20S Proteasomes are non-lysosomal, high molecular weight proteinases implicated in the degradation of misfolded proteins and several short-lived regulatory proteins. They have a well established role, as the core of the 26S proteasome complex, in the ubiquitin-dependent proteolytic pathway and in antigen processing. While correctly folded proteins are not degraded by the 20S proteasome, unfolding, for example by oxidation, may render them degradable. The 20S proteasome is a 700-kDa cylindrical particle, composed of 14 subunits of molecular masses 20-35 kDa. There is evidence that 20S proteasomes are in close proximity to or associate with the endoplasmic reticulum and nuclear and plasma membranes in vivo. To better understand the lipid association of 20S proteasomes in vitro, we used a lipid monolayer system as a simple model system for biological membranes. The structure and orientation of the monolayer lipid bound 20S proteasomes has been determined by electron microscopy. 20S proteasomes associated in an "end-on' configuration specifically on PI lipid monolayers forming large arrays, with their channels opposite the lipid headgroups. On ER and Golgi lipid films 20S proteasomes were oriented in the same way as on the PI lipid film but were monodisperse. Protein molecules were randomly oriented in the presence of PA, PG, PS, PC and mitochondrial lipid monolayers. We show that 20S proteasomes bind to phospholipids in vitro in a preferred orientation which places the proteasome channel perpendicular to the membrane.