Yeast Proteasome Each ring has 7 subunits. One subunit in an inner ring, and one in an outer ring are shown in cartoons. Empty space can be seen inside. The ends appear closed. Groll, M.,  Kim, K.B.,  Kairies, N.,  Crews, C. Crystal Structure of Epoxomicin:20S Proteasome reveals a molecular basis for selectivity of alpha,beta-Epoxyketone Proteasome Inhibitors J.Am.Chem.Soc. v122 pp.1237-1238 , 2000(JACS) 1g65 (PDB)