Interactome screening implicates BAG6 as a suppressor of UBQLN2 misfolding in ALS-dementia

Ubiquilin-2 (UBQLN2) is a ubiquitin (Ub)-binding shuttle protein that is mutated in X-linked forms of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). ALS/FTD-linked mutations in UBQLN2 disrupt its conformation, increasing its tendency to form cytoplasmic aggregates that may disrupt cellular regulation through loss-of-function (LOF) and gain-of-function (GOF) effects. To explore how ALS-associated mutations impact UBQLN2 function, we performed quantitative mass spectrometry (MS)-based interactome analysis using affinity-purified UBQLN2 from inducible pluripotent stem cells (iPSCs) and induced motor neurons (iMNs) expressing wild-type UBQLN2 (UBQLN2WT), a UBQLN2P497H clinical mutant, or a UBQLN24XALS allele harboring four disease mutations. Proteins showing enhanced association with ALS-mutant UBQLN2 proteins included PEG10, a known degradation target of UBQLN2, and BAG6, a chaperone involved in the triage of mislocalized proteins (MLPs). BAG6 knockdown inhibited the solubility recovery of both wild-type and ALS-mutant UBQLN2 proteins following heat stress (HS), suggesting it functions as a UBQLN2 holdase. In addition, knockdown of BAG6 or knockout of UBQLN2 led to PEG10 accumulation, implicating both in PEG10 turnover; however, neither BAG6 nor UBQLN2 was required for PEG10 degradation in response to HS. The aggregation prone UBQLN24XALS mutant showed increased PEG10 binding and modestly delayed PEG10 turnover while PEG10 degradation was not significantly different between UBQLN2WT and UBQLN2P497H iPSCs. The combined findings implicate BAG6 a UBQLN2 holdase and identify a suite of proteins whose altered binding may contribute to pathologic changes in UBQLN2-associated ALS/FTD.