure of TULP2 or its IFT complexes
• Causal hierarchy among transcript regulation, IFT trafficking, and ODF defects unknown