The 26S proteasome degrades polyubiquitinated proteins in the cytoplasm and nucleus. It is formed from a cylindrical catalytic core 20S proteasome capped at each end by a regulatory 19S particle. The 19S regulatory particle recognizes, unfolds, and translocates ubiquitinated proteins into the catalytic core particle in an ATP-dependent manner. The core particle is a highly conserved, barrel-shaped complex composed of four stacked heptameric rings. Narrow substrate entry channels are created by the two outer rings, which are each formed by seven alpha subunits. (Adapted from FBrf0215459 and PMID:28583440).