E3 ligase recruitment by UBQLN2 protects substrates from proteasomal degradation

Ubiquilins are a family of proteins critical to cellular proteostasis that are also linked to neurodegenerative diseases such as ALS, with specific mutations in UBQLN2 causing dominant, X-linked ALS. Despite an initial characterization as proteasomal shuttle factors, Ubiquilins have paradoxically been reported to stabilize numerous substrates. The basis of this triage decision remains enigmatic. Many other fundamental aspects of Ubiquilin function are unclear at the mechanistic level, such as the physiological significance of Ubiquilin phase separation, the unique role of each Ubiquilin paralog, and the mechanistic defects of ALS mutants. To address these questions, we utilized a library of triple knockout (TKO) rescue cell lines with physiological expression of single Ubiquilin paralogs or disease mutants in an isogenic background. Our findings reveal that UBQLN2 has a unique ability to protect substrates from degradation and that substrate stabilization correlates with the recruitment of multiple E3 ligases, including SCF ^bxo7 . Substrate stabilization is diminished in ALS mutants and this defect is exacerbated upon overexpression. We propose that UBQLN2 is a molecular chaperone that recruits multiple E3 ligases to modulate substrate stability. This proposal unifies many existing observations in the field and presents a new paradigm for understanding Ubiquilin function in health and disease.