Anomalies in dismantling the machinery of DNA replication can compromise genome integrity and contribute to tumorigenesis and aging. In this issue of Genes & Development, Dewar and colleagues (pp. 275-290) identified an E3 ubiquitin ligase, CUL2^LRR2, that modifies a subunit of the replicative CMG (Cdc45, minichromosome maintenance [MCM] subunits 2-7, and the GINS complex) helicase and triggers disassembly of the replication machinery. Their study offers critical insight into the mechanism of DNA replication termination while at the same time raising important questions for future research.

All eukaryotes require intricate protein networks to translate developmental signals into accurate cell fate decisions. Mutations that disturb crucial interactions between network components often result in disease, but how the composition and dynamics of complex networks are established is unknown. Here, we identify the tumor suppressor E3 ligase UBR5 as a quality control enzyme that helps degrade unpaired subunits of multiple transcription factors that operate within a single network. By constantly turning over orphan subunits, UBR5 forces cells to continuously replenish network components through new protein synthesis. The resulting cycles of transcription factor synthesis and degradation allow cells to effectively execute the gene expression program, while remaining susceptible to environmental signals. We conclude that orphan quality control plays an essential role in establishing the dynamics of protein networks, which may explain the conserved need for protein degradation in transcription and offers unique opportunities to modulate gene expression in disease.

Most quality control pathways target misfolded proteins to prevent toxic aggregation and neurodegeneration¹. Dimerization quality control further improves proteostasis by eliminating complexes of aberrant composition², but how it detects incorrect subunits remains unknown. Here we provide structural insight into target selection by SCF-FBXL17, a dimerization-quality-control E3 ligase that ubiquitylates and helps to degrade inactive heterodimers of BTB proteins while sparing functional homodimers. We find that SCF-FBXL17 disrupts aberrant BTB dimers that fail to stabilize an intermolecular β-sheet around a highly divergent β-strand of the BTB domain. Complex dissociation allows SCF-FBXL17 to wrap around a single BTB domain, resulting in robust ubiquitylation. SCF-FBXL17 therefore probes both shape and complementarity of BTB domains, a mechanism that is well suited to establish quality control of complex composition for recurrent interaction modules.