Ubiquilin
proteins facilitate delivery of
ubiquitinated proteins to the
proteasome for degradation. Interest in the
proteins has been heightened by the discovery that gene mutations in UBQLN2 cause dominant inheritance of
amyotrophic lateral sclerosis (ALS). However, the mechanisms by which the mutations cause ALS are not known. Here we report on the underlying defect of ubiquilin-2
proteins containing ALS-linked mutations in affecting
proteasome-mediated degradation. We found that overexpression of ubiquilin-2
proteins containing any one of five different ALS mutations slow degradation of Myc, a prototypic
proteasome substrate. Examination of coprecipitating
proteins indicated that the
mutant proteins are generally capable of binding polyubiquitinated
proteins, but defective in binding the
proteasome. GST-pulldown studies revealed that many of the mutants bind weaker to the S5a subunit of the
proteasome, compared with wild type (WT) ubiquilin-2
protein. The results suggest the
mutant proteins are unable to deliver their captured cargo to the
proteasome for degradation, which presumably leads to toxicity. Quantification of cell death is consistent with this idea. Measurement of
protein turnover further indicated the
mutant proteins have longer half-lives than WT ubiquilin-2. Our studies provide novel insight into the mechanism by which ALS-linked mutations in UBQLN2 interfere with protein degradation.