Membrane-less organelles in cells are large, dynamic protein/protein or protein/RNA assemblies that have been reported in some cases to have liquid droplet properties. However, the molecular interactions underlying the recruitment of components are not well understood. Herein, we study how the ability to form higher-order assemblies influences the recruitment of the speckle-type POZ protein (SPOP) to nuclear speckles. SPOP, a cullin-3-RING ubiquitin ligase (CRL3) substrate adaptor, self-associates into higher-order oligomers; that is, the number of monomers in an oligomer is broadly distributed and can be large. While wild-type SPOP localizes to liquid nuclear speckles, self-association-deficient SPOP mutants have a diffuse distribution in the nucleus. SPOP oligomerizes through its BTB and BACK domains. We show that BTB-mediated SPOP dimers form linear oligomers via BACK domain dimerization, and we determine the concentration-dependent populations of the resulting oligomeric species. Higher-order oligomerization of SPOP stimulates CRL3SPOP ubiquitination efficiency for its physiological substrate Gli3, suggesting that nuclear speckles are hotspots of ubiquitination. Dynamic, higher-order protein self-association may be a general mechanism to concentrate functional components in membrane-less cellular bodies.
Bibliographical noteFunding Information:
We thank BA Schulman, WK Pierce, EW Martin, and CR Grace for valuable discussions and N Clark for preliminary data collection. We also thank H Haecker (St. Jude Children's Research Hospital). This work was funded by a V Foundation Scholar Grant (T.M.), R01GM112846 (T.M.), R01GM101087 (S.K.O.), the National Cancer Institute Cancer Center Support Grant P30CA21765 (at St. Jude Children's Research Hospital), the American Lebanese Syrian Associated Charities (to J.P.T., S.K.O., and T.M.), and the Intramural Program of the National Institute of Biomedical Imaging and Bioengineering (P.S.). Microscopy images were acquired at the Cell & Tissue Imaging Center at St. Jude Children's Research Hospital.
- isodesmic self-association
- membrane-less organelle
- prostate cancer
- speckle-type POZ protein
- ubiquitin ligase