OBSOLETE. A large multisubunit protease found in the cytosol that recognizes, unfolds, and digests protein substrates that have been marked for degradation by the attachment of a ubiquitin group. Individual subcomplexes of the complete 26S proteasome are involved in these different tasks: the ATP-dependent 19S caps are believed to unfold substrates and feed them to the actual protease, the 20S proteasome.