Large-scale quaternary structural transitions underlie gain of function of SPOP cancer mutations.

Speckle-type POZ protein (SPOP), a substrate receptor for the Cullin-3-RING (CRL3) ubiquitin ligase, is mutated in different cancers. Both activating and inactivating mutations in SPOP drive oncogenesis, underscoring the need for precise regulation. Among substrate receptors, SPOP uniquely assembles into filaments that are multivalent for substrate binding. Conversely, many substrates contain multiple SPOP-binding motifs. How this unusual reciprocal multivalent architecture regulates ubiquitination and how mutations activate SPOP are unclear. Gain-of-function mechanisms are also generally poorly understood. Here, we reveal that SPOP assemblies exist in an equilibrium between an active filament and a large, autoinhibited, circular "double-donut" state. Activating mutations shift the equilibrium, resulting in aberrant substrate turnover. Combinations of activating and inactivating mutations can produce intermediate activities, thus uncovering a tunable regulatory axis with implications for targeted cancer therapies. Therefore, SPOP's ability to assemble into long filaments is required for its regulation in human cells and underlies a gain-of-function mechanism.