# Systematic Characterization of Cancer‐Associated SPOP Mutants Reveals Novel and Reprogrammable Degradative Activities

## TL;DR

This study explores how specific mutations in the SPOP protein affect its ability to degrade other proteins, revealing new therapeutic opportunities in cancer.

## Contribution

The study identifies distinct degradative activities of SPOP mutants and suggests their potential use as mutant-selective E3 ligases for targeted protein degradation in cancer.

## Key findings

- SPOP-F133L retains degradation of NUP153 and TPR but SPOP-F102C does not.
- SPOP-F133L partially down-regulates p53 through a Cullin-RING ligase-dependent mechanism.
- Both SPOP mutants support targeted protein degradation in engineered cells.

## Abstract

Speckle‐type POZ protein (SPOP) functions as the substrate adaptor of the Cullin3‐RING ligase complex and is recurrently mutated in multiple cancer types. Among these, F102C and F133L are frequent prostate cancer mutations within the substrate‐binding domain, yet their biochemical consequences remain incompletely understood. Using quantitative proteomics, we show that SPOP‐F133L, unlike SPOP‐F102C, retains degradative activity toward the nuclear basket proteins NUP153 and TPR, indicating substrate‐dependent loss‐of‐function. Moreover, SPOP‐F133L induces partial down‐regulation of p53 through a Cullin‐RING ligase‐dependent, post‐translational mechanism, revealing a potential neo‐substrate relationship. Finally, we demonstrate that both SPOP‐F102C and SPOP‐F133L support targeted protein degradation in an engineered cellular system. These findings define the degradative capacities of SPOP mutants and highlight opportunities to repurpose these variants as mutant‐selective E3 ligases for therapeutic applications.

Quantitative proteomics reveal distinct activities of speckle‐type POZ protein (SPOP) cancer mutants. SPOP‐F133L, unlike F102C, retains degradation of NUP153 and TPR, and partially down‐regulates p53 through a Cullin‐RING ligase‐dependent mechanism. Both mutants support targeted protein degradation in engineered cells, highlighting opportunities to exploit SPOP variants for ligand‐induced protein degradation in cancers harboring these mutations.© 2026 WILEY‐VCH GmbH