FIG. 3.

Transmission electron micrographs of negatively stained M. thermophila 20S proteasomes and subunits. (A) α subunit produced in E. coli independently of the β prosubunit. Seven-membered ring structures (α₇) with no apparent central pore are visible (arrowheads). (B) β prosubunit produced in E. coli independently of the α subunit. (C) Authentic 20S proteasome purified from M. thermophila. End-on views (single arrow and arrowheads) and lateral views (double arrowheads) of the cylindrical 20S proteasome comprised of four stacked (α₇β₇β₇α₇) rings. (D) 20S proteasome produced in E. coli. The structures are indistinguishable from those in C. (E) The 20S proteasome assembled in vitro from α₇ and β prosubunits independently produced in E. coli. The structures are indistinguishable from those in C and D. Bars, 50 nm.