Properties of the hybrid form of the 26S proteasome containing both 19S and PA28 complexes

Abstract

PA28 is a γ-interferon-induced complex that associates with the 20S proteasome and stimulates breakdown of small peptides. Recent immunoprecipitation studies indicate that, in vivo, PA28 also exists in larger complexes that also contain the 19S particle, which is required for ATP-ubiquitin-dependent degradation of proteins. However, because of its lability, the structure and properties of this larger complex remain unclear. Here, we demonstrate that, in vitro, PA28 can associate with 'singly capped' 26S (i.e. 19S-20S) proteasomes. Electron microscopy of the resulting structures revealed one PA28 ring at one end of the 20S particle and a 19S complex at the other. These hybrid complexes show e..