Ubiquitin conjugating enzymes participate in polyglutamine protein aggregation

Protein aggregation is a hallmark of several neurodegenerative diseases including Huntington's disease and Parkinson's disease. Proteins containing long, homopolymeric stretches of glutamine are especially prone to form aggregates. It has long been known that the small protein modifier, ubiquitin, localizes to these aggregates. In this report, nematode and cell culture models for polyglutamine aggregation are used to investigate the role of the ubiquitin pathway in protein aggregation.

Ubiquitin conjugating enzymes (Ubc's) were identified that affect polyglutamine aggregates in C. elegans. Specifically, RNAi knockdown of ubc-2 or ubc-22 causes a significant increase in the size of aggregates as well as a reduction in aggregate number. In contrast, RNAi of ubc-1, ubc-13, or uev-1 leads to a reduction of aggregate size and eliminates ubiquitin and proteasome localization to aggregates. In cultured human cells, shRNA knockdown of human homologs of these Ubc's (Ube2A, UbcH5b, and E2-25K) causes similar effects indicating a conserved role for ubiquitination in polyglutamine protein aggregation.

Results of knockdown of different Ubc enzymes indicate that at least two different and opposing ubiquitination events occur during polyglutamine aggregation. The loss of ubiquitin localization after ubc-1, ubc-13, or uev-1 knockdown suggests that these enzymes might be directly involved in ubiquitination of aggregating proteins.