Corepressor inactivation through tandem linear motif-induced polymerization

The transcriptional corepressor C-terminal binding proteins (CtBPs) form redox-dependent homo-tetrameric assemblies that regulate the expression of various epithelial, pro-apoptotic and pro-oncogenic genes through their involvement in the multi-component Polycomb repressive complex 2 (PRC2) 1. As CtBPs bind to short linear PxDLS-like sequence motifs 2, the presence of a tandem ALDLS motif on the putative metastasis suppressor Retinoic acid-induced 2 (RAI2) protein suggested a possible dual, non-canonical interaction with CtBPs 3. Here, we show that RAI2 induces CtBP polymerization through well-ordered filaments of stacked tetrameric CtBP layers. Consistently, we observed RAI2-mediated CtBP nuclear foci in RAI2-expressing cancer cells. RAI2-mediated CtBP polymerization synchronized with the relief of CtBP-mediated repression of the tumor suppressor gene CDKN1A. Conversely, we found a decline in RAI2 gene expression during the progression of prostate cancer to treatment-resistant subtypes, concomitant with a marked increase of neuroendocrine markers in a RAI2-depleted prostate cancer cell line. Taken together, our data demonstrate a previously unknown mechanism of multivalent short linear sequence motif-induced polymerization, which tips the balance of opposing oncogenic and tumor suppressive activities. Our findings serve as a model for a broad spectrum of related multivalent protein/protein interactions with diverse functional outcomes and provide a blueprint for the design of tandem motif-based inhibitor compounds targeting oncogenic CtBPs.