Filamentous aggregates are fragmented by the proteasome holoenzyme

Abstract

Filamentous aggregates (fibrils) are regarded as the final stage in the assembly of amyloidogenic proteins and are formed in many neurodegenerative diseases. Accumulation of aggregates occurs as a result of an imbalance between their formation and removal. Although there have been numerous studies of the aggregation process in vitro, far fewer studies of aggregate disassembly and degradation are available. Here we use single-aggregate imaging to show that large fibrils assembled from full-length tau are substrates of the 26S proteasome holoenzyme, which fragments them into small aggregates. TEM further revealed that these small aggregate species had no distinct structure. The intact proteasome holoenzyme is required to effectively target fibrils. Interestingly, while degradation of monomeric tau was not inhibited by ATP{gamma}S, fibril fragmentation was predominantly dependent on the ATPase activity of the proteasome. The proteasome holoenzyme was also found to target fibrils assembled from -synuclein (S), suggesting that its fibril fragmenting function may be a general mechanism. The fragmented species produced by the proteasome showed significant toxicity to human cell lines compared to intact fibrils. Together, our results indicate that the proteasome holoenzyme possesses a novel fragmentation function that disassembles large fibrils into smaller and more cytotoxic species.