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Systematic characterization of cancer-associated SPOP mutants reveals novel and reprogrammable degradative activities

Alana G Caldwell1, Harshil Parmar2, Xiaokang Jin2

  • 1Interdisciplinary Biological Sciences Graduate Program, Northwestern University, Evanston, Illinois 60208, United States.

Biorxiv : the Preprint Server for Biology
|December 3, 2025
PubMed

Insights

Speckle-type POZ protein (SPOP) mutations in prostate cancer show substrate-dependent functional loss. These SPOP variants can be repurposed for targeted protein degradation therapies.

Area of Science:

  • Biochemistry
  • Molecular Biology
  • Oncology

Background:

  • Speckle-type POZ protein (SPOP) is a crucial substrate adaptor for the Cullin3-RING ligase (CRL3) complex.
  • Recurrent SPOP mutations, particularly F102C and F133L in the substrate-binding domain, are observed in prostate cancer.
  • The precise biochemical impact of these specific SPOP mutations remains largely uncharacterized.

Purpose of the Study:

  • To elucidate the biochemical consequences of SPOP mutations F102C and F133L.
  • To investigate the substrate specificity and degradative activity of mutant SPOP proteins.
  • To explore the therapeutic potential of SPOP mutants in targeted protein degradation.

Main Methods:

  • Quantitative proteomics was employed to analyze protein degradation.
  • Functional assays were performed to assess the degradative activity of SPOP mutants on nuclear basket proteins (NUP153, TPR).
  • Cellular systems were engineered to evaluate the utility of SPOP mutants in targeted protein degradation (TPD).

Main Results:

  • SPOP-F133L retains degradative activity towards NUP153 and TPR, unlike SPOP-F102C, indicating substrate-dependent loss-of-function.
  • SPOP-F133L mediates partial p53 downregulation via a CRL-dependent, post-translational mechanism, suggesting a novel neo-substrate.
  • Both SPOP-F102C and SPOP-F133L demonstrated efficacy in supporting targeted protein degradation within engineered cellular systems.

Conclusions:

  • The study defines the specific degradative capacities of SPOP mutants F102C and F133L.
  • Findings reveal substrate-dependent functional alterations in SPOP mutations.
  • Mutant SPOP proteins present opportunities for developing mutant-selective E3 ligases for therapeutic applications in targeted protein degradation.
Keywords:
E3 ligaseProtein-protein interactionsProteomicsRecurrent mutationTargeted protein degradation

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